BIUST
Faculty of Engineering
Electrical, Computer and Telecommunications Department
EEN221
Tutorial for week 09
Covers material on chapter 18
Question 1
Find the voltage gain, the current gain and the power gain of the amplifier shown in Fig. A.
Answer:
25, 25, 625
Question 2
An amplification system consists of a source, an amplifier and a load. The source has an
open-circuit output voltage of 25 mV and an output resistance of 2 kΩ. The amplifier has an
open-circuit voltage gain of 975, an input resistance of 8 kΩ and an output resistance of 3 Ω.
The load resistance is 3 Ω.
Draw the equivalent circuit of this system and hence determine the voltage, current and
power gains of the amplifier. If the load resistance is reduced to 2 Ω, determine the current
gain of the amplifier.
Answer:
Vi =
8
× 25 mV = 20 mV
8+2
∴Vo/c = 975 × 20 = 19 500 mV
∴Vo =
3
× 19500 = 9750 mV
3+3
∴Gv =
Vo 9750
=
= 488.5
Vi
20
Ii =
25 × 10−3
= 2.5 µ A
(2 + 8) × 10−3
Io =
9750 × 10−3
= 3250 mA
3
∴Gi =
3250 × 10−3
2.5 × 10−6
= 1 300 000
Gp = GvGi = 488.5 × 1 300 000
= 634 000 000
Io =
∴G i =
Vo / c
3+ 2
=
19500
= 3900 mA
5
3900 × 10−3
2.5 × 10−6
= 1 560 000
Question 3
(a) An amplifier has the following parameters: input resistance, 2 kΩ; output resistance, 250
Ω; opencircuit voltage gain, 1000. It is used with a load resistance of 750 Ω. Draw the
Thévenin (constantvoltage generator) equivalent circuit of the amplifier and its load. For this
amplifier calculate: (i) the voltage gain; (ii) the current gain.
(b) Draw the Norton (constant-current generator) equivalent circuit of the amplifier and its
load.
(c) A 10 mV ideal voltage generator is applied to the amplifier input. Determine the current
of the source generator for the amplifier equivalent circuit.
Answer:
(a) (i) Vo =
750
× 100Vi = 750Vi
250 + 750
∴Gv = Vo/Vi = 750
(ii) Ii × 2000 = Vi
∴Vo = 750 × 2000 × Ii
∴Io =
750 × 2000
Ii
750
∴Gi = Io/Ii = 2000
(b)
Is/c = I i ×
2000 × 1000
= 8000 I i
250
∴Gv = 8000
Ro = 250 Ω as before
(c) For Vi = 10 mV, I i =
10 × 10−3
= 5 µA
2 × 10−3
∴Io = Gi × Ii = 2000 × 5 = 10 mA for voltage generator
∴IG = 8000 × 5 = 40 mA for current generator
Question 4
For the amplifier shown in Fig. B, determine the current gain and the voltage gain.
Answer:
20.6, 38
Question 5
An amplifier has a voltage gain of 50 dB. Determine the output voltage when the input
voltage is 2.0 mV.
Answer:
20 log
V2
= 50
V1
∴
V2
= 316
V1
∴ V2 = 2 × 316 = 632 mV
Question 6
Express in decibels the gain of an amplifier which gives an output of 10 W from an input of
0.1 W.
Answer:
20 dB