Nonlinear Circuits Interview Questions and Answers:

1. What is a comparator and where is it used ?

Ans. A comparator is essentially an op-amp operated in open-loop configuration. The output signal is either a high or low saturated voltage. It is used in digital interfacing, Schmitt triggers, analog­-to-digital converters, oscillators etc.

2. What are the limitations of op-amp as a comparator ?

Ans. Use of an op-amp as a comparator have several limitations, such as:

1. An op-amp typically has a lengthy recovery time from saturation as op-amps are usually designed to operate in linear mode with negative feedback. Almost all op-amps are provided with an internal compensation capacitor that imposes slew rate limitations for high frequency signal. Consequently an op-amp makes a sloppy comparator with propagation delays that can be of the order of tens of microseconds.
2. External hysteresis is always required for slow moving input signals. This is because op-amps do not have any internal hysteresis.
3. A comparator is designed to produce well limited output voltages that easily interface with digital logic. Compatibility with digital logic needs verification in use of op-amp as a comparator.
4. The quiescent current specification of an op-amp is valid only when the feedback is active. Some op-amps show an increased quiescent current when the inputs are different.

A general purpose op-amp, such as 741, can be employed in relatively less critical comparator applications in which speed and accuracy are not of vital significance. With positive feedback, the switching speed of op-amp comparator can be improved and false transition due to noise can be avoided. The output of an op-amp comparator is usually not compatible with a particular logic family like TTL that needs input voltages of approximately +5 V or 0 V. So op-amps are employed with externally wired components like zener diodes so as to keep the output voltage swing within specified limits.

3. What is zero-crossing detector ?

Ans. A comparator with a reference voltage of zero volt is called a zero-crossing detector. Diode clampers are often used to protect the comparator against excessively large input voltages.

4. What is Schmitt trigger?

Ans. A Schmitt trigger uses a comparator with a positive feedback and converts an irregular waveform to a square or pulse waveform. In the Schmitt trigger, the input voltage triggers the output every time, it exceeds certain voltage levels known as upper threshold and lower threshold voltages.

5. Why hysteresis is desirable in a Schmitt trigger?

Ans. Hysteresis is desirable in a Schmitt trigger because it prevents noise from causing false triggering.

6. What is the difference between a basic comparator and Schmitt trigger?

Ans. The basic comparator is a circuit with no feedback and it generates output based on the comparison of two inputs. If voltage at the noninverting terminal exceeds the voltage at the inverting terminal then output voltage is +V_sat. Otherwise the output is -V_sat , whereas the Schmitt trigger is an inverting comparator with positive feedback. This circuit converts any irregular shaped waveform to a square wave. It is also known as a squaring circuit.

7. Give the applications of Schmitt trigger.

Ans. Applications of Schmitt trigger are :

• As a comparator.
• As a zero-crossing detector.
• As a square-wave generator.
• Used in phase meter, pulse-time modulation and amplitude distortion analyzer.

8. What is window detector?

Ans. Sometimes it is required to be determined when an unknown signal is between two precise reference thresholds V_LTP and V_UTP. This determination can be made by a circuit known as the window comparator or detector. it is also called a double-ended limited detector.

9. What is logarithmic amplifier? Name the major applications of log amplifier.

Ans. The circuit, which has output voltage proportional to logarithm of the input voltage, is known as logarithmic amplifier. Log amplifiers have two major areas of applications viz in complex analog calculations and in signal processing.

10. What is antilog amplifier and for what purpose it is used?

Ans. The antilog amplifier is circuit that provides the output voltage proportional to antilogarithm of input voltage. It is used to convert log “encoded” signals back into real-world terms.

11. What is precision rectifier?

Ans. Precision rectifier is a small signal rectifier which is capable of rectifying signals of very small peaks (of the order of a few millivolts).

12. What is difference between conventional and precision rectifiers?

Ans. Conventional rectifiers are employed for rectification, when input voltages are significant in comparison to cut-in voltage of diodes but they can not be used for rectification of signals of voltage. For rectification of signals of very small peaks, precision rectifiers are employed. The circuit is essentially is a noninverting voltage follower when the diode of forward biased.

13. What is S/H circuit, how it can be formed and where is it used.

Ans. The S/H circuit samples an input voltage and holds on to its last sampled value until the input is sampled again. It can be formed by simply using an op-amp with an E-MOSFET as a switch. However, a monolithic S/H such as the LF 398 needs only one external capacitor. Such a circuit is used in digital interfacing and communications.

14. Give the applications of sample and hold circuits.

Ans. S/H circuits using analog multiplexers find wide application in analog signal processing and data conversion systems to store analog voltages accurately over time periods ranging from nanoseconds to several minutes. This finds many applications including data distribution system, simultaneous sample and hold designs, sampling oscilloscopes, digital voltmeters (DVMs), signal construction filters, and analog computational circuits.

15. What is peak detector?

Ans. The peak detector circuit computes the peak value of the input signal. The circuit has a property that it can follow the peak of the signal and the highest value is stored on a capacitor. If a higher value comes, its new value gets stored in the capacitor.

16. What are the applications of peak detector?

Ans. Peak detector finds extensive application in measurement instrumentation, especially in communication circuits like amplitude modulation (AM).

17. What is the name of the circuit that is used to detect the peak value of nonsinusoidal input waveform? Explain its operation.

Ans. The peak detector is name given to the circuit that is used to detect the peak value of nonsinusoidal input waveform.

18. What is clipper and how can it be formed?

Ans. A waveshaping circuit that controls the shape of the output waveform by removing or clipping a portion of the input wave is known as the clipping circuit. The clippers may be positive or negative.

19. Give the applications of analog multiplier.

Ans. The multiplier IC is too often overlooked as a direct, simple solu­tion to complex signal processing problems. Monolithic integra­tion has lowered the cost of multiplier ICs considerably. Applica­tions include signal multiplication for process instrumentation, guidance, chemical analyzers, and servomechanism control. Fre­quency doubling and phase angle detection as well as real power computation can be done with multipliers. Multipliers can be configured to output a dc whose value equals the true rms value of a signal. Voltage control of an amplifier’s, gain, an oscillator’s, frequency, and filter’s cutoff can all be obtained with multiplier. Multipliers can also be employed to improve data ac­quisition through ratioing two signals (to divide out mutual er­ror) and through transducer linearization.