Uploaded by Leonardo Biá

How to Make a Bandgap Voltage Reference by A. Paul Brokaw

advertisement
How to Make a
BANDGAP
VOLTAGE
REFERENCE
in ONE EASY Lesson
V+
(Not to be confused with
the Many Arduous Ones
to become proficient)
R=R
+
A
-
8A
VOUT
A
A. Paul Brokaw
© Copyright 2011, A. Paul Brokaw and Integrated Device Technology, all rights reserved. All diagrams
and figures are the sole property of the author and may not be used without permission.
A voltage reference is not only a convenience,
but is also a necessity for many electronic devices,
for example a voltage DAC which converts a digital input
to an output voltage. This result is the product of the
digital word, which is scalar with no dimension, and
some voltage to which the scaled output is referred.
The DAC needs to “know” how big a certain voltage
is, in order to set its full scale value.
We need a component which “knows” how big a volt is.
It turns out that the silicon all around us
can be persuaded to give up its secret knowledge
of the Volt if we will accommodate it.
The base emitter voltage of a bipolar transistor
versus temperature can, in theory, be extrapolated
to equal a known physical constant which has
the dimension Volt at a temperature of zero Kelvin.
This constant is called the
extrapolated bandgap of silicon.
Unfortunately, this voltage is temperature sensitive,
but predictably so.
However, silicon has a second property which also
relates temperature and voltage and this can be
combined with the change in Vbe to almost cancel
the temperature effects and make a voltage
approximating the bandgap at all temperatures,
or at least the temperatures most of us require.
LET ME BEGIN with a sort of hand-waving description of PN junctions and
bipolar transistors. This representation of a PN junction is shown forward
biased. Positive voltage on the P-type anode (blue,) drives holes toward the
junction with N-type (red) material. At the same time the negative voltage on
the N-type material repels electrons toward the junction where many recombine with the holes. Some of the electrons may cross over into the P-type,
where they are minority carriers that do not spontaneously arise in P-type
silicon. Similarly, holes cross the junction and become minority carriers in
the N-type. Minority carriers will recombine with carriers of opposite polarity, within the silicon.
The carriers that enter the silicon and combine with opposite polarity
carriers do not re-emerge at their injection terminal, and so constitute a net
current flow.
The quantities of the two minority carrier types aren’t necessarily equal, and
the ratio is related to the amount of doping of neutral silicon with the respective dopants which make the regions P and N.
The carrier injection will depend upon the voltage applied to the diode, and
so the current flow will be voltage dependent, though not linearly so, as you
should expect of a resistor.
If we reverse the polarity of the applied voltage the current flow will be greatly reduced to near zero. This results from the carriers being pulled away from
the junction, leaving the dopant atoms, which are fixed in the silicon lattice.
These fixed dopants aren’t free to move and recombine. The small current
that flows is due only to thermal ionization, and we will neglect it.
4
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Forward Biased
P-N-Junction
Anode
+
+ +
+
+
+ -
+
-
+
+
+
- -- - - -
+
+
+-
-
+
+
- - -
+ +
-
-++
-
- - -
Cathode
+
-
+
-
I=Is(e Vq/kT -1)
© APB junc1
Forward Biased P-N junction
I~0
Reverse Biased
P-N Junction
+
Reversed Biased P-N junction
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB junc2
5
Now, suppose we restore the forward bias to the junction, causing a current
to flow. Next, let’s add another N-type region within the P-type, and drive it
in until it is separated from the other N region by a thin layer of P-type material and apply to it a bias voltage greater than the voltage on the forward
biased junction.
Since the added N region is more positive than the P region, the new junction
is reverse biased, and we might expect very little current to flow. However,
some of the electrons pulled into the P region will be attracted by the added
N region, and they will cross that junction, become majority carriers in the N
region, and will constitute a current flow.
If we take as our goal to maximize the electron flow from bottom to top, there
are several things we can do. One is to reduce the size of the bottom N
region so that all the minority carriers will be less likely to recombine in the
P region. Another is to adjust the doping levels to favor more electrons and
fewer holes crossing the lower junction.
By now I’m sure you recognize the Emitter, Base and Collector of an NPN
and we have maximized electron flow from emitter to collector by making
the P-type base thinner and covering the emitter with the collector junction,
and have reduced the base current by doping to reduce the number of holes
injected into the emitter.
This seems a good place to interject a personal preference and the reasons
for it. First, please note that the story I just told shows the base voltage to be
the cause of both base and collector currents. The geometry and doping is
selected to minimize the base current as a fraction of emitter current, or to
raise beta, the ratio of collector current to base current.
Because the transistor construction will control beta, we are free to consider either base voltage or base current as the cause of collector current.
6
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
+ +
+
+
+ -
+
-
+
+
+
- + - +- +
+ - + +- +
+
+
- +
+
+
- -- - - - - - - - -
+
Additional Diffusion Forms
Additional diffusion forms
Crude
NPN
Transistor
Crude
NPN
Transistor
© APB junc3
Ic=Is(e Vbe(q/kT) -1)
Collector
-
+
-
-
--
Base
+
-
-
- -+
- + +
- - - - - - -
+
-
Emitter
Ib=Isb(e
Vbe(q/kT)
-1)
Adjustments to size and doping makes improved NPN transistor
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
7
However, I will assert that most of the time (with exceptions granted) it will
be easier to treat base voltage as the cause of collector current.
Here’s an example of the principle involved. Suppose we measure the base
and collector current of a transistor and record our measurements in Roman
numerals. Then to determine beta we need only divide one by the other to
discover that beta = CLXI+IV/X.
Now, that result is perfectly valid and as accurate as our measurements,
but wouldn’t it have been so much easier if we had used our modern number system?
Well, that’s similar to doing an analysis using beta vs. Vbe. You can get the
right answer either way, but as we’ll see one is much easier than the other.
An additional issue is that in most applications you want to minimize sensitivity to beta or base current. Why would you want to minimize the effects of
your major controlling transistor parameter?
8
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
VPLS
POS
+
-
NEG
V1
V V
Q1
V UA
I1
ic =DCCCVII uA
GND
Beta Measurement
beta = DCCCVI I ;
V
XX
XX
XX
C
I
V ) DCCCVI I
D
C
C
C
C
C
C
V
V
II
XX+XX +XX = LX & II / V = IV/ X = beta = CLXI + IV / X
http://www.guernsey.net/~sgibbs/roman.html
© APB romβ
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
9
With that observation in mind, let’s look at a simple current mirror. Q1 and Q2
have the same Vbe, so we can hope they will have the same collector current. The collector current of Q2 may be higher than Q1 since the collector
voltage is higher. The collector current of Q2 may also be reduced from the
current in R because the two base currents subtract from R current before
it drives the collector of Q1. In this case we would treat the base currents
as an error in current mirroring along with the error due to the mismatch of
collector voltage. These errors have opposite signs, but let’s see if we can’t
do better.
