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UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE
Department of Electrical and Computer Engineering
Experiment # 1 – BJT Differential Pair Amplifier with BJT Current Mirror

Overview: The purpose of this lab is to familiarize the student with the biasing and
operation of a BJT differential pair amplifier
...
The amplifier has several variations on this basic
configuration
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A detailed analysis of differential pair amplifiers can be found in any analog
electronics textbook
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Basic BJT Differential Pair Amplifier
The important characteristics for the differential pair amplifier to be studied in this
experiment are: differential voltage gain AVd, common mode gain AVcm, common-mode
rejection ration CMRR, and single-ended voltage gain AVse
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The single-ended gain (output taken at either Vout(+) or Vout(-), is then:

AVse =

α Rc
Vout (+) Vout (−) 1
= AVd =
=
Vin (−) Vin (+ ) 2
2re + 2 Re

Thse results are derived by looking at the output current ic flowing through the resistors
Rc, its relation to ie, and finding the resultant voltage at the output terminals (in small
signal terms)
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Vcc

Rc

Rc
Vout-

Vout+
Q2

Q1

Q2N3904

Q2N3904
Vcm

Re

Re

Iee

Vee

Figure 2
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The details of the
derivation can be found in any analog electronics textbook
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This
desirable characteristic is used to attenuate unwanted presences in the input signal such as
noise from a coaxial cable or from an audio cable
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For the circuit of Figure 1, solve for Rref and RC’s such that ICQA,B is
approximately equal to 2 mA and VCE1,2 = 6V
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2kΩ, and Re = 250Ω
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7V, Vt = 25mV, VA = 100V, β = 100, ICQ1,2 = 0
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Vcc

Rc

Rc
Vout-

Vout+

Q1

Q2

Q2N3904

Vin+

Q2N3904

Re

Vin-

Re

Rref
QB

QA

Q2N3904

Q2N3904

RA

RB

Vee

Figure 1
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Using the equations in the Overview and information from Problem 1, find the
small-signal differential voltage gain AVd, the differential circuit single-ended
gain AVse , the single-ended circuit common-mode voltage gain AVcmse , and the
CMRR (Common-Mode Rejection Ratio) for the circuit in Figure 1
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To determine ree assume that the emitter
resistor on the reference side of the current mirror is zero
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However, ambitous
students are encouraged to attempt the calculation without this assumption
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Observe the circuit in Figure 2
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Vcc

+

C 100uF

Rc

Rc
Vout-

Vout+

Q1

Q2

Q2N3904

Q2N3904
Rpot
500

Rref
QA

QB

Q2N3904

Q2N3904

RA

RB
-

C +
100uF

Vee

Figure 2
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Connect the circuit of Figure 2 with the resistor values calculated in the
Prelab
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7

3
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If VCE1,2 is not equal to approximately
6V, but ICQA,B = 2mA and VBE1,2 ~ 0
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Due to the differences in resistors RC and the
differences in β, the potentiometer is necessary to balance the differential pair
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Also,
measure and record the value of ICQ1,2 and VC1,2
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Apply a small signal input of 100mVp-p with a frequency of 1kHz at the base
of Q1 (i
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Vin(+) = 100mVp-p)
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Leave Vin(-) grounded
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Note which is in phase and which is out of phase
with Vin(+)
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Do this only for the
Vout(-) output point, it is not necessary to take measurements for both as they
will have the same frequency response
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Differential Amplifier with Single-Ended Input
*Note: Make sure that R1, R2 >> rinQ1

8

5
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Again use a
voltage divider network to attain a 100mVp-p input with a frequency of 1kHz
at Vin(-)
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Note which is in phase and which is out of phase with Vin(+)
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For the same reason as
before, it is necessary to do this only for the Vout(+) output point
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Using the same circuit from Step 5, connect channel 1 of the oscilloscope
to Vout(+) and channel 2 of the oscilloscope to Vout(-)
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Use the rotation
knob to adjust the scale
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7
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0Vp-p @1kHz) to both Vin(+) and
Vin(-), this connection will allow for common-mode measurements
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Vary the function generator frequency and record enough data to plot
the single-ended common-mode voltage gain AVse versus frequency
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Plot voltage gain vs
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Compare the two plots and note any
differences
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frequency for the data collected in Step
6, compare this plot with those obtained from Steps 4 and 5
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Using the data obtained for the single-ended common-mode gain and the
differential circuit single-ended gain (either measurement), calculated the
Common Mode Rejection Ratio (CMRR) for the differential pair amplifier
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3
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)?
Note: When plotting the frequency response curves make sure the x and y axes
are log (i
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, 1, 10, 100, 1000, etc
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