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ELEC 351L
Electronics II Laboratory
Spring 2002
Lab 8: BJT Differential Amplifier Measurements
Introduction
One of the main advantages of the differential amplifier is that it amplifies the differential
voltage from a desired signal source, such as a sensor or low-level radio circuit, but attenuates
the undesired common-mode signals that are induced on the cable or wires that carry the signal
to the amplifier. In order to take advantage of the common-mode rejection capability, however,
care must be taken in applying the desired signal to the input of the amplifier. It is also often
important to be aware of the small-signal input and output resistances of the amplifier. In this
lab experiment you will examine some of the practical aspects of diff amp design.
Theoretical Background
The basic diff amp circuit that was used in the previous experiment is shown in Figure 1. In the
configuration shown, the amplifier takes two “single-ended” input voltages v1 and v2, and it
VCC
R C1
R C2
vOUT1
vOUT2
Q1
I ref
R ref
v1
Q2
+
_
+
_
v2
Io
QA
QB
RA
RB
VEE
Figure 1. Difference amplifier with resistor loads.
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produces two “single-ended” output voltages vo1 and vo2. A “single-ended” voltage is one that is
referenced to ground. For example, we have represented the input voltage v1 in Figure 1 using a
voltage source between the base of Q1 and ground, as shown in the circuit fragment on the lefthand side of Figure 2. However, we could have represented the input voltage using the
alternative approach shown on the right-hand side of Figure 2. In the latter case, it is understood
that voltage v1 is referenced to ground; that is, v1 is the node voltage measured between input
terminal #1 and ground. Input terminal #1 is called a single-ended input because it can be
represented in a schematic diagram using a single circle (a “single end”) with a voltage label (v1)
next to it.
Q1
v1
v1
Q
1
+
_
Figure 2. Two alternative methods to represent single-ended input voltages.
Although the single-ended representation shown in Figure 1 is convenient for the purpose of
describing how a diff amp works, it is not the usual configuration. More typically, a single input
voltage is applied to the diff amp between the two input terminals; that is, the input voltage is not
referenced to ground but rather “floats” above ground. In the diff amp circuit, a voltage applied
across the two input terminals is called a differential input voltage. Similarly, the two output
voltages vo1 and vo2 labeled in Figure 1 are single-ended output voltages and are referenced to
ground. The voltage between the two output terminals can be taken as the output voltage instead
of one of the two single-ended output voltages. The voltage measured between the two output
terminals is called the differential output voltage.
In the laboratory it is somewhat difficult to apply a floating differential voltage to the input of a
diff amp or to measure directly the differential output voltage. This is because function
generators and oscilloscopes generally have one terminal of their outputs or inputs tied to
ground. If one were to attempt to use a function generator to apply a differential voltage to the
input of a diff amp, for example, the outer conductor of the cable, which is grounded, would
ground one of the inputs of the diff amp.
One solution to this problem is to use isolation transformers between the inputs and outputs of
diff amps and the single-ended (one terminal grounded) test equipment connected to them.
Recall that a transformer is made by winding two or more wires around a common iron or ferrite
core. A sinusoidal voltage applied to one of the windings causes a sinusoidal voltage to appear
across the other winding(s). The ratio of the input (primary) voltage to the output (secondary)
voltage is equal to the ratio of the number of turns on each winding. Isolation transformers
typically have turns ratios of 1:1, so whatever voltage is applied across one winding, the same
voltage appears across the other winding. This voltage relationship is shown in Figure 3a.
2
1:1
1:1
+
+
+
v
v
v
_
_
_
(a)
+
+
0.5v
_
v
+
0.5v
_
_
(b)
Figure 3. Isolation transformers with 1:1 turns ratios. The circles indicate winding connection
points. (a) Basic 1:1 isolation transformer. (b) Isolation transformer with center tap on one of
the windings.
Some isolation transformers have “taps,” or connection points, at intermediate locations between
the two ends of a given winding. A transformer with a center tap (half-way between the two
ends) on one winding is depicted in Figure 3b. Note that in a 1:1 transformer the voltage
between one end of a winding and a center tap is half that applied to the other winding. This is
because the number of turns between an end and a center tap is half the number of turns in the
other full winding. Since the turns ratio in this case is 2:1, the voltage ratio is 2:1 (or 1:0.5).
