linear integrated circuits interview questions and answers P-3

1.Discuss the need for an instrumentation amplifier? Give a detailed analysis for the same.

Ans:

In a number of industrial and consumer applications, the measurement of physical quantities is usually done with the help of transducers. The output of transducer has to be amplified So that it can drive the indicator or display system. This function is performed by an instrumentation amplifier.

Circuit diagram, instrumentation amplifier with transducer bridge,

Analysis, Expression for out put voltage.

2.Explain the operation of the Schmitt trigger. Ans:

Schmitt trigger is a regenerative comparator. It converts sinusoidal input into a square wave output. The output of Schmitt trigger swings between upper and lower threshold voltages, which are the reference voltages of the input waveform.

Circuit diagram, Analysis.

Expression for upper and lower threshold voltages with and without Vref. Hysterisis width. Waveforms.

3.Discuss in detail the operation of Astable multivibrator. Ans:

Astable multivibrator is a free running oscillator having two quasi-stable states. Thus, there is oscillations between these two states and no external signal are required to produce the change in state.

Circuit diagram, Analysis.

Expression for time period , T = 2RC ln(l + P)/(l-P)

Waveforms.

Circuit for asymmetric square wave generator.

4. Discuss in detail the operation of Monostable multivibrator.
Ans:

Monostable multivibrator is one which generates a single pulse of specified duration in response to each external trigger signal.   It has only one stable state. Application of a trigger causes a change to the quasi-stable state.An external trigger signal generated due to charging and discharging of the capacitor produces the transition to the original stable state.

Circuit diagram, Analysis.

Expression for time period , T = RC In (1 + Vd / Vsat) / (1 - p )

Waveforms.

5. What are the requirements for producing sustained oscillations in feedback
circuits?Discuss any two audio frequency oscillators.

Ans:

For sustained oscillations,

*                    The total phase shift around the loop must be zero at the desired frequency of oscillation, fo. ie, LAB =0 (or) 360°

*                    At fo, the magnitude of the loop gain | A | | should be equal to unity.

RC phase shift oscillator: Circuit diagram,Derive the condition for frequency of oscillation.

Gain,Av >= - 29

Wein bridge oscillator : Circuit diagram,Derive the condition for frequency of oscillation.

Gain, Av = 3

l.Mention some areas where PLL is widely used:

*Radar synchronisation
 *satellite communication systems
 *air borne navigational systems 
*FM communication systems *Computers.

2.List the basic building blocks of PLL:

*Phase detector/comparator
 *Low pass filter
 *Error amplifier
 *Voltage controlled oscillator

3.What are the three stages through which PLL operates?

*Free running
 *Capture

*Locked/ tracking

4.Define lock-in range of a PLL:

The range of frequencies over which the PLL can maintain lock with the incoming signal is called the lock-in range or tracking range.It is expressed as a percentage of the VCO free running frequency.

5.Define capture range of PLL:

The range of frequencies over which the PLL can acquire lock with an input signal is called the capture range. It is expressed as a percentage of the VCO free running frequency.

6.Define Pull-in time.

The total time taken by the PLL to establish lok is called pull-in time.It depends on the initial phase and frequency difference between the two signals as well as on the overall loop gain and loop filter characteristics.

7.For perfect lock, what should be the phase relation between the incoming signal and

VCO output signal?

The VCO output should be 90 degrees out of phase with respect to the input

signal.

8.Give the classification of phase detector:

*Analog phase detector 
*Digital phase detector

9.What is a switch type phase detector?

An electronic switch is opened and closed by signal coming from VCO and the input signal is chopped at a repetition rate determined by the VCO frequency.This type of phase detector is called a half wave detector since the phase information for only one half of the input signal is detected and averaged.

10.What are the problems associated with switch type phase detector?

*The output voltage Ve is proportional to the input signal amplitude.This is undesirable because it makes phase detector gain and loop gain dependent on the input signal amplitude.

*The output is proportional to coscp making it non linear.

11.What is a voltage controlled oscillator?

Voltage controlled oscillator is a free running multivibrator operating at a set frequency called the free running frequency.This frequency can be shifted to either side by applying a dc control voltage and the frequency deviation is proportional to the dc control voltage.

12.On what parameters does the free running frequency of VCO depend on?

*External timing resistor,RT *External timing capacitor,CT *The dc control voltage Vc.

 13.Give the expression for the VCO free running frequency. fo = 0.25 / Rt Ct

14.Define Voltage to Frequency conversion factor.

Voltage to Frequency conversion factor is defined as, Kv = Af₀/AV_c= 8f₀/V_cc where, A V_c is the modulation voltage required to produce the frequency shift A f₀

15.What is the purpose of having a low pass filter in PLL?

*It removes the high frequency components and noise. *Controls the dynamic characteristics of the PLL such as capture range, lock-in range,band-width and transient response.

*The charge on the filter capacitor gives a short- time memory to the PLL.

16.Discuss the effect of having large capture range.

The PLL cannot acquire a signal outside the capture range, but once captured, it will hold on till the frequency goes beyond the lock-in range.Thus , to increase the ability of lock range,large capture range is required.But, a large capture range will make the PLL more susceptible to noise and undesirable signal.

17.Mention some typical applications of PLL:

•         Frequency multiplication/division

•         Frequency translation

•         AM detection

•         FM demodulation

•         FSK demodulation.

18.What is a compander IC? Give some examples.

The term companding means compressing and expanding.In a communication system, the audio signal is compressed in the transmitter and expanded in the receiver. Examples : LM 2704- LM 2707 ; NE 570/571.

19.What are the merits of companding?

*The compression process reduces the dynamic range of the signal before it is transmitted.

*Companding preserves the signal to noise ratio of the original signal and avoids non linear distortion of the signal when the input amplitude is large.
 *It also reduces buzz,bias and low level audio tones caused by mild interference.

20.List the applications of OTA:

OTA can be used in

•         programmable gain voltage amplifier

•         sample and hold circuits

•         voltage controlled state variable filter

•         current controlled relaxation oscillator.