Another simple circuit will reduce by 4X the input current mirrored by Q2,
with the same approximations. Note that all the transistors are alike, with
the same Vbe so that they all operate at the same current density or current
per unit of active emitter area.
10
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
VPLS
R
POS
NEG
Ib1 + ib2
Q1
Ic=is(exp(Vbeq/kT)-1)
Q2
Ic=is(exp(Vbeq/kT)-1)
GND
Simple Current Mirror
Simple Current Mirror
© APB cm1
+V
R
ic
Q5
ic
Q4
ic
Q3
ic
Q1
VBE
Q2
ic
Common
A 4:1 mirror operates all transistors at the same current density
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
11
A sort of shorthand we might substitute for the explicit four transistors is to
show a single transistor collector and base with four emitters. The significance of this symbol is shown in the next picture, which is a cross section
of such a transistor.
This cross section shows a P-type wafer with N-type epi and a device including an N-type well with buried layer and a P ring to isolate this region from
the rest of the epi. A P-type base is diffused into the isolated well and heavily
doped N+ dots are diffused into the base. At the left of the base the N+ dot,
the P-type base, and the underlying n-epi form an NPN transistor. This transistor is confined to the region below the emitter and shares isolation and
base with three others
There are four emitter dots diffused into the base and so this device acts like
four NPNs the base and collectors of which are joined by construction, and
with four separate emitters.
The emitters are separated, rather than combined into one large one, so they
may be well matched to a device with a single identically sized emitter. Typically for bandgap circuit applications the four emitters will be joined by metallization, but in other applications they may be used individually.
12
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
+V
R
4ic
Q1
VBE
Q2
4A
ic
A
Common
Pictorial
shorthand
Pictorial Shorthand
Represents
Construction
represents construction
© APB cm3
Junction Isolated NPN With
Multiple Emitter Sites
n epi
P Isolation
n epi
n epi
P Isolation
P
Isolation
N+
n epi
P
N+
(Active) n epi (Active)
N+
n epi
P
Isolation
N+ Buried Layer
P Wafer
Junction Isolated NPN with multiple emitter sites
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB 4Xnpn
13
Even this simplification of the symbol becomes clumsy when more emitters
are wanted and so a multi emitter device may be indicated by notation nA
and to show what each emitter matches, one is designated A.
In this diagram both bases are driven by a common voltage V1, so the circuit is no longer a current mirror, but is a source of two collector currents,
instead. If we trust that the currents are a function of base voltage without
evaluating the direct relation, we can note that some voltage V1 applied to
Vbe 1 should result in a particular collector current from Q2. Since Q1 is effectively eight (I’ve chosen n=8, somewhat arbitrarily) transistors in parallel,
all of which have the same Vbe as Q2, I expect the net collector current of
Q1 to be eight times that of Q2. Since they share collector, base and emitter
voltage, the systematic error in this ratio should be very small, and in particular, unaffected by base current, which is supplied to both transistors by V1.
Now let’s introduce a resistor (1K) into the emitter connection of Q1. As the
current is adjusted by changing V1 the corresponding voltage drop across R1
will disturb the ratio of collector currents.
14
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
+V
8ic
8A
Q1
VBE
Q2
ic
A
+
V1
Variable
-
Common
further condensation uses 8A and A to indicate an 8:1 ratio of
AAtheFurther
Condensation Uses 8A and A to
two outputs of this voltage-driven current source
Indicate an 8:1 Ratio of the Two Outputs of
This Voltage Driven Current Source © APB 8:1
V
ic
8A
R1
1K
Q1
VBE2
Q2
ic
A
+
Common
-
V1
Variable
An Added Resistor Changes
the Emitter Voltage of Q1
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
An added resistor changes the emitter voltage of Q1
15
© APB 8:1R
For small values of V1 where currents are relatively small, the voltage across
R1 will be small and will hardly interfere with Vbe of Q1, so the collector currents will stay very nearly in the ratio eight to one. I hope you will take my
word for it below 350mV, and note that at 450mV the eight to one ratio is only
starting to be reduced by the difference in the two Vbes resulting from ~15
nA in R1.
i(Q1,C) i(Q2,C)
16
14
12
xle-9
10
8
6
4
2
0
0
50
100
150
200
250
variable, xle-3
300
350
400
450
WhenWhen
the the
Resistor
Voltageis small,
is Small,
resistor voltage
the Colloector
Current
Ratiois close
is Close
the collector
current ratio
to 8:1to 8:1
V
ic
8A
R1
1K
16
Q1
VBE2
Q2
ic
A
+
Common
-
V1
Variable
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB
But, as we will see, while the current in Q2 can rise exponentially with V1, the
rise of current in Q1 will be asymptotic to a linear slope defined by R1.
This means that as V1 rises, the ratio of collector currents falls until it is
below one. In the figure this happens as V1 crosses ~720mV in a room temperature simulation.
In order to more accurately set and maintain this crossover voltage an
opamp may be used to adjust Vbe2 to the value that makes the collector
currents equal.
160
i(Q1,C) i(Q2,C)
140
120
Exponential
Rise
xle-6
100
80
60
Slope Limited
by R1
40
20
0
440
480
520
560
600
640
variable, xle-3
680
720
760
HigherVbe1<Vb2
Currents
At higherAtcurrents
andVbe1<Vb2
the current ratio falls
and the Current Ratio Falls
V
ic
8A
R1
1K
Q1
VBE2
Q2
ic
A
+
Common
-
V1
Variable
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB 8:1R_2
17
An op-amp drives the common base line to maintain
Equal collector currents
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
-
Q1
VBE2
Q2
A
3
Common
An Op-Amp Drives the Common Base Line
In this to
arrangement
both collector
are drivenCurrents
up and down together,
Maintain
Equalcurrents
Collector
but their different slopes result in a differential signal at the opamp
© APBinput.
8:1force
There is a negative feedback path including Q2, node 2, and the opamp, as
well as a positive feedback path including Q1 with R1, node 1, and the opamp.
R1 reduces the effective Gm in the positive feedback loop which reduces
the gain of that path below that of the negative feedback path. So, when
the two currents match and nodes 1 and 2 are equal in voltage, the negative
feedback dominates and Vbe2 is maintained at the voltage which keeps the
collector currents balanced. It will be important to note that by making the
collector currents equal we have made the current densities unequal by a
factor n = 8.
While the feedback makes the two collector currents equal it does not answer the question of what value the currents actually have. However, it
should be apparent that the common value for the currents is closely approximated by the current in R1, which itself is determined by the difference
between Q1 Vbe and Q2 Vbe.
18
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
As indicated before, the temperature of the devices affects the result. This
figure shows that V3, the voltage across R1, changes in proportion to temperature. And the resulting current appears at the two collectors. The currents
track so well that the Q1 current line on the graph is obscured everywhere
by the Q2 current trace.