To see why isolation transformers are so useful, consider the transformer in Figure 3a. Suppose
that that output of a function generator is connected to the left-hand winding. One end of the
winding will be grounded, because one terminal of the generator is grounded. A sinusoidal
voltage v will be developed across the left-hand winding, so an identical sinusoidal voltage v will
appear across the right-hand winding. However, the secondary voltage “floats” above ground.
That is, neither end of the right-hand winding is grounded. Thus, an isolation transformer
provides a way to create a sinusoidal voltage across two nodes in a circuit without grounding
either node.
Tapped isolation transformers can be used to “move” the ground reference point in a circuit as
shown in Figure 4. Here, a pair of sinusoidal voltages that are equal and opposite with respect to
ground have been created by applying the output of a single-ended function generator to a
center-tapped transformer. When the upper end of the right-hand winding is positive with
respect to ground, the lower end is negative, and vice versa. This design technique can be very
useful when applied to diff amp circuits.
+
1:1
0.5v g
_
+
vg
+
_
0.5v g
_
Figure 4. Repositioning the ground reference using an isolation transformer.
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Experimental Procedure
 Reconstruct the diff amp circuit shown in Figure 1 using the same bias point and power supply
voltages as in the previous experiment. Remember to connect the COM terminal of the power
supply to the circuit ground. Use two matched pairs of 2N2222 or 2N3904 BJTs for the current
mirror and for the amplifying devices. The pin-outs of the 2N2222 and 2N3904 are shown in
Figure 5. As before, the quiescent output voltages (VOUT1 and VOUT2) should be around +1.4 V.
Verify that the quiescent output voltages and voltage drops across the current mirror emitter
resistors are correct before continuing.
Lead identification for
2N3904 in TO-92 package
Lead identification for
2N2222 in TO-18 package
(top view)
(top view)
E
B
C
E B C
Figure 5. Identification of terminals for 2N3904 and 2N2222 npn BJTs.
 Using a transformer, devise a way to apply a differential sinusoidal voltage to the input of the
diff amp without connecting either input terminal to ground. Keep in mind that proper operation
of the circuit depends upon the DC bias voltages at the bases of Q1 and Q2 each being equal to
0 V. Your design must satisfy both of these requirements (differential input voltage and zero
base bias voltage). Next, devise a way to use a second transformer to allow you to measure the
differential output voltage of the amplifier using the oscilloscope without grounding the collector
of either Q1 or Q2.
 Apply a differential sinusoidal voltage vidm at a frequency of 1 or 2 kHz to the input of the
amplifier, and measure the resulting differential output voltage vodm. Recall that vidm = v1 – v2,
and vodm = vo1 – vo2, where v1 and v2 are the single-ended input voltages to Q1 and Q2,
respectively, and where vo1 and vo2 are the single-ended output (collector) voltages of the two
BJTs. It may be necessary to use a voltage divider at the output of the function generator in
order to avoid clipping. If you detect oscillation in the output waveform (often characterized by
a “fuzzy bulge” along all or part of the displayed waveform), try connecting 0.001-F capacitors
between the bases of Q1 and Q2 and ground. The capacitors should break the positive feedback
path at high frequencies by shunting any high-frequency signals to ground. (What is the
reactance of a 0.001-F capacitor at, say, 100 MHz?) Calculate the differential-mode,
differential output gain Adm-diff of the amplifier, and compare the result to the theoretical value
given by
Adm  diff 
vodm
  g m RC .
vidm
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Recall that you can determine an approximate value for gm using
gm 
IC
,
VT
where the product VT has a value of approximately 0.025 V at room temperature and where IC is
the quiescent collector current flowing through either Q1 or Q2.
 Determine the differential-mode, single-ended output gains Adm-se1 and Adm-se2 of the amplifier.
It will be necessary to remove one of the transformers to do this. Compare the values you obtain
to the theoretical values
Adm  se1 
vo1  g m RC

vidm
2
and
Adm se2 
vo 2  g m RC

.
vidm
2
 Devise a way to measure the differential output resistance rout-diff, and determine its value for
your amplifier. Compare the value you obtain to the theoretical value given by
rout  diff  2 RC .
Based on the value you found for rout-diff, give one reason why this amplifier would make a poor
choice for driving an 8- speaker (i.e., what would happen if you were to connect the speaker
directly across the output terminals? How much of the signal power would be converted to
acoustic power by the speaker?)
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