Download Module – 5

Module – 5:
1
State and explain the condition of oscillation.
2
Discuss stability criteria for an oscillator.
3
Explain the working of (a) Hartley oscillator (b) Colpitis oscillator (c) Wien
Bridge oscillator (d) RC phase shift oscillator.
4
A phase shift oscillator with CE transistor has RL value of 2K ohms and R
value of 1K ohm. What is the minimum value necessary for hfe to obtain
sustained oscillation?
5
In a phase shift oscillator shown in the following figure 11 has hfe = 60. A
three stage ladder is used with R = RK ohm. What is the value of RL
necessary to achieve sustained oscillation? What value of capacitance
has to be used in each branch in order to obtain sustained oscillations at 5
KHz?
VCC
R2
R3
R1
C
C
Q1
R2
Re
C
When, R3 = R-Ri
Ri= hie
Assume that,
hoe. Re < 0.1
Neglact the effect of
R1, R2
Ce
Fig 11
6
For the Fig. 12, prove that
 = Vf’ / Vo = - 1 / [ 1- 5 2 – j (6- 3)]
where  = 1/RC
Assuming that the network does not load the amplifier. Prove that the
phase shift of is 180o for 2 = 6 and at this frequency  = 1/29.
7
Take into account the loading of the RC network in the phase-shift
oscillator of Fig. 12. If R0 is the output impedance of the amplifier ( assume
that Cs is arbitrarily large), prove that frequency of oscillation f and the
minimum gain A are given by
1
f 
2RC
R
R 
and A  29  23 0  4 0 
R
 R
R
6  4 0 
 R
1
2
VDD
C
Rd
R
V0
Rs
C
C
R
R
Vf
Fig 12
8
A two stage FET oscillator uses the phase shifting network as shown in
the Fig 13, Prove that
V’f / Vi = 1 / [3+ j(RC – 1/RC)]
9
In continuation with the above problem prove that the frequency of
oscillation is fo = 1/2 RC and that the gain must exceed 3.
R
C
C
Vo
R
V’f
Fig 13
10
Find V’f / V0 for the network shown in the fig. 14.
i.
Sketch the circuit of a phase shift FET oscillator using this network.
ii.
Find the expression for the frequency of operation (oscillation)
assuming that the network does not load down the amplifier.
iii.
Find the minimum gain required for oscillation.
C
R
c
O
C2
Vo
R2
V'f
u
o
Fig 14
11
Consider the two sections RC network shown Fig. 15, Find V’ f/Vi function
and verify that it is not possible to obtain 180o phase shift with a finite
attenuation.
Vo
C
C
R
R
Vf
Fig15
12
For the feedback network shown in Fig. 16, find (a) the transfer function,
(b) the input impedance, (c) If this network is used in a phase shift
oscillator find the frequency of oscillation and minimum voltage gain of the
amplifier. Assume that the network
R does not
R the amplifier.
R load down
C
Vo
C
C
V’f
13
Fig 16
Design the wien Bridge oscillator so that
the frequency of oscillation is f o =
1 KHz.
14
For the transistor phase shift oscillator of Fig.17 Show that frequency of
oscillation is given by
f 
1
2RC
1
6  4k
where
k
Rc
R
VCC
Rc
R1
C
R3
C
C
Q1
R
R2
C'
R
Re
Fig. 17
15
Show that a transistor with a small signal common emitter short circuit
gain less than 44.5 can not be used in the phase shift oscillator of Fig. 17
16
Describe briefly a) Series - operated crystal oscillator
b) Shunt - excited crystal oscillator
17
Verify
jX  
j  2   s2
C '  2   p2
for the reactance of the crystal, where the
symbols have their usual meanings.
18
a)
Prove that the ratio of the parallel to series resonant frequencies is
1
given approximately by 1  C
2 C'
b)
If C= 0.04 pF and C’ = 2.0 pF, by what percent is the parallel
resonant frequency greater than the series- resonant frequency?
19
A crystal has the following parameters: L = 0.33 H, C= 0.065 pF, C’ = 1.0
pF, and r = 5.5 K.
a) Find the series resonant frequency.
b) By what percent does the parallel resonant frequency exceed the
series resonant frequency?
c) Find the Q of the crystal.
20
A FET phase shift oscillator having gm = 6000 S, rd = 36 KΩ and
feedback resistor R=12 KΩ is to operate at 2.5 KHz. Select C for specified
oscillation operation.