80
i(Q1,C) i(Q2,C)
xle-6
70
60
50
40
V
R4
40K
R3
40K
2
1
8A
R1
1K
+
-
Q1
VBE2
Q2
A
3
Common
V(3)
80
xle-3
70
60
50
40
-60
-40
-20
0
20
40
tdegc
60
80
100
120
120
The two collector currents track almost perfectly as temperature is
swept – And their common value increases with rising temperature
as indicated by the voltage across R1
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB 8:1force_2
19
To see what and why the voltage across R1 is what it is, let’s examine a
little experiment. Take two identical NPN transistors and bias each into
conduction with a voltage applied to the base, using a different voltage for
each device.
Arrange to measure each collector current and the difference of the base
voltages.
Now, using the junction equation for the two transistors, the form is the
same, but allowance is made for the ratios of current and area. One of the
factors in the collector current is an exponential function of Vbe and q, electron charge, over k, Boltzmann’s constant, and T, Absolute Temperature, all
minus one. Since the exponential will be on the order of 1e14 to 1e18 for the
transistors and environment we will use, the subtraction of one can safely
be neglected.
Take the ratio of current densities and with a bit of manipulation you will find
the difference of the Vbes or ΔVbe as it is generally referred to, depends
upon the current ratio which is controlled in the circuit, the emitter area ratio,
Kelvin temperature and the constants.
20
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
V
ic1
A1
Q1
VBE1
VBE2
+
s
Q2
+
A2
ic2
s
V2
V1
VBE1 Common
VBE2
Current Density vs. Base Voltage Difference
Current Density vs. Base Voltage Difference
© APB ∆Vbe
V
ic1
Ic1 = A1Is(e
Vbe1q/kT
ic2
-1) A1
Ic1 ~ A1Is(eVbe1q/kT)
s
Q1
VBE1
V1 +
VBE1 -
Common
VBE2
+ V2
- VBE2
Q2
A2 Ic2 = A2Is(eVbe2q/kT-1)
s
Ic2 ~ A2Is(eVbe2q/kT)
Then: (ic1/A1)/(ic2/A2)=(e Vbe1q/kT)/(e Vbe2q/kT)=e (Vbe1-Vbe2)q/KT
So: (q/KT)(Vbe1-Vbe2)=In((ic1/A1)/(ic2/A2))
∆Vbe=Vbe1-Vbe2=(kT/q) In((ic1/A1)/(ic2/A2))=(kT/q) In((ic1/ic2)(A2/A1))
Current
Ratio
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Area
Ratio
21
The Bottom Line:
DVbe = (kT/q)ln((ic1/ic2)(A2/A1))
...and if the Currents are Equal:
DVbe = (kT/q)ln(A2/A1)
Setting the current ratio equal to one and noting
that the area ratio and the constants are unlikely
to change, we find that DVbe is Proportional To
Absolute Temperature …
22
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
... or PTAT.
(as it's often referred to)
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
23
Current Density vs. Base Voltage Difference
V(Vbe2) V(VBE2, 3) 1.2385
1.25
1.2
1.15
1.1
1.05
1
.95
.9
.85
.8
.75
.7
.65
.6
.55
.5
.45
-60
-40
-20
0
20
40 60
tdegc
80
100 120 140
If we extrapolate temperature to zero Kelvins we should find that ΔVbe also
goes to zero despite the fact that the transistors may be differently sized
and operated at different currents. This figure shows a linear interpolation
of Vbe back to zero. Ideally this extrapolation would go to VG0, the extrapolated bandgap of silicon. But the actual value of the two Vbes would meet
at a slightly lower voltage than the extrapolation because the slope of Vbe
vs. temperature is not quite linear. That is, the linear extrapolation reaches
about 1.2385V at zero Kelvin, but tracing out the complete theoretical characteristics of actual transistors would find they meet at a slightly lower voltage.
The curvature of Vbe is difficult to see in the figure, unless you look closely to
see the departure from the linearity of the extrapolation in the region where
the transistors were accurately simulated as indicated by the colored traces.
In practice this curvature over the temperature range of common interest
is small enough to often neglect. However, if better temperature behavior is
needed several methods may be used to compensate it, one of which will be
mentioned later.
24
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB Xtrap
For now, let me neglect Vbe curvature and treat it as a linear function of temperature. As we have seen, the PTAT voltage across R1 can be scaled to be
larger as it is by R3 (40K) and R4 (40K). So, we can generate a PTAT voltage
scaled as we wish to add to the Vbe of Q2. Near our temperature range of interest we should be able to combine Vbe2 with a PTAT voltage scaled to give
a wide range of results for the sum. However, as we approach zero Kelvin
the PTAT voltage approaches zero, so that whatever Vbe it is combined with
will approach VG0, the extrapolated bandgap voltage.
The combined Vbe2 and PTAT voltage, being the sum of two functions linear
in temperature, should itself be such a linear function. And since we know
one value it will take on, we can make that function constant at all temperatures by adjusting the PTAT voltage to make the sum the same as VG0 at any
other temperature, including room temperature. And since these voltages
complement one another over temperature Vbe2 is often called Complementary To Absolute Temperature, or CTAT.
1.2385 1.2385- (tdegc+273) *2m (tdegc+273) *2m
1.3
1.2
Temperature Invariant Sum of Linear
Functions of Temperature
1.1
1
.9
.8
.7
.6
A PTAT Voltage Scaled
to be Complemented by Vbe
A Vbe With Negative Slope
Complementing A PTAT
Voltage (CTAT)
.5
.4
.3
.2
.1
0
-300
-200
-100
0
100
tdegc, from -273 to 345
200
300
© APB Invar
Current Density vs. Base Voltage Difference
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
25
A simple circuit can be used to generate the sum of both the PTAT and
CTAT voltages.
The amplifier used to drive VBE2 until its inputs balance will try to do that
even if there is some additional impediment, such as R2, in the way. That is,
the amplifier will drive VBE2 positive and the current necessary to drive R3
and R4 will be supplied by a rising voltage on R2.
When the two collector currents match, their sum will pass through R2 (actually a bit more when we include the base currents of Q1 and Q2, but they can
often be neglected, or made negligible by additional circuitry) making it take
on the PTAT voltage characteristic of current in R1.
Since we are free to make R2 whatever value we choose, it can be made
large enough to be complemented by the CTAT Vbe at the output of the amplifier.
Since the CTAT voltage is not strictly linear, a slight additional PTAT voltage
component will tilt up the downward curvature for a best fit to our desired
temperature range. This linear addition acts like the extrapolation which,
as we said before, passes through a voltage which is a small amount larger
than the bandgap.
