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# AE_Tutorial 13_Tuned Amplifiers

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Tutorial 13 gives problems on Tuned Amplifiers.

AE_Tutorial 13_Problems on TUNED AMPLIFIER

Problems have been taken from “Microelectronic Circuit Design” by R.C.Jaeger & T.N.Blalock, McGraw Hill Publibcation,2006.

Problem(1)

Design a tuned amplifier having fo=1MHz, 3dB BW=10kHz and resonance gain= -gmRC= -10, Use a FET which at the given bias point gm=5mA/V and ro=10kΩ.

Solution:

These L and C values are not realizable hence Autotransformer will have to be used along with L’ and C’ where turns ratio is chosen to get L = L’/n2 and C = C’n2.

If n = 3 and L’=28.6µH and C’=0.9nF then it is possible to realize the tank circuit as required in this problem. The actual Autotransformer Configuration is described in the Text (Lecture 12).

Problem(2):

Repeat Problem 1 but with new Q of the tank circuit as 150, find Rp. Then find the new value of RL so that Q of the circuit remains unchanged at resonance frequency 1MHz.

Let the new value of RL be RL’.

If Q should remain unchanged then

Problem(3)

Part(a)

L=5μH . Determine C=?

Design an Intermediate Frequency Amplifier which resonates at fo=455kHz & -3 dB BW=10kHz.

Determine C* to give a resonance frequency at fo=455kHz. [Ans: C1*=24.45nF]

Part(b):- The IF tuned amplifier is coupled to a second IF stage.

Equivalent Circuit:-

Rin=1kΩ, Cin=24.27nF

Determine C1 required and the resulting B.W.

To give fo=455kHz

The resulting resistance= 1kΩ||Rin=1kΩ||1kΩ=500Ω=

This is larger than desired. This is due to loading caused by Zin of the subsequent stage.

Part(C) By using tapped transformer the loading can be removed. Let gm=40mA/V

To achieve a BW of 10kHz, find the value of n=?, C1=? And current gain at resonance.

Ans:- n=1.36, C1=24.36nF, IC/I1=19.1 A/A

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