Single Supply Op Amp Circuits
ROCHESTER INSTITUTE OF TECHNOLOGY
MICROELECTRONIC ENGINEERING
Single Supply Op Amp Circuits
Dr. Lynn Fuller
Webpage: http://people.rit.edu/lffeee
Microelectronic Engineering
Rochester Institute of Technology
82 Lomb Memorial Drive
Rochester, NY 14623-5604
Tel (585) 475-2035
Fax (585) 475-5041
Email: Lynn.Fuller@rit.edu
Department webpage: http://www.microe.rit.edu
3-1-12 OpAmpSingleSupply.ppt
Rochester Institute of Technology
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© March 1, 2012
Dr. Lynn Fuller
Page 1
Single Supply Op Amp Circuits
OUTLINE
Introduction
Basic Dual Supply Op Amp Circuits
Power Supplies
NJU 703X Op Amp
LTC 6078 Op Amp
Single Supply Op Amp Circuits
Virtual Ground
Inverting Amplifier
Non Inverting Amplifier
Comparator
Multivibrator
Current to Voltage Converter
Differential Amplifier
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
INTRODUCTION
This document discusses single-supply, low-voltage, rail-to-rail,
Operational Amplifier (Op Amp) circuits. Although all op amps can
operate with single supply or dual supply, most engineers are
familiar with dual-supply Op Amp circuits such as those shown on
the following page. The dual supply allows the input and output to
be easily referenced to zero volts. (analog ground = earth ground)
Single supply Op Amps usually refers to low voltage Op Amps
using voltages of 5, 3.3 or smaller and ground. Some types of Op
Amps will not work at these voltages. (some Op Amps use BJT
current source biasing that takes a couple of diode drops of voltage
to work thus the output voltage of these Op Amps can only get
within 1.4 volts of the supply rails. For example at 5 volts, output is
limited between 1.4 volts and 3.6 volts and with 3.3 volts supply
some Op Amps may not work at all. With single supply Op Amp
circuits we Rochester
also Institute
can ofnot
have negative output voltages. There are
Technology
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several techniques
for working with these limitations.
© March 1, 2012
Dr. Lynn Fuller
Page 3
Single Supply Op Amp Circuits
SOME BASIC DUAL SUPPLY OP AMP CIRCUITS
These dual supply circuits should be familiar:
R2
+V
+V
R1
R1
-
Vin
Vo
Vin
+
Vo= Vin (1 + R2/R1)
-V
Non-Inverting Amplifier
Inverting Amplifier
C
+V
R
-
Vo
+
Vo= - Vin R2/R1
-V
Vin
R2
-
Vo
Vin
+V
-
Vo
+
+
Vo= Vin
-V
-V
Unity
Buffer
RochesterGain
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Integrator
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Dr. Lynn Fuller
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Vo= -1/RC Vin dt
Single Supply Op Amp Circuits
VOLTAGE SUPPLIES
Single Supply
Dual DC Power Supply
Multiple
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Supplies
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
BASIC TWO STAGE CMOS OPERATIONAL AMPLIFIER
1. Low Voltage operation
2. Rail to Rail input and output voltages
3. Low Input bias ~ 1pA or smaller
4. Low Output Current (depends on M6 and M7)
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5. Unity Gain Bandwidth depends on Cc
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
LOW VOLTAGE, RAIL-TO-RAIL OP AMP
1.
2.
3.
4.
5.
6.
3 to 16 Volt operation
Rail to Rail input and output voltages
Low Input bias ~ 1pA
Output Current ~1mA
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Unity Gain Rochester
Bandwidth
1.5 MHz
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Power Dissipation 1mA at 3 V = 3000uW
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
NJU703X OP AMP DATA SHEET
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
LTC6078 OP AMP
1.
2.
3.
4.
5.
6.
2.7 to 5.5 Volt operation
Rail to Rail input and output voltages
Low Input bias ~ 1pA
Output Current ~5mA
Unity Gain Bandwidth ~350Khz
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Power dissipation
54 uA at 3 V = 162uW
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© March 1, 2012
Dr. Lynn Fuller
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Single Supply Op Amp Circuits
LTC6078 OP AMP
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Single Supply Op Amp Circuits
CREATING A SPLIT SUPPLY FROM A SINGLE SUPPLY
Simple Voltage Splitter
+V/2
The simple voltage
splitter draws a lot of
power if R’s are low.
C’s ensure AC short
(for AC signals). C’s
might not be needed
for DC signals.
R
R
C
C
Single
Supply
-V/2
Common
Ground
Earth
Ground
Signal
Ground
Virtual
Ground
Analog
Ground
Chassis
Ground
Digital
Ground
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Dr. Lynn Fuller
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Floating
Ground
Single Supply Op Amp Circuits
VIRTUAL GROUND / VOLTAGE SPLITTER
Virtual Ground Using Op Amp
+V
+V
100K
Vout = V/2
+
100K
Virtual ground is simply a voltage reference
typically half of the supply voltage.
This virtual ground can supply/sink only as
much current as the maximum Op Amp output
current.
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
TLE2426 RAIL SPLITTER (COMMERCIAL VIRTUAL GND)
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
VOLTAGE FOLLOWER EXAMPLE
+V
Vin
+
Vo = Vin = Vac + Vdc
Vac
Vdc
1.
2.
3.
4.
5.