This small amount will be process dependent and the total is often called the
“Magic Voltage” for the process. The magic voltage is the output voltage
setting at some selected temperature which gives the best temperature performance. Designs to hit the magic voltage can also be trimmed, for example
by adjusting R2 at a single temperature.
26
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Disclaimer: This configuration is optimized for
presentation, not necessarily for integration
V
Bandgap
Voltage
R4
40K
R3
40K
(well, almost)
2 +
1
8A
PTAT
∆Vbe
R1
1K
R2
4.619K
-
Q1
VBE2
Q2
A
3
R5
20K
CTAT Vbe
4
Adding R2 provides a
fully adjustable
PTAT Voltage
Common
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Multiple
Image
of ∆Vbe
© APB Invar_3
27
PTAT, CTAT, and Resulting
ZTAT (~OTC) Sum
V
R4
40K
R3
40K
2
1
8A
R1
1K
+
-
Q1
VBE2
3
4
R2
4.619K
Common
Q2
A
1.25
1.2
1.15
1.1
1
1.05
.95
.9
.85
.8
.75
.7
.65
.6
.55
.5
.45
.4
.35
.3
.25
.2
.15
.1
.05
0
-60
V(4) V(VBE2, 4) V(VBE2)
-40
-20
0
20
40 60
tdegc
80
100 120 140
An example of the simulated result for the process used in the other examples shows the low Temperature Coefficient (TC) result at the node VBE2, of
adding Vbe2 to the voltage across R2. The apparent TC, at this viewing scale,
appears flat and so is often abbreviated ZTAT.
The magic voltage for a given process may be less than some desired reference level such as 1.5V or 2V. And/or it may be desirable to make a design
easily reusable in other processes which have different magic numbers.
28
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
A Small Additional Burden on the OP-Amp Makes Available
Any Desired Output Voltage Greater Than the Bandgap
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
-
Q1
VBE2
Q2
3
A
OUT
R5
20K
Vout=VBE2*(R5/R6+1)
R6
20K
4
R2
4.619K
Common
This requirement can often be accommodated with the addition of a simple
voltage divider. As we saw before the amplifier will do whatever it can to
balance the two collector currents. So, attenuating the output before driving
the common base line will require the amplifier output to rise in proportion
to the attenuation to restore the magic voltage to VBE2. As shown in the
figure, a resistor divider of one half will roughly double the output voltage.
The green trace shows twice the magic voltage at VBE2 while the red trace
shows the simulated output.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
29
2.5
V(Out) 2*V(VBE2)
2.0
1.5
1.0
.5
0
-60
-40
-20
0
20
60
40
tdegc
80
140
120
100
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
-
Q1
VBE2
Q2
A
OUT
R5
20K
R6
20K
3
2X
4
R2
4.619K
Common
30
© APB multpl_2
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Our Little Circuit Can Reveal Two Fundamental Error Sources
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
Q1
-
VBE2
Q2
A
OUT
R5
20K
R6
20K
3
4
R2
4.619K
Common
One is Due to Base
Currents in R5, with
No Corresponding
Current in R6
The Other is Due to
Non-linearity of Vbe
as a Function of
Temperature, the
So-called Bandgap
Curvature
Two errors contributing to non-zero TC can be visualized with this circuit.
The effects of base current on the basic reference can often be neglected. However, depending on the current you can afford to spend making the
voltage divider from small value resistors, the base current effects on the
multiplied output may not be negligible. We can also use this example to
investigate curvature compensation.
Up until now we have been looking at results on a scale where the total error
spanned a pixel or two and looked pretty small. But if we examine the results
and the scale of the errors we’ll find they look worse and may require reduction to meet our performance specification.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
31
The green trace represents about as well as we can do with the simple cell
and the process at hand. It’s multiplied by two for comparison with V(Out).
A similar arrangement should help you determine both the magic voltage
and the intrinsic curvature due to your process. The blue curve is twice the
magic voltage, which is ideally tangent to the real or simulated result near
the midrange temperature. The red trace is the simulated output with a 2X
feedback attenuation.
The reason it is higher than the others is that the base currents of both transistors flow in R5, slightly raising its voltage, without a corresponding component of current in R6. Neglecting a few things, like the possibility that the
base currents differ because of collector current density, we can make an
approximate correction for this error.
32
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
2.500
V(Out) 2*V(VBE2) 2*1.2385
2.495
2.490
2.485
2.480
2.475
2.470
2.465
-60
-40
-20
0
20
40
tdegc
60
80
100
120
140
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
-
Q1
VBE2
Q2
A
OUT
R5
20K
R6
20K
3
2X
4
R2
4.619K
Common
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
© APB multpl_433
R7 introduced between Q1 and Q2 carries the base current of only Q1 and
introduces a slight offset into the loop, making ΔVbe, which changes the current in R1 and, hence in R2. The formula given in the figure gives a good first
approximation which can be refined with a bit of experimentation.
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
-
Q1
VBE2 Q2
R7
2.164K
3
A
OUT
R5
20K
R6
20K
4
R2
4.619K
Common
Adding R7=(R1/R2)*R5*R6/(R5+R6) creates a base current
proportional voltage within the ∆Vbe Loop, reducing the reference
voltage, slightly, to compensate the error due to base currents in R5
34
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
2.478
2.477
2.476
2.475
2.474
2.473
2.472
2.471
2.470
2.469
2.468
2.467
2.466
OUT 2.465
2.464
R5
2.463
20K
2.462
2.461
2.460
R6
20K
2.459
2.458
2.457
2.456
2.455
2.454
2.453
2.452
2.451
2.450
2.449
2.448
2.447
2.446
2.445
0
-60
Ideal
V(Out) 2*(-1:V(VBE2)) 2*V(VBE2)
Previous 2X VBE2
V
R4
40K
R3
40K
2
1
8A
R1
1K
-
Q1
VBE2 Q2
A
R7
2.164K
3
+
4
R2
4.619K
Common
The Approximation Used
For R7 Results in
Slight Over-correction
2X Error
Correcting Output
-40
-20
0
20
40 60
tdegc
80
100 120 140
© APB βcomp_2
Using the formula with the schematic values gives a value for R7. The simulation in black shows the effect of R7 and base current on the nominal voltage
at VBE2.
The red trace shows the simulated output of the circuit to compare with
twice the value of VBE2 from the previous simulation.
The black trace shows twice the pre-compensated value to compare with
the simulated output which results from base current in R5.
The simulated output is a bit less than twice the nominal magic voltage, since
the approximation of the formula slightly overcompensated the base current.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
35
R7 can be reduced slightly to take into account a well-modeled difference in
the base currents of Q1 and Q2 as well as the approximations in the formula
for R7.
