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(Q1Q.6) An amplifier is measured to have an internal resistance of 10 kΩ, voltage gain of 100V/V and output resistance of 100 Ω. Also, when a load resistance of 1 kΩ is connected between the output resistance if found to decrease to 8 kΩ. If the amplifier is fed with the signal source having an internal resistance of 2 kΩ, then
Q. Find Gm.
Gm = (voltage gain in V/V) / (output resistance) or 100/100 A/V.
Av = Avo X Rl/Ro+Rl or 100 X 1000/1000+100 or 90.9 V/V.
Gvo = (Avo X input resistance) / (input resistance + signal resistance).
Gv = (Gvo X Av) / Avo or 83.3 X 90.9 / 100 V/V.
Rout = Rl (1 – Gvo/Gv). Put in the respective values and solve.
(Vo X R in) / (Vi X Rl) gives the required value of Ai.
(Q.7Q.10) The circuit shown below is a small sine wave signal with average zero and transistor ß =
Q. Find the value of R(E) to establish a dc emitter current of about 0.5 mA.
Find R(C) to establish a dc collector voltage of about +5V.
V_{c} = 15 – R_{c}.I_{c}
5 = 15 – R_{c} * 0.99 * 0.5m
R_{c} = 20.2kΩ
= 20kΩ.
For R (L) = 10 kΩ and transistor Ro = 200 kΩ, determine the overall voltage gain.
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