Electronic Circuit Analysis lab viva questions with Answers part - II

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Oscillators:

1. What is an Oscillator?
It is a circuit which acts as a generator, generating the output signal which oscillates with constant amplitude and constant frequency. 
which feedback used in oscillators?
+ve feedback
2. What is the output of an oscillator if transistor is ideal?
Zero
3. what is Barkhausen criterion ?
|AB| = 1, <AB =360 degrees
where A , B represents amplifier and feedback gain respectively
4. How an oscillator generates oscillations without any input?
Due to the noise exist at the input terminal of forward path (amplifier).
5. Classify oscillators?
Based on feedback : LC oscillators, RC oscillators
Based on Frequency : AF oscillator, RF oscillators
6.What are LC oscillators?
Colpitts oscillator, Hartley oscillator.
7. What are RC oscillators?
RC phase shift oscillator, wein bridge oscillator.
8. What are the output signal frequency of different oscillators?

9. what are the various crystals?

Rock salt, Quartz , Gallium Arsenide.

10. what are the advantages of crystal oscillators?

Excellent frequency stability, high frequency operation and automatic amplitude control

11. which oscillators are AF oscillators?

Audio frequency oscillators are RC phase shift and wein bridge oscillators

12. which are RF oscillators?

Radio frequency oscillators are Colpitts, Hartley and crystal oscillators

13. why can't we use LC oscillator for low frequency oscillations?
Because to produce low frequency oscillations the inductor value should be very high i.e no. of turns in inductor should be more so circuit becomes bulky

POWER AMPLIFIERS:

1. What is the main difference between general amplifier and power amplifier?
General amplifier can amplify voltage or current or both, but output power. power amplifier can increase the output power.
2. Why can't we get more current and voltage gains using general amplifier?
In general amplifier voltage gain is more but current gain is less because as current increases heat dissipation increases across transistor increases so transistor will be burned.
3. Is the power amplifier amplifies the power of input signal?
yes.4. What are the classifications of Power amplifiers?
Basically 2 types: Audio power amp and Radio power amplifier.
Based on mode of operation: Class A, Class B, Class AB, Class C, Class D.
5. Which power amplifier has more efficiency?
Class D6. Define efficiency?
It is defined as ratio of output power and input power.
7. What are the efficiencies of various power amplifiers?
Class A   =   50%
Class B   =   78.5%  
Class C   =   90%
Class D   =   100%
8. Define cross over distortion, and how it can be overcome?
Due to the switching times of transistor a flat top occurs in output of Class B power amplifier. To avoid it the two transistors will normally be biased at a level that is slightly above cut-off.9. What are the percentage of active region in various Power amplifiers?
Class A   =   100%
Class B   =   50%
Class AB =   50-100%  
Class C   =   less than 50%
10. What is total harmonic distortion?
It is the ratio of the sum powers of all harmonic components to power of  the fundamental frequency. 11.Define power dissipation in power amplifiers?
It is defined as product of collector current and voltage across collector and emitter. 12. Specify Q-point location on load line for different power amplifiers?
Class A   -   Middle point DC load line
Class B   -   on Vce axis on load line
Class AB -   nearer to Vce axis on load line  
Class C   -    down side of Vce axis on load line.
13. Where we can use Class A power amplifier?
Low power amplifiers where efficiency is not important
14. Where we can use Class B power amplifier?
Output power amplifiers.
15. Where we can use Class C power amplifier?
Tuned RF amplifiers but can not used as audio amplifiers due to high distortion.

Tags: Viva question on power amplifiers and oscillators, ECA lab viva questions with answers.