V
R7 Reduced to Avoid
Overcompensation
R4
40K
R3
40K
2
1
8A
R1
1K
+
-
Q1
VBE2 Q2
R7
2.009K
3
See JSSC, Vol SC-9, #6,
Dec 1974 page 390
A
OUT
R5
20K
R6
20K
4
R7 Value Empirically
Optimized for Best Fit
(Carefully Tinkered
in Simulation)
R2
4.619K
Common
© APB βcomp_3
The result of this adjustment can be seen here to almost exactly match the
simulated output to an ideal doubling of the output of the single bandgap, indicating that the output error due to base currents has been greatly reduced.
The pre-distorted voltage at VBE2 is also doubled and shown in black to compare with the ideal doubling.
As caveat, I want to point out that while this error reduction technique works
well in practice you may prefer to find an alternative if noise at the output is
an issue. The addition of R7 can easily double the intrinsic minimum noise
of this circuit and must be considered along with other substantial noise
sources.
All of the outputs we have seen show a non-linear voltage vs. temperature
component. Unlike PTAT errors, which can be corrected with PTAT voltages
that are easily generated, the PTAT output adjustment can only correct for
the linear baseline of the nonlinearity resulting from Vbe.
36
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
V(Out) 2*(-1:V(VBE2) 2*1.2385 2*V(VBE2)
2.480
2.475
Ideal
2.470
Compensation
Adjusted
2.465
2.460
2.455
2.450
2.445
-60
-40
-20
0
20
40
tdegc
60
80
100
120
140
V
R4
40K
R3
40K
2 +
1
8A
R1
1K
-
Q1
VBE2 Q2
A
R7
2.009K
3
OUT
R5
20K
R6
20K
4
R2
4.619K
Common
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
37
© APB βcomp_4
So what about
the Curvature?
Where does THAT
come from??
The origin of the curvature is with Vbe, as is almost everything in this bandgap circuit. An expression for Vbe derived from first principles allows us to
compare Vbe of a transistor with a second transistor which matches it or
with itself when operated under different conditions. For a given process and
device design, then, the expression for Vbe lets us say “If you’ve seen one,
you’ve seen ‘em all.”
Vbe of a Transistor as a function of Current and Temperature may be inferred
from a measurement of Vbeo, its nominal value at temperature T=To, and
current i=io, using the relation:
Vbe = VG0 +(T/To)(Vbeo-VG0)+(kT/q)ln(i/io)+m(kT/q)ln(To/T)
Where VG0 is the Bandgap Voltage extrapolated to 0K and m is a property of
the Transistor determined by its design and processing. m is approximated
by XTI in the GP SPICE Model
Note that Vbe Consists of:
• a Constant
• a Term Linear in Temperature
• a Term that May be Zero or nonlinear
• a Term which is a non-zero, non-linear, function of temperature
This last is the origin of the “Bandgap Curvature.”
38
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Since Vbeo will be less than VG0, the dominant linear term will fall with increasing temperature ... And, the third term, (kT/q)ln(i/io), will be zero, if the
current is held invariant at io.
However — if i=io(T/To), as is the PTAT current in our bandgap cell, then this
term may be re-written as:
(kT/q)ln(io(T/To)/io)) = - (kT/q)ln(To/T)
which may be combined with the fourth term,
which is the curvature term, to yield:
(m-1)(kT/q)ln(To/T)
The Vbe of Q2 Operating with PTAT Current is combined with the voltage
across R2 scaled up from ΔVbe to produce the nominal output voltage:
Vout= VG0 +(T/To)(Vbeo-VG0)+(m-1)(kT/q)ln(To/T)+ R2((kT/q)ln8)/R1
The second and fourth terms are both temperature proportional and are of
opposite sign, so their sum may be made zero by setting the R2/R1 ratio. This
leaves Vout as:
Vout= VG0+(m-1)(kT/q)ln(To/T)
VG0 is the ideal bandgap voltage of silicon extrapolated to zero, and would
be a fine reference value if the total was not corrupted by the second, curvature, term.
This term is generally curving downward in the temperature range of interest to us so we add a PTAT component (by adjusting R2/R1 to leave a residual + PTAT) to remove the net negative slope of the curvature. As a result
we see the peak of the curvature in our temperature range, slightly above
the ideal VG0.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
39
A Diferent Perspective on the Basic Circuit Using a Different Scale
V(VBE2)
1.2390
1.2385
1.2380
1.2375
1.2370
1.2365
1.2360
1.2355
1.2350
1.2345
-60
-40
-20
0
20
40
tdegc
60
80
100
120
140
V
R4
40K
R3
40K
2
1
8A
R1
1K
+
-
Q1
VBE2
Q2
A
3
4
R2
4.619K
Common
40
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Going back to the basic bandgap circuit and looking at the output on a different scale shows the curvature as if superimposed on our desired temperature invariant circuit. Although the curvature term is a logarithmic function of
temperature in a series expansion it has a large quadratic term, for which we
can compensate and reduce the remainder to a third and higher order error.
The way in which the current variable term, (kT/q)ln(i/io), reduces the final
curvature term may be exploited by generating a larger multiple to combine
with the fundamental curvature of the same form to reduce the sum to zero.
A more simple and cost effective compensating curvature can be made by
adding a resistor with a more positive TC to the PTAT voltage multiplier.
Note that the residual curvature appears to be a parabola, indicating that it
has a large quadratic component.
A temperature proportional resistor driven by the PTAT current results in a
T2 voltage which can be sized to cancel the quadratic component of the
residual curvature.
The small remainder will be seen to be dominated by a third order term.
If we put a PTAT current in a temperature proportional resistor, the result is
a quadratic function of temperature. We can do that by putting a diffused
resistor with a, relative to R1 and R2, large positive TC in series with R2. Of
course R2 must be reduced, not only to “make room” for the R8 voltage, but
to also reduce the total voltage across R2 and R8. As we will see, curvature
correction gives us better performance, and also brings the magic voltage
closer to the actual extrapolated bandgap.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
41
This figure hides the zero TC component of the previous plotted output voltage to show the curvature due to Vbe of Q2. Over the same temperatures we
can see the quadratic compensating voltage appearing across R8, an added
+TC resistor.
The summation of the quadratic correction with Vbe curvature substantially
reduces the remaining temperature error.
