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# AE_Tutorial 12_Oscillators

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Tutorial 12 gives the design problems on Audio & Radio Frequency Oscillators.

AE_Tutorial 12. Problems on OSCILLATORS

Problems have been taken from:

“Microelectronic Circuit Design” by R.C.Jaeger and T.N. Blalock, 2nd Edition, McGraw Hill,2006.

“Microelectronic Circuits- Analysis & Design”, Muhammad H. Rashid, Thomson, Indian Edition, 1999.

Problem (1)Wien Bridge Oscillator

This is called Diode amplitude stabilization of a Wien Bridge Oscillator.

Determine frequency of oscillation & the amplitude of the oscillation.

[Ans:- 9.95 kHz,3.0V]

When the diodes are off,the gain of the amplifier is

So that loop gain >1 and the oscillations grow.

When diode turn ON, the gain is

and the oscillations decay.

PROBLEM(2)

Design the Colpitt’s for oscillation frequency

Step(1) Choose

Step(2) Calculate L from

Step(3) Calculate RF & RL.

Let RL=100kΩ Therefore RF=

Step(4) Determine R1______Let A=10

Step(5) Determmine gm

PROBLEM(3) Colpitts Oscillator Using BJT

Calculate (i) Frequency of oscillation.

(ii)Check the Barkhausen criteria.

(iii)Determine R2.

Solution:-

RFC acts as a open circuit And CB and CE acts as a short circuit.

Therefore the Equivalent Circuit is :-

From the nodal equations we get equating the imaginary parts

Equating Real parts we get

For large values of

Gain

Subs

in Eq(2)

For

Sub (4) in (3)

(i)This

can be provided by BJT hence Barkhausen Criteria is satisfied

(ii)Frequency of oscillation=

(iii)

PROBLEM(4)LC-Tuned MOS Oscillator

Design an oscillator at

For oscillation Loop Gain= 1∠0˚

Step(1) Choose a suitable value of C:

Let C=0.01μF

Step(2) Calculate the value of L from

Step(3) Find the value of R1 from Eq

Step(4)

PROBLEM(5)

Design a Hartley Oscillator at fo=5MHz.USE 2N3822 n-JFET whose parameters are

The circuit is:-

Practically it is implemented in the following manner:-

The incremental circuit is given in Figure 10.

Solution:-

BY analysis:-

We take

PROBLEM(6)CRYSTAL OSCILLATOR

This is a Colpitts-derived op-amp crystal oscillator which uses crystal as an inductor.

This uses a 2 MHz crystal oscillator

Find the frequency of oscillator.

Given

Equivalent Circuit of Crystal Oscillator:

This can also be expressed as:

Effective Capacitance Ceq is parallel with

is given by

It is given that 2MHz crystal is used. Hence the result is self consistent.

This is the frequency of the Crystal Oscillator.

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