Electronics II Laboratory
Experiment No. (2)
Feedback in Amplifiers
Object
To study the effects of feedback on the amplifier characteristics
(gain, bandwidth, input and output impedance).
Theory
Feedback is a method in which a portion of the output returned to
the input in order to modify the characteristics of the device. Feedback
can applied to transistor amplifier circuits to modify their performance
characteristics such as gain, bandwidth, input and output impedance etc.
An amplifier in which feedback is incorporated known as feedback
amplifier. Block diagram of typical feedback amplifier shown in Fig. (1).
Feedback can divide in two categories depending upon the phase of the
returned (feedback) signal with respect to the input signal. If the returned
signal is in phase with input signal, feedback is known as positive
feedback. It increases the gain of the amplifier but reduces the bandwidth
and stability of the circuit. It used to produce oscillation. If the feedback
signal is out of phase with respect to the input signal, it is know as a
negative feedback. Negative feedback improves the performance of an
amplifier but reduces the Over all gain. It helps to stabilize the gain,
increases bandwidth: reduces distortions and assures the repeatability of
the circuit performance.
There are number of ways by which a signal can be derived from
output and can be returned to input. Therefore feedback amplifiers can be
classified in the following four groups depending upon the
interconnections of the basic amplifier and the input and output terminals
of feedback network as shown in Fig.(2).
Jassim K. Hmood 1 3-2011
Electronics II Laboratory
Basic
A Ampilfier io B
Vs is S ii Vo
Av
Rif Ri
Vf Vo
Fig.(1) Block diagram of basic feedback amplifier
(A) Voltage-Shunt Feedback
Voltage-Shunt configuration of feedback amplifier is shown in
Fig.(2-a). The feedback network consists of a single resistance R f.
Voltage developed across RL is sampled and feedback to input through
Rf. The shunt connections at input and output terminals reduce input and
Output impedance. The amplifier works as trans-resistance type voltage
amplifier. Fig.(3-a) is a Voltage-Shunt feedback amplifier with:
If
Vo
(B) Voltage-Series Feedback
Voltage-Series topology of the feedback amplifier is shown in
Fig.(2-b). Voltage developed across load resistance is sampled and
feedback to input through resistance Rf and RE (potential divider) as
shown in Fig.(3-b). Sampled voltage is proportional to the output voltage
and feedback in series with the input voltage.
Series connection at input increases input resistance and shunt
connection at output reduces output resistance. The resulting amplifier is
a true voltage amplifier.
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Electronics II Laboratory
R2
Vf Vo
R1 R2
Vf
Vo
R2
So;
R1 R2
(C) Current-Shunt Feedback
Current-Shunt topology of the feedback amplifier is shown in Fig.(2-
c). Feedback signal is proportional to the output current and feedback to
input in shunt. The series connection at the output increases output
resistance and shunt connection at input decreases input resistance. The
amplifier works as a true current amplifier.
If
Io
D) Current-Series Feedback
In Current-Series configuration feedback signal is proportional to the load
current and fed to input through a resistance RE in series with the input
signal as shown in Fig.(2-d). The series connection at the input and the
output increases the input and output impedance. Amplifier- circuit works
as trans-conductance type current amplifier.
I = Ic
Vf = Ic . R
V I R
f c
Io Ic
R
Tables (i) and (ii) show the different topologies of the feedback
with their analysis and the effect of negative feedback on amplifier
characteristics respectively.
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Electronics II Laboratory
Vs Av Vo Vs Av Vo
a) b)
Vs Av Vo Av Vo
c) d)
Fig.(2) Feedback amplifier
Procedure:
1. Connect the circuit as shown in Fig.(4-a).
2. Measure dc collector current Icl without input signal (if the value of I c1
is not between 4 and 4.5mA, change the value of RB.
3. With disconnect 22k, Set the input voltage at 5 kHz, and then change
the input voltage until the output becomes 4 V p-p. Then calculate the
voltage gain.
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Electronics II Laboratory
4. Measure the output resistances.
5. Find frequency response by measuring voltage gain to different
frequencies (30, 100, 200, 500, lk , 10k, 30k etc). Determine upper
and lower half power frequencies and bandwidth.