A Quadratic Function
Approximates the
Curvature Function
V
Output
Curvature
Component
R4
40K
R3
40K
2
1
8A
R1
1K
-
Q1
VBE2
Q2
A
3
4
R2
4.085K
R8
414
5
+
/
PBASE
Common
A Small + TC Resistor
Produces a Squared
Response to the
PTAT Current
4.50
4.25
4.00
3.75
3.50
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
.75
.50
.25
0
-.25
-.50
-.75
-1.00
-1.25
-1.50
-1.75
-2.00
-2.25
-2.50
-2.75
-3.00
-3.25
-3.50
-3.75
-4.00
-4.25
-60
nonlin(V(VBE2, 4)) nonlin(V(5))
-40
-20
0
20
40 60
tdegc
80
100 120 140
© APB curvpm_2
42
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
V
Uncorrected
Result
R4
40K
R3
40K
2
1
8A
R1
1K
+
-
Q1
VBE2
Q2
A
3
4
R2
4.085K
R8
414
5
/
PBASE
Common
Net Corrected
Result
Approximate Compensation
for Curvature Causes
“Magic Voltage” to
Approach the Bandgap
1.2390
1.2385
1.2380
1.2375
1.2370
1.2365
1.2360
1.2355
1.2350
1.2345
1.2340
1.2335
1.2330
1.2325
1.2320
1.2315
1.2310
1.2305
1.2300
1.2295
1.2290
1.2285
1.2280
1.2275
1.2270
1.2265
1.2260
1.2255
1.2250
1.2245
-60
(-1:V(VBE2)) V(VBE2)
-40
-20
0
20
40 60
tdegc
80
100 120 140
© APB curvpm_3
Note that the magic voltage that results from curvature correction is substantially reduced, making it closer to VG0 where it would be without the
effects of Vbe curvature.
So far, the example circuits and simulations have been based on the SPICE
models of an analog bipolar process. Although the Bandgap Reference principles were developed using similar transistors, the need for voltage references persists on circuits made on processes that lack isolated vertical
transistors.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
43
... and Now, for the
Device Deprived …
From time-to-time some of us may find ourselves Deprived of real (vertical,
isolated, bipolar) transistors and forced to make do with crumbs from the
CMOS table.
Fortunately, even parasitic vertical substrate transistors will exhibit the
ΔVbe = (kT/q)ln((ic1/ic2)(A2/A1)) relationship,
and the Emitter Current ratio MAY be a satisfactory substitute for (ic1/ic2)
Moreover, these ill suited thick-base substitutes may be more uniform in
manufacture than their thin-base counterparts that serve us so well. This
uniformity is welcome, though it hardly compensates for the lack of individually usable collector currents.
At the heart of the bandgap reference is a junction, or two or three. On CMOS
processes, diffusions may be arranged to form transistors that have a common collector in the substrate. The opportunity to directly control the collector current ratio by comparing them is not available. Additionally, since the
collector currents are not driving a second stage, ΔVbe must be measured
directly rather than inferred from a higher gain common base input stage.
44
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
This paper shows in some detail a way to build a temperature stable reference using parasitic Vertical PNP transistors. And to generate an output
voltage which may be freely chosen within a useful range. A similar and instructive circuit can be used for illustration.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
45
Simple sub-Bandgap uses CMOS Parasitic PNPs
VIN
MP13
1/40
1s
4s
MP1
MP2
MP3
80/4
80/4
80/4
4s
4s
2
1
PP 5P
MP7 C1
MP5 MP6
2s 40/4
2s
4
5
3
40/4 40/4
MN2
MN6
10/1
80/2
1s
ZTAT
MN5
VPLS -
VREF
MP7
10/.6
R4
100K
R3
123.06K
/
TF
/
TF
R1
123.06K
/
TF
Q1
Q2
Startup
MN1
80/2
PTAT
7
16
CTAT
GND
R2
15.96K
/
TF
6
MV
4s
Diff In
ZTAT Driven to 0
ZTAT
V3 +
2s
MN4
CTAT
1
MV
4s
6/2
MV
80/2
4s
9
8
MN3
12/2
1s
Core
2s
Amp
This figure shows a relatively simple scheme to derive a 1V reference using
VPNP transistors Q1 and Q2. They are part of the core, the center section of
the diagram, which is flanked by an Amplifier and a Startup circuit.
To simplify the explanation, we can strip away
the Startup and part of the core.
But First, A Simplified Sub-Block
VIN
2
4s
MP2
MP3
80/4
80/4
4s
2
MP5 MP6
2s 40/4
2s
4
5
3
0.4750942
0.7882932
0.4750931
40/4 40/4
1.08813
MN2
80/2
Equal Currents
In Unequal Sizes
Result In
V3 +
∆ Vbe Forced VPLS Across R2
2s
6
1.087784
MV
4s
Diff In
Driven to 0
/
TF
R2
18.25K
7
0.3622341
GND
24
MN5
MN1
80/2
4s 80/2
MV
9
PTAT
Q1
Q2
Core
46
PP 5P
MP7 C1
0.2357213
1
1s
MN4
6/2
MV
4s
8
MN3
0.2363653
12/2
2s
Amp
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
The Amplifier, at the right, drives the gates of two matched PMOS transistors
MP2 and MP3, causing them to deliver equal currents to Q2 and Q1 which
are in the ratio 24:1 of matched emitters. As long as the currents in the two
transistors are equal and non-zero the difference in their Vbes should be
given by:
ΔVbe = (kT/q)ln24
depending only on the ratio of the currents, not their magnitude.
That means that when the matched currents are small the emitter voltage
of Q1 is larger than the emitter voltage of Q2 by almost ΔVbe. This will make
the gate voltage of MN1 more positive than the gate of MN2 and the drain of
MN1 will pull down the gate of MP3 as well as of MP2 connected to node 5.
Increasing MP3 current will, of course, increase Vbe of Q1 as a positive
feedback.
However, as currents increase, so does the voltage across R2 until it eventually is equal to ΔVbe. At that point, the difference between the gate voltages
of MN1 and MN2, the inputs to the amplifier, will be zero. Considering the
gain of the path from MP3 gate (node 5) to node 1 at the emitter of Q1 it will be
limited by the transconductance of MP3 and the impedance of Vbe1.
The gain of the path from MP2 gate to node 2 at the top of R2 will be limited by
MP2 Gm and the combined impedance of R2 with Q2. Since the impedance
of Q1 and Q2 are the same at equal currents, the negative feedback path will
dominate the positive feedback path and the circuit will rest at equilibrium
with currents set by ΔVbe/R2.
Simulation over temperature shows the PTAT voltage appearing across R2.
The second trace is the input voltage of the amplifier which is within 1uV
over temperature. The amplifier topology yields these results, but of course
is corrupted by device matching errors.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
47
We can be certain that when the MP3 and MP2 currents are equal, the amplifier inputs will be balanced and bring their drains to the voltage required
to set up the PTAT currents.
