Download ECE_SEM_1_ENGG_GRAPHICS_CLASS_NOTES

Reg. No.
MODEL –I EXAMINATION JAN 2011
SET - II
Subject
Branch
Duration
: Electronic Circuit – II
: ECE
: 3 Hrs.
PART – A
Code
: EC 2251
Sem
: IV
Max Marks: 100
(10x 2 =20 )
1. Mention two advantages of Negative Feedback
The advantages of negative feedback are higher fidelity and stabilized gain, Increased
bandwidth, less distortion and reduced noise and input & output impedances can be modified as
desired.
2. Addition of Negative feedback to an amplifier reduces its voltage gain from 300 to
60. Determine the feedback factor.
Feedback factor = 0.013
3. Define desensitivity factor
The reciprocal of sensitivity is called as de-sensitivity D
D = 1+A
Where sensitivity is defined as the fractional change in amplification with feedback divided by
the fractional change without feed back and is equal to 1/1+A.
4. List the four basic types of negative feedback
(1) Voltage series feedback (2) Voltage feedback (3) Current series feedback
(4) Current shunt feedback
5. Why LC oscillators are not used for low frequency range?
For the generation of low frequency signals, the LC circuits become impracticable and the RC
phase shift oscillators are more suitable. With the advantage of IC technology RC network is the
only feasible solution. It is very difficult to make an inductance that too of very high value in an
IC. Therefore RC oscillators are increasingly popular.
6. What is an Oscillator?
An oscillator is a circuit which generates output signal without giving any input.
7. State Barkhausen criteria
The loop gain should be unity.
The loop phase shift should be 0 or 360 . This criterion is known as Barkhausen’s criterion for oscillation
8. Give the frequency of oscillation of RC phase shift oscillator
1
fo = -------------------Where K = Rc/R
2RC 6+4K
9.Distinguish: Amplifier & Oscillator
In an amplifier circuit, the frequency, waveform, and magnitude of a.c power generated is
controlled by a.c signal voltage applied at the input of the amplifier. On the other hand in an
oscillator, the frequency, waveform, and magnitude of a.c power generated is controlled by the
circuit itself i.e no external controlling voltage is required. Thus an oscillator may be considered
as an amplifier which provides its own input signal.
10. What is piezo electric effect?
When a mechanical stress is applied between the two phases of the crystal potential is developed
across it and vice versa.
PART – B
( 5x 16 =80 )
11. (a) Draw the circuit diagram of an amplifier with current series feedback and derive
expression for gain,input& output impedance (16)
An amplifier whose fraction of output is fed back to the input is called feedback amplifier. A feedback amplifier consists
of two parts namely amplifier circuit and feedback circuit. Depending upon whether the feedback signal
increases or decreases the input signal it is classified into two i. Positive Feedback – If the feedback signal is in
phase with the input signal .ii Negative feedback – If the
feedback signal is out of phase with the input signal. The positive feedback increases the gain of the amplifier
whereas the negative feedback decreases the gain. In the current series feedback connection a fraction of the
output current is converted into a proportional voltage by the feedback network and then applied in series with
the input.
Af = A / 1 + Aβ , zif = zi(1+Aβ ) , zof = zo(1+Aβ )
(Or)
(b) Draw the block diagram of four types of feedback topology and compare them with
respect to Rif & Rof. ( 8+8)
Current Series : zif = zi(1+Aβ ) , zof = zo(1+Aβ )
Current Shunt: zif = zi / (1+Aβ ) , zof = zo(1+Aβ )
Voltage Series: zif = zi(1+Aβ ) , zof = zo / (1+Aβ )
Voltage Shunt: zif = zi / (1+Aβ ) , zof = zo / (1+Aβ )
12. (a) (i) Elaborate the method of identifying feedback topology (8)
(ii) Write short notes on Nyquist criterion for stability of feedback amplifier (8)
I.If the feedback is taken from the point of output branch then it is voltage sampling otherwise
current sampling
In the input side if the feedback is directly connected to the input node then shunt mixing
otherwise series mixing.
ii.The nyquist criteria says that how stable the circuit works.
