Prelab 10: Differential Amplifiers
Name:
Lab Section:
For this lab, assume all NPN transistors are identical 2N3904 BJTs and all PNP transistors are identical
2N3906 BJTs.
Component
2N3904 NPN BJT
2N3906 PNP BJT
IS (A)
6.734 × 10−15
1.41 × 10−15
VA (V)
74.03
18.7
Table 1: Transistor properties
A differential pair with a resistive load is shown in Figure 1; use this circuit to answer the following
questions. Note: You can ignore base currents for this analysis, but you must use the device parameters
given in Table 1. Assume VCC = 9 V and that the inputs are biased at 0 V DC.
VCC
R2
R3
+ vout −
R1
vin+
Q3
Q1
Q4
vin−
Q2
−VCC
Figure 1: Differential pair with resistive load
1. What value of R1 corresponds to IC1 = 2 mA?
R1 =
1
2
2. In lab, we have 500 Ω, 1 kΩ, 5.1 kΩ, and 10 kΩ resistors. Suppose you only wanted to use one resistor
for R1 , which value would give the IC1 closest to 2 mA? What current would you get with the resistor
you chose? Use this value for R1 in the remainder of this prelab.
R1 =
IC1 =
3. We would like to bias the output half-way between 0 V and 9 V to achieve maximum voltage swing;
calculate the values for R2 and R3 that will let us achieve an output bias of 4.5 V. (For this example,
assume R1 is the resistor you chose for the previous question). Among the available resistors mentioned, which ones should we use for R2 and R3 , respectively? What output bias does this combination
achieve? Use these values (of the available resistors) for R2 and R3 in the remainder of this prelab.
(Exact) R2 = R3 =
(Available) R2 = R3 =
Vout,DC =
4. What is the output resistance of the circuit (be sure to take into account the Early Effect)? Assume
R2 and R3 have the values you chose for them (from among the available resistors).
Rout =
5. What is the differential-mode gain of the circuit?
ADM =
6. What is the common-mode gain of the circuit?
3
ACM =
DM 7. What is the common-mode rejection ratio of the circuit? Use CM RR = A
ACM .
CM RR =
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
Experiment 10: Differential Amplifiers
1
Objective
Differential amplifiers are designed to amplify the difference between two signals; thus, such amplifiers are
capable of reducing noise that is common to both inputs. We can quantify the differential-mode versus
common-mode gain in a quantity called the common-mode rejection ratio (CMRR). Differential amplifiers
also lend themselves to use in feedback, though we will not explore that usage in this lab. A typical differential
amplifier with a single-ended output is the op-amp.
2
Materials
The items listed in table 1 will be needed. For this lab, assume all NPN transistors are identical 2N3904
BJTs and all PNP transistors are identical 2N3906 BJTs. Note: Be sure to answer the questions on the
report as you proceed through this lab. The report questions are labeled according to the sections in the
experiment. CAUTION: FOR THIS EXPERIMENT, THE TRANSISTORS CAN BECOME
EXTREMELY HOT!!!
Component
LM741 op-amp
2N3904 NPN BJT
2N3906 PNP BJT
1 kΩ resistor
5.1 kΩ resistor
10 kΩ resistor
0.1 µF capacitor
Quantity
1
4
2
2
2
1
1
Table 1: Components used in this lab
Component
2N3904 NPN BJT
2N3906 PNP BJT
IS (A)
6.734 × 10−15
1.41 × 10−15
VA (V)
74.03
18.7
Table 2: Transistor properties
3
3.1
Procedure
Generating a differential signal
Before building a differential amplifier, let us first generate a differential signal, which would require inverting
an analog signal. One way we can do this is by using an op-amp in negative feeback, as shown in Figure 1.
1
3
2
PROCEDURE
1kΩ
vin
VCC
1kΩ
−
vout
+
−VCC
Figure 1: Inverting amplifier
1. Construct the circuit in Figure 1 using the LM741 op-amp. The pin layout for the LM741 op-amp is
in Figure 2. Note: If your LM741 doesn’t have a notch as shown in the figure, check for a small dot;
this dot labels pin 1.
Figure 2: LM741 pin layout
2. Apply a 60 mVpp , 1 kHz sine wave to the input. Display the input and output on the oscilloscope and
verify that the output is the inverse of the input.
3.2
Differential pair with resistive load
1. Construct the circuit in Figure 3 using 2N3904 transistors for the NPN BJTs. Use R1 = 10 kΩ,
R2 = R3 = 5.1 kΩ, and VCC = 9 V. Note that this is the same circuit you analyzed in the prelab.
VCC
R2
R3
+ vout −
R1
vin+
Q1
Q3
Q4
vin−
Q2
−VCC
Figure 3: Differential pair with resistive load
3
3
PROCEDURE
2. Ground the inputs and measure IC1 , IC2 , IC3 , and VOUT,DC . How do these values compare to your
hand calculations?