Developing a PTAT voltage across R2
by forcing equal Node 2 and 3 voltages
V(2,7)
115
110
105
100
95
90
85
80
75
70
65
60
550
xle-6
∆Vbe = (kT/q)ln((i1/1)/(i2/24))
Q1 and Q2 operate at equal currents set by MP2 and MP3, but they are
sized 1:24, resulting in a 24X current density ratio and a difference
in Vbe given by: ∆Vbe = (kT/q)ln((i1/1)/(i2/24))
The amp forces ∆Vbe to appear across R2
V(2,3)
Equal currents in MP2 and MP3
increase the R2 voltage
to balance the amp
xle-6
1.1
1.0
.90
.80
.70
.60
.50
.40
.30
.20
.10
0
-.10
-.20
-.30
-.40
-60
-40
-20
0
20
40
60
80
100
120
2
MP2
PP 5P
MP7 C1
MP3
80/4
4s
80/4
4s
2
40/4
2s
3
0.4750942
4
0.4750931
MP5 MP6
2s
0.7882932
40/4 40/4
5
1.08813
MN5
V3 +
/
TF
R2
18.25K
PTAT
7
0.3622341
GND
24
Q1
Q2
Core
48
MN1
MV
80/2
4s
9
0.2357213 MN4
1
1s
6/2
80/2
2s
6
MN2
1.087784
80/2
MV
Diff In
Driven to 0
VPLS -
140
vpls
VIN
4s
MV
4s
8
MN3
0.2363653
12/2
2s
Amp
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Loading Nodes 2 and 3 requires the Amp to make
more MP2 and MP3 Current to preserve equilibrium
VIN
4s
MP1
MP2
MP3
80/4
80/4
80/4
4s
4s
2
PP 5P
MP7 C1
2s
4
5
3
0.4750941
MP5 MP6
2s 40/4
0.4750931
0.8520052
40/4 40/4
1.042865 MN2
80/2
ZTAT
MN5
VPLS -
VREF
0.9983732
R4
100K
GND
R3
123K
/
TF
/
TF
CTAT
R2
18.25K
24
MN1
MV
80/2
/
TF
PTAT
7
/
0.3622341
TF
6
1.04257
MV
4s
Diff In
ZTAT Driven to 0
ZTAT
V3 +
2s
Q1
Q2
1
R1
123K
CTAT
4s 80/2
9
0.2155659 MN4
1s
6/2
Core
MV
4s
8
MN3
0.2159596
12/2
2s
Amp
Kids! Don't try this at home!
(It's a non-starter)
If we load, equally, nodes 2 and 3 the amplifier must drive MP2 and MP3 to
supply this additional current in order to maintain the input voltages which
force the PTAT current.
Equal valued R1 and R3 added to the schematic will require more current
from MP2 and MP3, and the amplifier will drive them to provide it.
Notice that the currents in Q1 and Q2 should remain PTAT while the currents
in R1 and R3 are proportional to Vbe and must, therefore, be CTAT. MP2 and
MP3 are supplying both. Remembering that the sum of a CTAT and PTAT
quantity can be made temperature invariant by using the correct proportions,
we can adjust either or both of R2 with R1and R3 to make a temperature invariant current in MP2 and MP3. Such a quantity is sometimes referred to
as ZTAT, in this context.
The ZTAT current should also flow in MP1, a transistor matched to MP2. This
current can drive the load resistor, R4, to a voltage resulting from the ZTAT
current flowing in it.
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
49
Loading CTAT Voltages With R1 and R2 Requires
Complementary Currents From MP2 and MP3
This figure illustrates the PTAT and CTAT currents and their almost constant
sum at scale that reveals an error and a consequence. The error is due to
the curvature of Vbe vs. temperature, and will remain in the final output. The
consequence is a slight net positive TC to the current, due to a small TC in
the thin film resistors used for simulation. This TC should not appear in the
output, since R4 has the same TC.
The output voltage results from approximately 10µA flowing in R4, so that it
can be set over a wide range by changing the nominal value of R4.
The small difference seen between i(R4) and i(R2) + i(R3) is mostly a result
of the difference in the drain voltage of MP1 from that of MP2 and MP3. In
many applications this may be neglected, or if Vin has a wide range of values, the three ZTAT transistors can be cascoded to reduce supply voltage
sensitivity.
This circuit shares a common “gotcha” with most self biasing circuits. In addition to the desired behavior described here, the circuit can take on another
stable state when all currents and the reference voltage are at zero. Once in
this state the circuit will remain there.
50
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
i(R2) i(R3)
7.0
R2 Current is Still PTAT
6.5
6.0
xle-6
5.5
5.0
4.5
4.0
3.5
New R3 Current (and R1, too) is CTAT
3.0
i(R2) + i(R3) i(R4)
10.10
10.08
Sum of PTAT and CTAT and
as Mirrored by MP1
xle-6
10.06
10.04
10.02
10.00
9.98
9.96
V(VREF)
1.0020
1.0015
1.0010
1.0005
1.0000
.9995
Nominal 1V Ref over Temp
.9990
(Your Results May Vary)
.9985
.9980
-60
-40
-20
0
20
40
vpls
60
80
100
120
140
VIN
MP1
4s
MP2
80/4
80/4
4s
PP 5P
MP7 C1
MP3
80/4
4s
2
2s 40/4
2s
4
5
3
0.4750941
MP5 MP6
0.4750931
40/4 40/4
1.042865 MN2
0.8520052
80/2
ZTAT
MN5
VREF
0.9983732
R4
100K
GND
R3
123K
/
TF
/
TF
CTAT
R2
18.25K
24
MN1
MV
80/2
/
TF
PTAT
7
/
0.3622341
TF
6
4s
Diff In
ZTAT Driven to 0
ZTAT
V3 +
VPLS -
2s
1.04257
MV
Q1
Q2
Core
1
R1
123K
CTAT
4s
9
0.2155659
1s
80/2
MV
4s
8
MN4
MN3
0.2159596
6/2
12/2
2s
Amp
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
51
So, most self-biased circuits require a startup which is sometimes easily arranged, other times not. My personal preference is to avoid dynamic starts,
since once started and switched off, for example by noise, your circuit may
then fail to restart.
This circuit includes a starter circuit that operates when power is applied
to VIN.
If the circuit does dynamically start (depending on the dV/dt of VIN among
other things), fine, but if not, VREF will be low ~ 0. As a result, MP13 will come
on as power ramps up. It will turn on M6 which drives the common PMOS
gate line near the top turning on MP2, 3, and-so-forth. Once there are some
non-zero currents the circuit will come on regeneratively.
The current in MN6 is likely to be very much larger than the nominal current
provided by MN1 to node 5, and it is important to prevent it from corrupting
the reference. As the circuit starts and VREF rises, it will drive MN7 on. Note
that while MN7 has a relatively large gate aspect ratio, MP11 has a small
one and is easily overcome by MN7. When node one is pulled low, MN6 will
be turned off and the small current from MP13 will continue to flow in MN7.
This arrangement works well and can be used in other applications. But, a
word of warning: The startup circuit works by triggering the positive feedback in the core and amplifier loop. However, the startup circuit includes
another negative feedback path in addition to the one that stabilizes the
bandgap output. Normally this path is unstable and gives control to the desired loop. However, it may be stabilized by the addition of a low frequency
dominant pole, such as one formed by a large capacitor from VREF to GND
that might be added to reduce noise. That can cause the startup to stabilize
VREF at a voltage lower than the main loop.