6. Connect a resistance Rf (feedback network) = 470Kohm between
points A and B, and repeat steps 3 and 4. Determine bandwidth by
measuring frequencies at which gain =0.707 times the gain at 5 kHz.
7. Replace resistance Rf=470Kohm by 1M and repeat steps 4 and 5.
Determine bandwidth as in step 6.
8. Connect circuit as shown in Fig. (4-b), and repeat steps 2, 3, 4 and 5.
9. Connect a resistance Rf of 22Kohm between points A and B as shown
in Fig.(4-a) and repeat steps 3, 4 and 6.
10. Replace Rf =22Kohm by 47kohm and repeat step 6.
11. Remove Rf and CE[ as shown in Fig.(4-d) and repeat steps 3,4, and 5.
12. Connect the circuit as shown in the Fig.(4e), and repeat steps 2, 3, 4
and 6.
13. Remove Rf and repeat step.3, 4 and 6.
14. Connect the circuit as shown in Fig. (4c) and repeat steps 2, 3, 4 and 6
15. Remove Rf and Cf and repeat steps 3, 4 and 6.
Discussion:
1. On the basic of experimental results, state the effects of negative and
positive feedback on gain, input resistance and output resistance.
2. State the difference between current and voltage feedback.
3. List five characteristics of an amplifier, which are modified by the
negative feedback. Support your answer by experimental results.
4. On the basis of experimental results, explain the effects of topology on
the characteristics of an amplifier (gain, input, and output resistance)
5. Verify your experimental results by theoretical calculations.
Jassim K. Hmood 5 3-2011
Electronics II Laboratory
6. Explain why and when the feedback amplifier will oscillate.
7. Mention the application and of the negative and positive feedback?
Table (i): The different topologies of the feed back with their analysis
Characteristics Voltage- Current- Current- Voltage-
series series shunt shunt
=Xf/Xo Vf/Vo Vf/Io If/Io If/Vo
A=Xo/Xi Av=Vo/Vi Gm=Io/Vi Ai=Io/Ii Rm=Vo/Ii
D=1+A D=1+Av D=1+Gm D=1+Ai D=1+Rm
Af Av/D Gm/D Ai/D Rm/D
Rif Ri x D Ri x D Ri/D Ri/D
Rof Ro/(1+Av) Ro(1+Gm) Ro(1+Ai) Ro/(1+A)
Table(ii): The effect of negative feedback on amplifier characteristics
Characteristics Voltage- Current- Current- Voltage-
series series shunt shunt
Rof Decreases Increases Increases Decreases
Rif Increases Increases Decreases Decreases
Gain Decreases Decreases Decreases Decreases
bandwidth Increases Increases Increases Increases
nonlinear Decreases Decreases Decreases Decreases
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Electronics II Laboratory
Vcc Vcc
RL RL
Vo
Vo
Vin Q1
NPN R1 Q1
NPN
R3
R2 RE
C1 +
RE
Vs
-
a b
Vcc Vcc
RL RL
R1
R Vo C
Vo
Q1
NPN Q1 Q2
NPN NPN
Rs
Rs
+
Vs + RE
- Vs
-
c Rf
d
Fig. (3)
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Electronics II Laboratory
Vcc Vcc
10V 10V
220k 1k
320k 1k
0.1uF 0.1uF Vo
Vo
C2 0.1uF
1uF Q
Q
Rs 6.8k
6.8k
RE
220
CE RE 5kHz +
+
100uF 100 Vs
Vs 50mV
-
-
Rf
22k
(a) (d)
Vcc
10V
320k 320k 1k
1k
Vo
0.1uF 0.1uF
0.1uF
Q1 Q2
6.8k
+ RE1 RE2
5kHz 470 100 100uF
Vs
50mV
-
Rf
56k
(b)
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Electronics II Laboratory
Vcc
10V
320k 320k 1k
1k
0.1uF 0.1uF Vo
0.1uF
Q1 Q2
6.8k
+ RE1 RE2
5kHz 470 100uF 470 100uF
Vs
50mV
-
Rf
0.1uF 10k
Fig.(4) Feedback amplifier circuit configuration
Jassim K. Hmood 9 3-2011
