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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:

Figure 1
Figure 1 (graphics1.png)
Figure 2
Figure 2 (graphics2.png)
Figure 3
Figure 3 (graphics3.png)
Figure 4
Figure 4 (graphics4.png)
Figure 5
Figure 5 (graphics5.png)
Figure 6
Figure 6 (graphics6.png)
Figure 7
Figure 7 (graphics7.png)
Figure 8
Figure 8 (graphics8.png)
Figure 9
Figure 9 (graphics9.png)

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.

Figure 10
Figure 10 (graphics10.png)
Figure 11
Figure 11 (graphics11.png)

Let the new value of RL be RL’.

If Q should remain unchanged then

Figure 12
Figure 12 (graphics12.png)

Figure 13
Figure 13 (graphics13.png)

Problem(3)

Part(a)

Figure 14
Figure 14 (Picture 1.png)

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]

Figure 15
Figure 15 (graphics14.png)
Figure 16
Figure 16 (graphics15.png)

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

Figure 17
Figure 17 (Picture 2.png)

Equivalent Circuit:-

Figure 18
Figure 18 (Picture 3.png)
Figure 19
Figure 19 (Picture 4.png)

Rin=1kΩ, Cin=24.27nF

Determine C1 required and the resulting B.W.

To give fo=455kHz

Figure 20
Figure 20 (graphics16.png)
Figure 21
Figure 21 (graphics17.png)

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

Figure 22
Figure 22 (graphics18.png)

Figure 23
Figure 23 (graphics19.png)
Figure 24
Figure 24 (graphics20.png)
Figure 25
Figure 25 (graphics21.png)

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

Figure 26
Figure 26 (Picture 5.png)

Figure 27
Figure 27 (Picture 6.png)

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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