Lab 8: Bipolar Differential Pair with an
Active Load
ELE 344
University of Rhode Island, Kingston, RI 02881-0805, U.S.A.
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Amplifiers with Active Loads
In this lab you will be making measurements on a bipolar differential amplifier
with an active load. This amplifier is shown in figure 1. Notice that the output
is single-ended. Thus, this amplifier converts its output from differential to
single-ended.
V
cc
V
cc
V
o
V
in−1
V
in−2
I
EE
(a)
Figure 1. Bipolar differential pair with an active load.
The differential gain is given by
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Tel: (401) 874-5482; fax: (401) 782-6422; e-mail: davis@ele.uri.edu
ELE 344 Spring 2009
5 April 2009
Ad =
vo2
= −gm2 (ro2 kro4 )
vid
(1)
where gm2 is the transconductance of Q2 , ro2 & ro4 are the output resistances
of Q2 & Q4 , respectively.
Assuming that the current source has a finite resistance, REE , the common
mode gain is given by
Acm =
ro4
vo2
≈−
vic
β3 REE
(2)
where β3 is the βac value for Q3 .
The common mode rejection ratio, CMRR, is given by
ro2
Ad
≈ gm2 β3 REE
CMRR =
Acm
ro2 + ro4
note that the
1
2
∼
1
gm2 β3 REE
2
(3)
assumes that ro2 ≈ ro4 .
These approximations are quite useful because each parameter can be directly
computed from the tail current, IEE .
The tail current source, IEE , will set the tail current for this amplifier. You
will need to set a current source and mirror the current to the emitters of Q1
and Q2 . The amplifier using a resistor and a simple current mirror to set IEE
is shown in figure 2; the current source, IT ail = IEE .
V
cc
V
cc
V
cc
I
Tail
V
o
V
in−1
V
in−2
V
bn
(a)
Figure 2. Bipolar differential pair with an active load.
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Lab Instructions
1) Design the bipolar differential amplifier stage described in figures 1 and 2.
This can be accomplished by replacing the current source in figure 2 with a
resistor. Select the resistor, Rx , to set the bias current in Q1 & Q2 to 1mA.
The circuits used for the differential and common mode gain measurements
are shown in figure 3, (a) & (b). The schematic in figure 3 (a) shows the
set up for a differential measurement. Not all of the schematic has been
shown. Notice, that the base of Q1 and Q2 , must be set to a DC level which
sets the proper DC value of VBE . Thus, a signal generator must provide
the input signal for Vin1 and an inverted version of the signal for Vin2 . The
common mode gain measurement set up in figure 3 (b) provides a resistor
biasing scheme that will work. You must pick the values of R1 and R2 .
Think about the voltage drops, VCE in Q5 and VBE in Q1 & Q2 in order
to set the voltage in the resistive voltage divider.
Vcc
Vcc
Q3
Rx Ω
Q4
Q3
Rx Ω
C
Vo.1
Vo.2
Q4
C2
Vo.1
Vo.2
RL
Vin.1
Q1
Q2
RL
Vin.1
Vin.2
Q1
Q2
Vin.2
Vcc
R1
Q6
Q6
Q5
R2
Q5
C1
+ Vic
−
(b)
(a)
Figure 3. Amplifiers: Common Emitter Stage: (a) as drawn in the text book and
(b) with active biasing.
The MP Q6XXX, Quad Dual-In-Line Complementary transistor Pairs,
will be used for this lab. Use the 2219 for the npn SPICE model and the
3906 for the pnp SPICE model. The data sheet with pin connections will
be provided in the lab.
2) Simulate your amplifier design.
i) Verify the operating point.
ii) Estimate the differential voltage gain.
iii) Estimate the output voltage swing.
iv) Perform the AC analysis (SPICE) for the differential gain.
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v) Based upon the results of your AC analysis select an input frequency
for the transient analysis which is within the mid-frequency range for
this amplifier.
vi) Select some differential input voltage levels in order to estimate the
linear range of the output voltage swing for this amplifier.
vii) Perform the AC analysis (SPICE) for the common mode gain.
viii) Select some common mode input voltage levels in order to estimate
the linear range of the output voltage swing for this amplifier with a
common mode input.
ix) Does the differential mode gain and the common mode gain agree with
your estimates ?
3) Assemble a prototype using the proto board.
i) Verify the operating point.
ii) Estimate the differential voltage gain.
iii) Estimate the linear output voltage range for several differential input
voltage levels using the spectrum analyzer. Determine the maximum
peak-to-peak input voltage swing.
iv) Estimate the common mode voltage gain.
v) Estimate the linear output voltage range for several common mode
input voltage levels using the spectrum analyzer. Determine the maximum peak-to-peak input voltage swing.
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Write Up
Include the following results in your write up:
1) Record the values of the resistors used in your design.
2) Record the SPICE simulation results.
3) Record the values of ro1 , ro2 , ro4 , ro5 , gm1 , gm2 , βac3 and βac1,2 from the
SPICE simulations.
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4) Record the measurement results.
5) Record the calculations, simulated measurements of the amplifier voltage swing vs. the proto board measurements.
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Questions
1) How does the DC value of VBE for Q1 and Q2 relate to the common
mode voltage ?
2) Can you estimate the input impedance of this amplifier ?
3) How is the amplifier gain affected with a 5kΩ load ? (hint: What are
the values of ro2 and ro4 ) ?
4) Does the common mode gain come close to your approximated value ?
5) This amplifier stage is typically followed by a second amplifier stage;
the second amplifier stage should have a high input impedance. Why
must the input impedance of the second stage be high ?
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