3. Apply a 60 mVpp , 1 kHz sine wave to vin+ and ground vin− . Use the oscilloscope to display the input
waveform (vin+ ) and the output waveform (vout+ ); sketch the results on your lab report. If the input
signal is noisy, use the averaging feature of the oscilloscope to get a more accurate result.
4. Use the oscilloscope to measure the peak-to-peak voltages of vin+ and vout+ .
5. Now display vout+ and vout− on the oscilloscope. Do they appear as you would expect?
6. Now use the oscilloscope to display vout+ − vout− . Measure the peak-to-peak voltage of the signal and
calculate the differential gain of the circuit. Does this match the gain you calculated in the prelab?
7. Apply a 30 mV amplitude, 1 kHz sine wave to both vin+ and vin− . Use the oscilloscope to display the
output waveforms from vout+ and vout− . What do you see at the output? Why?
8. Use the inverting amplifier (from section 3.1) to apply a 40 mVpp , 1 kHz differential sine wave to
the inputs (i.e. a 20 mVpp sine wave applied to vin+ and the inverted sine wave to vin− ). Use the
oscilloscope to measure the peak-to-peak voltage of the differential input and output. Now use these
measurements to determine the gain. Does this gain match your prelab calculations? Does it match
the gain you observed in step 3.2.6?
3.3
Differential pair with active load
VCC
Q5
Q6
vout
R1
vin+
Q1
Q3
Q4
vin−
Q2
−VCC
Figure 4: Differential pair with active load
1. Construct the circuit in Figure 4 using 2N3904 transistors for the NPN BJTs and 2N3906 transistors
for the PNP BJTs. Use R1 = 10 kΩ and VCC = 9 V.
2. Apply a 60 mVpp , 1 kHz sine wave to vin+ and ground vin− . Use the oscilloscope to display the output
waveform from vout and sketch the result. Why is the output not sinusoidal?
3. We would like to reduce Rout by loading the amplifier with a small resistor. Attach a load to the
amplifier as shown in Figure 5. Use CL = 0.1 µF and RL = 5 kΩ.
4. Calculate the differential gain for the amplifier with the new load resistance.
5. Apply a 40 mVpp , 1 kHz sine wave to vin+ and ground vin− . Use the oscilloscope to display vin+ and
vout . Sketch vout . What is the measured differential gain of the circuit? How does this compare with
your hand calculations? Does the gain match the differential gain you measured in step 3.2.6? Should
they match?
3
4
PROCEDURE
VCC
Q5
Q6
+
CL
R1
vin+
Q3
Q4
vin−
RL
vout
−
Q1
Q2
−VCC
Figure 5: Differential pair with reduced output resistance
3.4
SPICE Analysis
1. Write a netlist for the circuit in Figure 3. Apply a differential input of 20 mV amplitude, 1 kHz as you
did in step 3.2.8. Hint: Generate a 20 mV amplitude, 1 kHz sine wave and use dependent sources to
generate the non-inverted and inverted 10 mV amplitude sine waves.
2. Use SPICE to find IC1 , IC2 , IC3 , and VOUT,DC . Compare these values with your prelab calculations
and lab measurements.
3. Plot the differential input and differential output signals in Awaves. Print the plot and attach it to
your lab report. Use the plot to calculate the gain. Does it match your hand calculations? Does it
match your lab measurements?
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
Report 10: Differential Amplifiers
Name:
Lab Section:
3.2.2 Measure IC1 , IC2 , IC3 , and VOUT,DC . How do they compare with your hand calculations?
IC1 =
IC2 =
IC3 =
VOUT,DC =
3.2.3 Sketch the waveforms at vin+ and vout+ .
3.2.4 Measure the peak-to-peak voltages of vin+ and vout+
vin+,p−p =
vout+,p−p =
1
2
3.2.5 Qualitatively describe vout+ and vout− (i.e their similarities and differences). Is this result agreeable?
3.2.6 Measure the peak-to-peak voltage of vout+ − vout− and calculate the differential gain of the circuit.
Does this match the gain you calculated in the prelab?
vout,p−p =
ADM =
3.2.7 What do you see at the output? Why?
3.2.8 Measure the gain. Does it match your prelab calculations? Does it match your result from 3.2.6?
ADM =
3.3.2 Sketch the output waveform. Why is it not sinusoidal?
3
3.3.4 Calculate the differential gain of the amplifier with the added load resistor.
ADM =
3.3.5 Sketch vout . What is the measured differential gain of the circuit? How does it compare with your
hand calculations? Does it match the gain you observed in step 3.2.6? Should they match?
ADM =
3.4.1 Attach your netlist to this report.
3.4.2 Use SPICE to find IC1 , IC2 , IC3 , and Vout,DC . Compare these values with your prelab calculations
and lab measurements.
IC1 =
IC2 =
IC3 =
VOUT,DC =
4
3.4.3 Attach your plot to this report. What is the gain according to the plot? Does it match your hand
calculations? Does it match your labs measurements?
ADM =
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.