(Or)
(b) For the circuit diagram shown in fig., find the voltage gain, feedback factor, input
impedance & output impedance with hfe= 50 & hie = 1.1K (16)
β = - 2 x 10-5 , Rmf = 50 , Ri = 0.971K , D = 2.854 , Rif = 340.22Ὡ , Rof = 735.8Ὡ
13. (a) ) Explain the working principles of Hartley oscillator with neat sketch and derive its
frequency of oscillation (8+8)
It is a sinusoidal oscillator which uses a single tapped coil having two parts named L1 and
L2.The tank circuit also includes a capacitor. The centre point of L1 and L2 is
grounded. They are inductively coupled and form an auto transformer or a split tank
inductor. Feedback between the input and output circuit is accomplished through the
autotransformer action which gives 180 phase shift .The transistor introduces 180
phase shift, therefore the total phase shift is 360 and hence the feedback is positive or
regenerative. In The circuit R1 and R2 form a voltage divider for providing base bias
and RE is an emitter swamping resistor to add stability to the circuit. When Vcc is
applied, an initial bias is established by R1 and R2 and oscillations
The Oscillators using resonant LC tank circuits are most often used for sources of radio frequency(RF) energy
are called as resonant circuit oscillator.
f = 1/( 2  (LeqC) )
Leq= L1+L2;
The condition of oscillation for Hartley oscillator is
hfe = X1/X2 = L1 +M / L2 + M
(Or)
(b) Explain the working of Wein bridge oscillator with neat sketch and derive its frequency
and condition for oscillation (8+8)
The Wien bridge oscillator is the most commonly used audio frequency oscillator because of its simplicity and stability.
Figure shows the Wien bridge oscillator in which Wien bridge circuit is connected between the amplifier input terminals and
the output terminal. The bridge has a series RC network in one arm and a parallel RC network in the adjoining arm. In the
remaining two arms of the bridge, resistors R1 and Rfare connected.
The phase angle criterion for oscillation is that the total phase shift around the circuit must be 0. This condition
occurs only when the bridge is balanced. The frequency of oscillation fo is exactly the resonant frequency of the balanced
Wien bridge and is given by,
Frequency = f0 = 1/(2π R C )
14. (a) Explain the working principles of clapp oscillator with neat sketch and derive its
frequency of oscillation (8+8)
A clapp, oscillator is to generate sine wave in the frequency range 1-500Mhz.It uses a LC tuned circuit with a CE
amplifier to obtain oscillations. The feedback consists of two capacitors C1 and C2 and an inductor Land
C3.The Resistors R1 , R2 and RE provide dc bias to the transistor. The feedback between the output and input
circuit is accomplished by the voltage developed across the capacitor C2Feedback between the input and output
circuit is 180 phase shift .The transistor introduces 180 phase shift, therefore the total phase shift is 360 and
hence the feedback is positive or regenerative. When Vcc is applied, an initial bias is established by R1 and R2 ,
the capacitors C1 and C2 are charged . The capacitors discharge through the coil(L) which sets up the frequency
of oscillations f = 1/( 2  (L Ceq))where Ceq= C1 C2 / C1+ C2.The oscillations across the capacitor C2 is fed
back to the base-emitter junction and appear in an amplified form at the collector.
(Or)
(b) Give the principle of RC oscillator. Explain in details about RC phase shift oscillator and
derive its frequency of oscillation & condition for oscillation (8+8)
Oscillator is a feed back circuit where a fraction of output voltage of an amplifier is fed back to the input in the same phase.
RC phase shift oscillators are a sine wave oscillator which is used in the audio frequency range. It has a CE amplifier ,which
provides 180.phase shift to the input signal and three frequency selective RC phase shift networks provides a phase shift of
60of each , a total of 180 for a signal with frequency equal to specific value, which corresponds to the output of the
oscillator. Thus the total phase shift between the input and output is360.
The frequency of oscillation is given by
f = 1/( 2  RC (6+4K) )
15 a)(i) A crystal has L = 2 H, C = 0.01 pF and R = 2 KΩ. Its mounting capacitance is 2 pF.
Calculate its series and parallel resonant frequencies and quality factor. (8)
(ii)
Brief about frequency ranges of RC and LC oscillators (8)
I. Fs = 1.1MHz
fp = 1.13MHz
Q = 7037
(Or)
(b) Derive the general equation for the LC oscillator
hfe Z1Z2+Z1Z2+Z1Z3+hie(Z1+Z2+Z3) = 0