ELG 3136 Lab Experiment #2 Differential Amplifiers
Lab Experiment #2
Differential Amplifiers
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Student 1
Student 2
Student 3
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ELG 3136 Lab Experiment #2 Differential Amplifiers
Objective: The objective of this lab is to construct a crude differential amplifier from
discrete components.
Prelab Work
1) A current source is to be implemented using a current mirror. The circuit is
shown in Figure 1. Assuming the transistors to be matched each with a β of 200,
what value of R is needed so that the current I = 2 mA?
Figure 1 Current Mirror
2) Suppose that we replace the transistors in Figure 1 with two matched transistors
with β = 100. For the same R found in 1), what is the new value of I? What
happens to I if we change transistors again so that β = 500?
3) For the differential amplifier shown in Figure 2, find gm, rπ, re, Rid and Ad.
ELG 3136 Lab Experiment #2 Differential Amplifiers
10 V
10 V
3k
3k
+
vd
-
2 mA
-10 V
Figure 2. amplifier / Amplificateur différentiel
4) Repeat 3) for a differential amplifier with 20 Ω resistors connected to the each
emitter.
5) For the circuit shown in fig. 2, find the single ended common mode gain if the
current source has a 100 kΩ input resistance.
6) Suppose that the load resistance on Q2 is Rc + ∆Rc, where ∆Rc = 0.05Rc, what is
the common mode gain for the differential output? What is the CMRR (in dB)?
ELG 3136 Lab Experiment #2 Differential Amplifiers
Procedure / Procédure
Part 1 Current Mirror
1) 2 2N3904 transistors are needed for this part of the experiment. β is measured for
the transistors. Try to select two transistors whose β values are very close.
2) Build the circuit shown in fig. 3. What current I, do we expect if the transistors
are assumed to be perfectly matched? Vary the resistance Rx from 1 kΩ in steps
of 1 kΩ and measure the voltage across Rx. From this determine I. Stop
increasing Rx when I has decreased to one-half its original value. Graph I as a
function of Rx. Use the graphs sheet given on the following page.
3) At what value of Rx does I start decreasing and why does this happen?
I starts decreasing at Rx =
10 V
6.8 kΩ
Rx
-10 V
I
-10 V
Fig. 3 Circuit to implement in Part 1
ELG 3136 Lab Experiment #2 Differential Amplifiers
I
Rx
ELG 3136 Lab Experiment #2 Differential Amplifiers
Part 2 Differential Amplifier
1) 2 new 2N3904 transistors are needed. Once again students must select 2 which
have very similar values for β.
2) Build the circuit in fig. 4 by removing the resistance Rx from the circuit in fig. 3
and building the differential amplifier part on top of it. The resistance RB is
needed to supply the transistor with a DC base current. The capacitors are
needed to force the DC current into the base. Measure the DC voltages in the
transistors’ collectors and bases. What is the linear range of the output signals;
what is the voltage offset of your amplifier? Write those values in the table below.
10 V
RB
RC
RC
RB
Rs
Q1
C1
Vs
V+d
-
+ V - Q
o
2
B
C1
A
Rid
RK
-10 V
RB= 2.2 MΩ
RS= 100 KΩ
VC1
VC2
VB1
Fig. 4 Differential Amplifier Circuit
VB2
Linear Range of the output signal
Voltage Offset
ELG 3136 Lab Experiment #2 Differential Amplifiers
3) Using a 100 mV amplitude (200 mVp-p) 5 kHz sine wave input, measure
the vd. Since
vd/vs= (Rid||RB)/(Rs+(Rid||RB)). Use this to find Rid.
Rid =
4) Measure the amplitude of the output. Find the differential gains Ad = vo/vd as
well as Ads = vo/vs.
Make sure that you correctly measure the differential output vo1 – vo2.
Assuming the transistors to be matched, what differential gain is expected?
Vo
Ads
Ad
Expected Voltage Gain
5) Disconnect the base of the second transistor from ground and connect it to point B
in fig. 4. This is common mode.
Observe the output vo and measure its amplitude.
If the output appears as a clipped or distorted sine wave, you may be required to
reduce the amplitude of vs.
Find the common mode gain Acm = vo/vcm.
Compute the CMRR in dB (20log(Ad/Acm)).
vo
Acm
CMRR in dB