LABORATORY 9
THE DIFFERENTIAL AMPLIFIER
OBJECTIVES
1. To understand how to amplify weak (small) signals in the presence of noise.
1. To understand how a differential amplifier rejects noise and common mode signals.
2. To evaluate the roles of the components of a differential amplifier.
3. To test the operation of a current source in place of an emitter resistor.
4. To simulate the dc conditions of various differential amplifier circuits using MicroCap
software.
INFORMATION
A differential amplifier is a basic building block seen as the first stage of an operational
amplifier. Its importance to the success of the op-amp is paramount. A differential
amplifier provides two input and two output terminals. The amplifier is powered by “split
supplies,” also the norm for op-amps. Although the op-amp has two input terminals, the
input to be amplified is usually connected between one input and circuit ground. The other
input is used for feedback and other purposes. This experiment will consider the dc biasing
and ac small signal aspects of differential amplifier operation, beginning with the circuit of
Figure 9.1.
Icc
Rc1
Ic1
Ic2
Vcc
Rc2
Vo1
Vo2
Rb1
Vs1
Ib1
Q1
Q2
Ie2
Ie1
Rb1
Ib2
Vs2
Re
Iee
Vee
Figure 9.1. Basic BJT differential amplifier circuit.
Ideally both transistors Q1 and Q2 should have equal parameters, so the Equations (9.1)
and (9.2) could describe the currents values in this circuit.
I c1 = I c 2 =
I cc
2
9-1
Equation (9.1)
I ee
Equation (9.2)
2
A preferred simple circuit of a real differential amplifier is given in Figure 9.2 and it is
using a current source (in the emitters) to DC bias Q1 and Q2. This current source is
designed to overcome variations in the transistors parameters and for temperature
compensation. The design of a current source is provided below.
I e1 = I e 2 =
Vcc
Ic1
Rc1
Rc2
Vo1
Vo2
Q1
Vs1
Q2
Rb1
R1
Rb1
Vs2
Vb3
Q3
R2
Iee
Re
Vee
Figure 9.2. Differential amplifier with a current source.
For DC biasing of the differential amplifier shown in Figure 9.2 following equations could
be used.
Vcc = I c1 Rc1 + VCE1 + VCE 3 + I ee Re + Vee
Equation (9.3)
With some approximation the following equation could be used for dc biasing of this
circuit:
Vb 3 = VBE 3 + I ee Re + Vee
Equation (9.4)
PRE-LABORATORY PREPARATION
The lab preparation must be completed before coming to the lab. Show it to your TA for
checking and grading (out of 15) at the beginning of the lab and get his/her signature.
1. Select the BJT pair
You will be provided with four 2N3904 NPN transistors. You need to select two NPN’s
that are reasonably matched for the basic differential amplifier, and one NPN’s for the
9-2
current source. Use the curve tracer software on the computer in ESB3108 to obtain the
input and output characteristics for each of the transistors. Try to match two 2N3904
transistors to be used as the basic two transistors in the differential pair. A note of
warning, do not damage one of the transistors in the differential pair. If you loose one you
may have to start all over again!
2. Circuit design
Use the circuit of Figure 9.3 as a model to design a differential amplifier with a
current source. Set the DC power supply voltage at ±10 VDC. Include a base resistance
Rb1=Rb2=10 kΩ in each input. Design the circuit such that VCE = 4.5 V for the two
transistors in the differential pair. Similarly set VCE = 4.5 V for the current source
(transistor Q3). Select IC1 = IC2 =2 mA through each transistor in the differential pair, the
constant current source will then have to provide an ICC = IEE = 4 mA. Assume the DC hFE
= 175 (unless you match two NPN’s and have a different value), and VBE = 0.7 V.
Set the voltage VB3 = -5V, using resistors R1 = R2 = 10 kΩ.
The Equations (9.3) and (9.4) should be of help in determining the RC1 = RC2 and
Re component values. Remember the standard values for the resistors that are available in
the box on the windowsill in ESB3107 and use only one resistor for each location. You
may have somewhat different voltages and currents in the lab.
3. Small signal models of the circuit
The DC operating point values for VCE, IC, and IB for each transistor in the circuit were given.
Use the small signal model for the transistor and calculate the low frequency (1 kHz)
voltage gain for the following case:
3.1. From the input of Q1 to the output of Q1 and Q2, with the input of Q2 connected
to ground.