If Vin is between the +V and –V supply values then Vo = Vin
Vin can not go negative in the example shown because –V supply is ground.
If the signal generator has a DC offset (Vdc) that is larger than the signal
amplitude then this circuit will work because Vin will never go negative.
The output
can be referenced to ground or to analog ground
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Vdc doesMicroelectronic
not need
to be V/2
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© March 1, 2012
Dr. Lynn Fuller
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Single Supply Op Amp Circuits
INVERTING AMPLIFIER EXAMPLE
Inverting Amplifier
R2
+V
Vo= - Vin R2/R1
R1
-
Vin
These Grounds are not the same
+
-
+
Vo
1.
2.
3.
4.
5.
This is a DC and AC amplifier
The input is referenced to the virtual ground.
The virtual ground needs to source or sink the current Vin/R1
The Op Amp needs to source or sink Vo/R2
The output voltage is referenced to the virtual ground which may
be ½ of +V
6.Rochester
If aInstitute
loadof Technology
is connected to Vo the virtual ground needs to source or
Microelectronic Engineering
sink the load current.
© March 1, 2012
Dr. Lynn Fuller
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Single Supply Op Amp Circuits
INVERTING AMPLIFIER EXAMPLES
R2
+V
2R1
-
Vin
+
Vo= - Vin R2/R1
AC Inverting Amplifier
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with capacitor coupling
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
INVERTING AMPLIFIER EXAMPLES
DC Inverting Amplifier
R2
+
+V
Vin
-
-
2R1
+V
+
+
Vo
-
2R1
+V
100K
100K
Vo= - Vin R2/R1
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
NON-INVERTING AMPLIFIER EXAMPLES
+3.3
10K
+V = 3.3
Vin
+
-
10K
Sensor To
10.07K
Vo
+V=3.3
20K
100K
1.
20K
2.
3.
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4.
© March 1, 2012
The two 20K resistors can be replaced by
its Thevenin equivalent of V/2 and 10K
This sets up the analog ground at V/2 and
the voltage gain to 11
Vin is V/2 (or zero if referenced to
analog ground) if the sensor is 10K
If the sensor is not exactly10K then Vo
will have a value of 11 x Vin
Dr. Lynn Fuller
Page 18
Single Supply Op Amp Circuits
COMPARATOR
Vo
+V
+V
+
Vin
Theoretical
Vo
Vref
Vref
0
+V
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Dr. Lynn Fuller
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Vin
Single Supply Op Amp Circuits
BISTABLE CIRCUIT WITH HYSTERESIS
Vo
+V
Theoretical
R
+V
R
R
+V
VT
VTH = 2/3 V
0
+
-
Vo
Vin
VTL =1/3 V
Vin
1.
2.
The R’s set up the threshold voltage at V/3 and 2V/3
Vout is either +V or Ground
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
SINGLE SUPPLY OSCILLATOR (MULTIVIBRATOR)
R3
R1
+V
Vo
R2
+V
VTH
VTL
+V
VT
0
+
-
C
t
Vo
t1
Let R1 = 100K, R2=R3=100K
and +V = 3.3
Then VTH = 2.2 when Vo = 3.3
VTL = 1.1 when Vo = 0
R
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Dr. Lynn Fuller
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Single Supply Op Amp Circuits
SINGLE SUPPLY PHOTO DETECTOR I TO V AMP
470K
I
Light
Vishay BPW46
Digikey No. 751-1017-ND
+5
Vout
+
NJU7024
The voltage across the diode is zero
volts in the dark and the current is
zero
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© March 1, 2012
In the light I is 5uA (in direction
shown, i.e. out of p-side)
What is Vout?
Dr. Lynn Fuller
Page 22
Single Supply Op Amp Circuits
WHEATSTONE BRIDGE AND DIFFERENTIAL AMP
Vs
Rf
Rst
R1
+V
R3
+V
Rin
-
Vo-
Vo1
+
+
Vo+
R2
R4
Rin
Rsb
Rf
+V
+
1.
2.
The R1=R2=R3=R4 make a Wheatstone bridge and are sensor resistors
that will change in response to pressure.
Vo+
and Vo- should be equal to each other and ~Vs/2 with no pressure.
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© March 1, 2012
Dr. Lynn Fuller
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Single Supply Op Amp Circuits
SUMMARY
Low voltage Op Amps are often used with a single supply.
Some circuits work just fine with single supply such as the
comparator.
Other circuits use a virtual ground typically ½ of the supply voltage.
Since signal generators and oscilloscopes are referenced to earth
ground. Op Amp circuits need to consider this if powered by a
single supply referenced to earth ground. In that case earth ground
and virtual ground are at different voltages.
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© March 1, 2012
Dr. Lynn Fuller
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Single Supply Op Amp Circuits
REFERENCES
1. Using Single Supply Operational Amplifiers – from Microchip
2. Designing Single Supply, Low-Power Systems – from Analog
Devices
3. Designing Circuits for Single Supply Operation – from Linear
Technology
4. Single Supply Design – from TI
5. Design Trade-Offs for Single-Supply Op Amps – from Maxium
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© March 1, 2012
Dr. Lynn Fuller
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Single Supply Op Amp Circuits
HOMEWORK – SINGLE SUPPLY OP AMP CKTS
1. Pick one of the example circuits above and do SPICE analysis.
2. More…
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© March 1, 2012
Dr. Lynn Fuller
Page 26