But, assuming that problem is avoided, the startup can be observed in a
sweep of VIN.
52
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Origin of the expression “Awakened with a Start”
VIN
MP13
1/40
MP1
1s
4s
MP2
80/4
80/4
4s
80/4
4s
2
1
PP 5P
MP7 C1
MP3
MP5 MP6
2s 40/4
2s
4
5
3
40/4 40/4
MN2
MN6
10/1
80/2
1s
ZTAT
MN5
VPLS -
MN7
10/.6
GND
Startup
VREF
R4
100K
R3
123K
/
TF
CTAT
/
TF
/
TF
R2
18.25K
24
MN1
MV
80/2
/
TF
PTAT
7
6
MV
4s
Diff In
ZTAT Driven to 0
ZTAT
V3 +
2s
Q1
Q2
1
R1
123K
CTAT
4s 80/2
9
1s
Core
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
MV
4s
MN4
MN3
6/2
12/2
8
2s
Amp
53
Initially R4 holds VREF low for any small VIN voltage. When VIN is below the
threshold of any MOS device, they must all be off or operating sub-threshold
at low currents. The green trace shows the simulator’s guess at the node 1
voltage for this condition. As VIN reaches the threshold voltage of MP13 it
comes on and pulls node 1 to VIN. Initially this voltage is insufficient to turn
on MN6, but as VIN continues to rise MN6 connects node 5 (the common
PMOS gate line) to node 1 at Q1.
VIN continues to rise so that MP2 and MP3 start to turn on the core, and
when VIN reaches about 1.6V the positive feedback of the main loop comes
on and drives VREF to the stable operating point. As VREF rises it turns on
MN7 which pulls node 1 low and cuts off MN6.
This circuit using VPNPs does not explicitly express the bandgap voltage,
although other arrangements can be made to do so. A simple thought experiment may give a different perspective on how the bandgap still remains the
reference. At zero Kelvin we should not expect the circuit to work correctly,
but we can extrapolate room temperature behavior to that temperature.
54
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
Start-up and Regulation vs. Supply Voltage
V(VREF) V(V1) V(VIN)
5
4
3
2
1
0
0
.5
1
1.5
2
2.5
vpls
3
3.5
4
4.5
5
MN6 energizes the PMOS Gate Line until VIN provides
headroom to start. Then VREF turns on MN7 to sink
small drain current of Long Channel MP13
VIN
MP13
1/40
MP1
1s
4s
MP2
80/4
80/4
4s
80/4
4s
2
1
PP 5P
MP7 C1
MP3
MP5 MP6
2s 40/4
2s
4
5
3
40/4 40/4
MN2
MN6
10/1
80/2
1s
ZTAT
MN5
VPLS -
MP7
10/.6
GND
Startup
VREF
R4
100K
R3
123K
/
TF
CTAT
/
TF
/
TF
R2
18.25K
24
80/2
/
TF
PTAT
7
6
MV
4s
Diff In
ZTAT Driven to 0
ZTAT
V3 +
2s
Q1
Q2
1
R1
123K
CTAT
MN4
6/2
MN1
MV
4s 80/2
9
1s
Core
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
MV
4s
MN3
12/2
8
2s
Amp
55
In a Thought Experiment Extrapolating to 0K
ZTAT Currents Remain ~ 10µA
VIN
MP13
1/40
MP1
1s
4s
MP2
80/4
80/4
4s
80/4
4s
2
1
PP 5P
MP7 C1
MP3
MP5 MP6
2s 40/4
2s
4
5
3
40/4 40/4
MN2
MN6
10/1
80/2
1s
ZTAT
MN5
VPLS -
MP7
10/.6
GND
VRef
Remains
~ 1V
Startup
VREF
R4
100K
R3
123K
/
TF
CTAT
/
TF
/
TF
R2
18.25K
24
MN1
MV
80/2
/
TF
PTAT
7
6
MV
4s
Diff In
ZTAT Driven to 0
ZTAT
V3 +
2s
Q1
Q2
1
R1
123K
CTAT
4s 80/2
9
1s
Core
MV
4s
MN4
MN3
6/2
12/2
8
2s
Amp
PTAT Currents ~ 0
CTAT Voltage ~ 1.23V ~ “Magic Voltage”
Suppose we take this circuit and imagine the linear approximation of the
PTAT and CTAT voltages across R2 and R1, R3, respectively, will continue to
zero Kelvin. As we approach zero the PTAT component will approach zero
volts, while Vbe should approach the linear extrapolation to VG0. If we also
imagine that the ZTAT current will remain invariant at 10µA, all of this current
must now be in each of R1 and R3. Using the values which gave the “correct”
ZTAT current at room temperature we see that the 10µA in 123K indicates the
Magic Voltage should be about 1.23V, for these transistors.
56
HOW TO MAKE A BANDGAP VOLTAGE REFERENCE IN ONE EASY LESSON
R=R
VOUT
+ A
-
ll Folks !
A
s
’
t
ha
T
h-th
A
th-t
8A
I hope this presentation has been useful
and answered a few questions – and I'd like to think
it may have evoked even more questions.
VIN
We have some of the answers at IDT
MP13
– perhaps you could help us find more.
1/40
1s
MP1
MP2
4s 80 /4
4s 80 /4
1
2
MN6
10 /1
80 /4
2s
3
4
ZTAT
VPLS -
VREF
R4
100K
R3
123K
ZTAT
/
TF
/
TF
D
ZTAT Driveiff In
n to 0
R2
18.25K
MN5
7
/
PTAT
TF
HOW TO MAKE
IN ONE EASY
Q2 LESSON
GNAD BANDGAP VOLTAGE REFERENCE
CTA
Startup
4s
1s
V3 +
MP7
10 /.6
MP3
T
Q1
24
Core
/
TF
80 /2
R1
123K
57
1
CTAT
Amp Trick
9
1s
MV
4s
MN4
6// 2
V+
V+
R=R
VO
+ A
-
VIN
MP13
1/40
A
8A
1s
1
MN6
10 /1
1
V3 +
VPLS -
MP7
10 /.6
1
GND
Startup
VOUT
1s
MP1
4s
MP2
80 /4
4s
4s
2
6
1
MP3
80 /4
PP 5P
MP7 C1
80 /4
2s
3
ZTAT
R4
100K
2
4
1s
VREF
40 /4
R3
123K
/
TF
CTAT
ZTAT
/
TF
/
TF
24
D
ZTAT Driviff In
en to 0
R2
18.25K
7
MN5
/
TF
PTAT
Q2
Q1
Core
5
1
R1
123K
CTAT
80 /2
Amp Trick
9
1s
MV
MN1
4s 80 /2
MN4
6// 2
MV
4s
MN3
12/2
Amp
2
V+
+
R=R
+
A
-
8A
A
Integrated DeviceTechnology
Learn more: www.idt.com
VOUT