3.2. Similarly from the input of Q2 to the output of Q1 and Q2 with the input of Q1
connected to ground. Keep track of the phase angle in each situation. Use this value to
compare with the simulation.
3.3. Connect the two inputs together to the same voltage source. Calculate the voltage
gain from the input voltage to one or both outputs (Q1 and Q2). Compare your results
with the simulation below. (Common mode signals and/or noise)
3.4. Calculate the input impedance at one input with the other input connected to
ground.
4. MicroCap simulations
4. 1. Set the circuit in Figure 9.2 using calculated components values for resistors RC1 =
RC2 and Re.
4.2. Determine the DC quiescent values for VCE, VBE and IC for each transistor Use a
Table 9.1 to compare the experimental data, the calculated values and the results obtained
from the simulation.
4.3. With the input signal connected to one input and the second input grounded, determine
the frequency response of the voltage gain (magnitude in dB and phase) to the collectors of
9-3
Q1 and Q2. Use a frequency range from 50 Hz to 1 MHZ. Plot two separate sets of curves
and bring them to your lab session.
Attention: You must plot the Bode plots, i.e. the ratio of the output voltage over the input
voltage, not the output voltage by setting the input voltage equal to 1.0∠0°!
MicroCap simulations tips:
• To provide a power supply to the circuit use two “Battery” sources from the
MicroCap library. Connect them as Vcc and Vee voltage sources with common
ground and set them to a 10VDC.
• To obtain the values of the all the bias currents and voltages on your schematic
choose the Probe AC mode from Analysis menu and click on Node Voltages and
Currents icons on the toolbar.
• For a sine wave signal source (used for simulating the Vin), use a 1MHz Sinusoidal
Source from the Micro–Cap library. Set the AC Amplitude to A= 0.02(V) in the
model description area of the signal source. Note that A=0.02V corresponds to a
magnitude of Vp-p=0.04V.
• To obtain the gain and phase frequency response plots for this circuit you must run
“AC ANALYSIS”. To get best results for your plots set the AC Analysis Limits as
follows:
Frequency Range: 1E6,10 which corresponds to a frequency range from 10 Hz to 1MHz
Plot parameters:
P*
1
2
3
4
Y-Expression**
dB(V(3)/V(1))
ph(V(3)/V(1))
dB(V(5)/V(1))
ph(V(5)/V(1))
X-Expression
F
F
F
F
X-Range
1e6,10,1k
1e6,10,1k
1e6,10,1k
1e6,10,1k
Y-Range
30,0,5
200,60,20
30,0,5
10,-90,20
Voltage Gain Q1
Phase Q1
Voltage Gain Q2
Phase Q2
Note:
* Set parameter P (plot #) to plot separate diagram for each curve
**V(1), V(3) and V(5) are the AC voltages at corresponding nodes(1) at the input and (3)
and (5) at the output of the simulation circuit set up. In your particular case they could have
different numeration.
4.4. With the input signal connected to both inputs in parallel, determine the frequency
response of the voltage gain (magnitude in dB and phase) to the collectors of Q1 and Q2.
Use a frequency range from 50 Hz to 1 MHZ. Plot separate sets of curves and bring them to
your lab session.
• To get best results for your plots set the AC Analysis Limits as follows:
Frequency Range: 1E6,10 which corresponds to a frequency range from 10 Hz to 1MHz
Plot parameters:
Voltage Gain Q1
Phase Q1
Voltage Gain Q2
Phase Q2
P*
1
2
3
4
Y-Expression**
dB(V(3)/V(1))
ph(V(3)/V(1))
dB(V(5)/V(1))
ph(V(5)/V(1))
X-Expression
F
F
F
F
9-4
X-Range
1e6,10,1k
1e6,10,1k
1e6,10,1k
1e6,10,1k
Y-Range
0,-60,10
-100,-200,20
0,-60,10
-100,-200,20
EQUIPMENT
1.
2.
3.
4.
5.
6.
Digital multimeter (Fluke 8010A, BK PRECISION 2831B)
Digital oscilloscope Tektronix TDS 210
Function Generator Wavetek FG3B.
PROTO-BOARD PB-503 (breadboard).
BJT 2N3904 – 4.
Resistors 10 kΩ− 4.
PROCEDURE
1. Connect the differential amplifier shown in Figure 9.3 using calculated values of
RC1 = RC2 and Re. You will obtain better results if you ‘match’ the two transistors
in the differential pair using the curve tracer output characteristics.
