LABORATORY 4
FEEDBACK AMPLIFIER CIRCUITS
OBJECTIVES
To learn about the effects of feedback on amplifier circuits by simulation and by
experimental work.
INFORMATION
Negative feedback structures are used in electronic amplifiers mainly to:





Desensitize the amplifier gain from variations in circuit elements values (for
example, variations in transistor beta have negligible effect on feedback amplifier
gain)
Extend the bandwidth of the amplifier
Control the input and output impedance of the amplifier
Reduce the nonlinearity of the amplification
Reduce the noise generated by amplifier components or external interference
In this experiment, the shunt-shunt (or current-mixing voltage-sampling) topology will be
investigated.
1. Shunt-Shunt Feedback Amplifier
The basic structure of a shunt-series feedback amplifier is shown in Figure 4.1. The input
source is either a current source or its Norton equivalent circuit and the output is a voltage
signal; therefore, it is a transresistance amplifier.
The basic idea of this feedback scheme is to sense the output voltage then adjust the input
current, depending on what the output voltage is. The output voltage is sensed (or sampled)
by a circuit connected in shunt and a feedback signal is generated that is fed to the input.
The feedback signal is a current that can be mixed with the input current in shunt.
4- 1
Figure 4.1. Basic shunt-shunt feedback amplifier structure (from Sedra and Smith).
An example of a shunt-shunt feedback amplifier is shown in Figure 4.2. The feedback
network in this circuit is the resistor Rf, which connects the output of the circuit to the
input side.
Vcc
Rc
Vo
Vcc = 12 V
Rs = 10k
Rc = 3.3k
Rf = 56k
Rf
Is
Q1
Rs
Rof
Vs
Rinf
Figure 4.2. A shunt-shunt feedback amplifier circuit.
The overall shunt-shunt feedback amplifier can be analyzed by separating the feedback
network from the basic amplifier or A-circuit, then modeling the effect of the feedback
network on the A-circuit (called "loading effect") using appropriately placed resistors. The
analysis procedure is shown below.
4- 2
Figure 4.3. General analysis procedure for shunt-shunt feedback amplifiers (from Sedra
and Smith).
The closed loop gain of the feedback amplifier (
) is calculated based on
(Equation 4.1)
where
is the gain of the basic amplifier without feedback but with considering
the loading effects of the feedback network and
is the feedback network gain.
This type of feedback decreases the input and output impedances of the amplifier
according to:
(Equation 4.2)
and
(Equation 4.3)
4- 3
EQUIPMENT
1.
2.
3.
4.
5.
6.
7.
8.
Digital Multimeter (Fluke 8010A, BK PRECISION 2831B)
Digital Oscilloscope Tektronix TDS 210
Function Generator Wavetek FG3B.
Two DC Power Supply (one is ungrounded)
PROTO-BOARD PB-503 (breadboard)
BJT 2N3904 and 2N2222, two each
Resistors according to the circuit figures
Capacitors 5 x 47F.
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. Shunt-shunt feedback amplifier
1.1. Calculate the DC quiescent conditions ( ,
and ), the voltage gain (
), the
feedback gain (
), the input resistance (
), and the output resistance
(
) for the feedback amplifier of Figure 4.2. Use
(transistor forward beta).
1.2. Figure 4.7 shows the basic amplifier (including the loading effects of the feedback
network) and the feedback network for the shunt-shunt feedback amplifier of
Figure 4.2. The power supply
is used to bias the transistor to the same operating
point as in the previous step (Step 1.1). Calculate the amplifier gain (
),
feedback gain (
), input resistance ( ), and output resistance ( ).
Compare
to
,
to
, and
to
according to Equations 4.1
to 4.3.
Vcc
Vcc = 12 V
Rs = 10k
Rc = 3.3k
Rf = 56k
C1 = C2 = 47
Rc
Vo
Is
Q1
Vb
Rs
Rf
Vo
Rf
Vs
C1
Rf
Ro
If
C2
Rin
(a)
(b)
Figure 4.7. The shunt-shunt feedback amplifier: (a) basic amplifier (A-circuit). (b)
feedback network
4- 4
1.3. Simulate the circuit in Figure 4.2 by MicroCap and record the operating point DC
values in Table 4.1, and the small signal parameters in Table 4.2. Note that
is
determined by
(Equation 4.5)
Compare the results of simulation to the results of your calculations in Step 1.1.
1.4. Run AC Analysis in MicroCap and plot and print the frequency response of the
feedback amplifier circuit.
1.5. Simulate the basic amplifier circuit in Figure 4.7(a) by MicroCap and record the small
signal parameters in Table 4.3. Compare the results of simulation to the results of
your calculations in Step 1.2.
1.6. Run AC Analysis in MicroCap and plot and print the frequency response of the basic
amplifier circuit.
