(a) A feedback amplifier has an open-loop low-frequency gain of A O = 5 × 10 4 , an open-loop bandwidth of ω H = ( 2 π ) ( 5 ) rad / s , and a closed-loop low-frequency gain of A f ( 0 ) = 80. Determine (i) β and (ii) the closed-loop bandwidth. (b) Using the results of part (a), if β is reduced by 50 percent, determine the percent change in (i) A f ( 0 ) and (ii) ω f H ? (Ans. (a) (i) β = 0.01248 , (ii) ω f H = ( 2 π ) ( 3.125 × 10 3 ) rad / s ; ( b ) ( i ) + 100 % , (ii) − 50 % )
(a) A feedback amplifier has an open-loop low-frequency gain of A O = 5 × 10 4 , an open-loop bandwidth of ω H = ( 2 π ) ( 5 ) rad / s , and a closed-loop low-frequency gain of A f ( 0 ) = 80. Determine (i) β and (ii) the closed-loop bandwidth. (b) Using the results of part (a), if β is reduced by 50 percent, determine the percent change in (i) A f ( 0 ) and (ii) ω f H ? (Ans. (a) (i) β = 0.01248 , (ii) ω f H = ( 2 π ) ( 3.125 × 10 3 ) rad / s ; ( b ) ( i ) + 100 % , (ii) − 50 % )
Solution Summary: The author explains the feedback transfer function and the closed loop bandwidth of an amplifier.
(a) A feedback amplifier has an open-loop low-frequency gain of
A
O
=
5
×
10
4
,
an open-loop bandwidth of
ω
H
=
(
2
π
)
(
5
)
rad
/
s
,
and a closed-loop low-frequency gain of
A
f
(
0
)
=
80.
Determine (i)
β
and (ii) the closed-loop bandwidth. (b) Using the results of part (a), if
β
is reduced by 50 percent, determine the percent change in (i)
A
f
(
0
)
and (ii)
ω
f
H
?
(Ans. (a) (i)
β
=
0.01248
, (ii)
ω
f
H
=
(
2
π
)
(
3.125
×
10
3
)
rad
/
s
;
(
b
)
(
i
)
+
100
%
,
(ii)
−
50
%
)
2-) The AC equivalent of a feedback amplifier circuit is given in the figure on the right. (Hfe100, Va = ∞, Ic1 = 15 mA, Ic2 = 5mA and Ic3 = 5 mA)
a) State the type of feedback used in the circuit, explaining the reason.
b) Draw the small signal equivalent of the amplifier circuit.
c) Calculate the value of β for the feedback by drawing the β circuit.
d) Find the Avf = Vo / Vs closed loop gain of this circuit.
e) Find the Rif and Rof values.
Given the feedback circuit below and assuming the voltage amplifier has gain A₂, input resistance
R₁ and output resistance Ro, answer the following questions:
+
a. Loop gain
b. Closed loop gain
Re
C. Input resistance
d. Output resistance
Rin
м
+
RE
A) Sketch the feedback small signal model of this circuit. (Hint: You may convert the input VIN
and source resistance Rs to its Norton current equivalent if needed)
B) Find the following in terms of the amplifier parameters and resistances R, and RF:
Rout
A closed-loop amplifier is to have a 75° phase margin for B 1. What is the required unity-gain frequency f, if
feq
= Woq/(2n)
= 50MHZ? What is the required fia?
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