2. Use a dual voltage Power Supply and connect its POS terminal as Vcc, NEG
terminal as Vee and COM terminal as a common ground. Set the power supply
voltage to 10V DC. Measure the DC quiescent point values. Use a Table 9.1 to
compare the voltages and currents from your calculations and from the simulation
with the experimental data. If your results are significantly different (more than
15%) from your calculated and simulated values, try to find out and eliminate the
reason for that discrepancy.
VCE
Q1
VBE
IC
VCE
Q2
VBE
IC
Q3
VBE
VCE
Calculation
Simulation
Experiment
Table 9.1.
CH1
CH1
CH2
PHASE METER
CH2
OUT
Vcc
Rc2
Rc1
INPUT
10k
Vs1
Vo1
Q1
FG
10k
Vo2
Q2
10k
Q3
10k
Re
Vee
Figure 9.3. Differential amplifier frequency response measurements.
9-5
IC
3. Connect the signal generator (FG) to the Q1 input with the Q2 input connected to
ground as it is shown in Figure 9.3. Increase the input signal to one input (Q1).
Determine the maximum output signal the circuit will produce without distortion.
Use a frequency of 1 kHz. Compare this result with the VCE of the two transistors in
the differential pair and with VCC.
4. Set the frequency at 1 kHz and determine the input resistance as seen at Q1 with the
input of Q2 connected to common ground.
5. With the input signal connected to one input and the second input grounded,
determine the frequency response of the voltage gain (magnitude in dB and phase)
to the collectors of Q1 and Q2. Set the amplitude of the Function Generator (FG)
to 50mV. Sweep a frequency range from 50 Hz to 1 MHz and collect all data in
Table 9.2.
6. Connect both Ch1 of the oscilloscope and the Phase meter in parallel to the input
signal. Connect both Ch2 of the oscilloscope and the Phase meter in parallel to the
output 1 (Q1) signal. Read the both instruments and then switch the “out”
measurement terminal to the output 2 (Q2) signal. Change the frequency of the
signal generator and repeat the measurements for both outputs. Note which output
provides the inverting output and which one the non-inverting output.
7. Calculate the voltage gain AQ1[dB] and AQ2[dB] for both outputs of the
differential amplifier, using Equation (9.5).
AV (dB) = 20 log
f [Hz]
50
100
200
500
800
1k
10k
50k
100k
200k
500k
800k
1M
Vin [V]
VO1 [V]
Θ1[deg]
Vout
Vin
AQ1[dB]
Equation (9.5).
VO2 [V]
Θ2[deg]
AQ2[dB]
Table 9.2 Frequency response of a differential amplifier.
8. Plot obtained voltage gain and phase data on top of your simulated Bode plots and
compare the results.
9. Set the frequency at 1 kHz and interchange the two input connections. Determine
the voltage gain to the collectors of Q1 and Q2.
9-6
10. Connect both inputs to the same voltage source as it is shown in Figure 9.4 and
make the necessary measurements to plot the frequency response of the common
mode signal. Increase the input signal up to 6-7 V to be able to measure the output
voltage. Vary the frequency from 50 Hz to 1 MHZ and fill all data in a table similar
to the Table 9.2. These results will give you an indication how well the differential
amplifier rejects the common signal.
11. Plot obtained voltage gain and phase data on top of your simulated Bode plots and
compare the results.
CH1
CH1
CH2
PHASE METER
CH2
OUT
Vcc
Rc2
Rc1
INPUT
10k
Vo1
10k
Vo2
Q1
Q2
10k
Q3
10k
Vs1
Re
Vee
FG
Figure 9.4. The common mode signal frequency response measurements.
REPORT
Your Lab report is due one week later. Please submit it to your TA in the beginning of
your next lab session.
Note: You must copy/print the Signature and Marking Sheet from your manual
before coming to the lab session.
9-7
SIGNATURE AND MARKING SHEET – LAB 9
To complete by TA during the lab session
Student Name:____________________
TA Name:___________________
Student # : _____________________
Check
boxes
Task
Max. Granted
TA
Marks Marks Signature
Pre-lab completed
15
Diff. Amplifier DC bias Test completed
5
Diff. Amplifier Test completed
10
Common Mode Test completed
5
Overall Report Preparation
65
TOTAL MARKS
100
9-8