PROCEDURE
1. Shunt-shunt feedback amplifier
1.1. Build the circuit in Figure 4.2 using BJT transistor 2N3904 and the other provided
components.
When the layout has been completed, have your TA check your breadboard for errors
and get his/her signature on the Marking Sheet.
1.2. Initially apply only DC power to the circuit, measure the amplifier's Q point using the
Digital multimeter, and record them in Table 4.1. Compare the measured data to the
DC quiescent conditions from the MicroCap simulations.
VCE
[V]
VBE
[V]
IB
[A]
IC
[A]
Simulation
Experiment
Table 4.1. DC quiescent conditions for the shunt-shunt feedback amplifier
1.3. Turn ON the Function Generator to supply the input AC signal to the amplifier
circuit. Set the frequency to 10 kHz.
1.4. Connect CH1 of the oscilloscope in parallel with the input AC source to measure VS.
Connect CH2 of the oscilloscope in parallel with the transistor collector-emitter
terminals to measure the parameters of the output signal Vo.
1.5. Set the input voltage level to VS = 50 mV (RMS), as measured by the CH1 of the
oscilloscope. Since the values are small, read the peak-peak values of the VS (CH1)
and Vo (CH2) from the oscilloscope display and record the data in Table 4.2.
4- 5
1.6. To determine the input current, IS, first measure the voltage across the resistor RS. To
do so, measure the voltage at each of its two terminals using CH1 of the oscilloscope.
Subtract the lower value of the higher one, and divide the result by the actual
measured value of RS. Record the calculated IS in Table 4.2.
1.7. Calculate voltage gain (VO / VS), feedback amplifier gain (Af = VO / IS), and the input
resistance (Rinf = VS / IS – RS), and record in Table 4.2.
VS
[V]
VO
[V]
IS
[A]
VO / VS
Af
[V/A]
Rinf
[ohm]
Simulation
Experiment
Table 4.2: Shunt-shunt feedback amplifier parameters
1.8. Turn OFF the AC signal generator and DC power supply, replace the existing BJT
transistor with 2N2222, turn ON the DC power supply and AC signal generator,
measure the input and output voltages and ) and calculate the voltage gain ( /
). Compare this value with the
/
value from Table 4.2 and provide an
explanation in your report.
1.9. Change your circuit to the one shown in Figure 4.7(a) (the basic amplifier). Replace
back the BJT transistor with 2N3904. Use the DC power supply for Vb, which
provides the required bias voltage for the BJT.
NOTE: Please note that the value for Vb is small around 0.8 V. Its value can be calculated
through:
Vb= VBE + RS IB
(Equation 4.6)
where VBE and IB are the measured values from Table 4.1.
1.10. Turn ON the DC power supplies and measure the DC operating points. If needed,
change Vb slightly to get the same VCE you had measured and recorded in Table 4.1.
1.11. Turn ON the Function Generator to supply the input AC signal to the amplifier circuit.
Set the frequency to 10 kHz.
1.12. Connect CH1 of the oscilloscope in parallel with the input ac source to measure VS.
Connect CH2 of the oscilloscope in parallel with the transistor collector-emitter
terminals to measure the parameters of the output signal Vo.
1.13. Set the input voltage level to VS = 50 mV (RMS) or any other value which will not
result in clipping at the output voltage (CH2). Since the values are small, read the
peak-peak values of the VS (CH1) and Vo (CH2) from the oscilloscope display and
record the data in Table 4.3.
1.14. Similar to Step 1.6, measure the input current IS and record in Table 4.3.
VS
[V]
VO
[V]
IS
[A]
A
[V/A]
Simulation
Experiment
Table 4.3: Parameters for the amplifier circuit in Figure 4.3.
4- 6
Rin
[ohm]
REPORT
1. For the shunt-shunt amplifier, compare Af to A/(1+AB), Rinf to Rin, and Rof to Ro
resulting from pre-lab simulations in Steps 2.3 and 2.4. Are the results consistent with
feedback theory?
2. For the shunt-shunt amplifier, compare the bandwidth of the feedback amplifier to
that of the basic amplifier. Comment on any differences between the two and briefly
explain.
3. When an amplifier with feedback has its transistor(s) changed, what is the effect on
its operation? Briefly explain why this is so?
4. Compare simulated and experimental results and comment on any differences or lack
thereof.
Your lab report is due one week later.
Note: You must copy/print the Signature and Marking Sheet from your manual
before coming to the lab session.
4- 7
SIGNATURE AND MARKING SHEET – LAB #4
To be completed by the TA during your lab session
Student Name:____________________
Check
boxes
TA Name:___________________
Task
Max.
Marks
Pre-lab completed
20
Shunt-Shunt Feedback completed
35
Overall Report Preparation
45
TOTAL MARKS
100
4- 8
Granted
TA
Marks Signature