Frequency Response of an Op-Amp Interview Questions and Answers:
1. What is meant by frequency response?
Ans. The manner in which the gain of an op-amp responds to different frequencies is known as the frequency response. A frequency response plot is a graph of the magnitude of gain with respect to frequency.
2. What is meant by frequency compensation?
Ans. Measures taken to prevent oscillations of amplifiers are known as frequency compensation.
3. Name the conditions which are usually to be fulfilled for a circuit to oscillate.
Ans. For a circuit to oscillate two conditions normally have to be fulfilled—the loop gain should be equal or greater than unity, and the loop phase-shift should be equal to 360°.
4. What is meant by loop gain?
Ans. The loop gain is the gain around the loop from the inverting input terminal to the amplifier output and back to the input via the feedback network.
5. What is meant by open-loop gain?
Ans. The gain from the inverting input to the amplifier output is known as open-loop gain.
6. What is meant by loop phase shift?
Ans. The loop phase shift is the total phase shift around the loop from the inverting input terminal to the amplifier output and back to the input via the feedback network.
7. What is meant by pole frequency?
Ans. The pole frequency fp is the frequency at which the gain is down by 3 dB from its midband value.
8. Why are there variations in gain and phase-shift with respect to frequency in op-amps?
Ans. In op-amps the variations in gain and phase shift with respect to frequency are due to the internal capacitors and stray capacitances, which are due to physical characteristics of semiconductor devices (BJTs and FETs) and the internal construction of the op-amp.
9. How can the rate of change of gain and phase-shift of an op-amp be controlled?
Ans. The rate of change of gain as well as phase shift of an op-amp can be controlled by adding specific components, most common of which are resistors and capacitors.
10. What is meant by compensating network provided in an op-amp?
Ans. The network formed by specific components like resistors and capacitors provided in an op-amp to improve its performance over desired frequency range by controlling its gain and phase shift is known as compensating network.
11. Why is overcompensation usually provided in op-amps?
Ans. Overcompensation results in better amplifier capability, but it also produces an upper cutoff frequency which is not as high as could be achieved with smaller compensating components.
12. What is relation between unity gain frequency and break frequency?
Ans. The unity gain frequency f1 and break frequency (or –3 dB frequency) are related by equation
f1 = Avdf0 and where Avd is dc gain.
13. What is meant by gain-bandwidth product of an op-amp?
Ans. Unity-gain frequency is also called the gain bandwidth product.
14. Why is open-loop configuration of little practical use?
Ans. The open-loop configuration is of little practical use because it has small bandwidth and very high (saturated) gain.
15. What is meant by stable system?
Ans. A circuit or a group of circuits connected together as a system is said to be stable if its output attains a fixed value in a finite time.
16. How SR is specified and measured?
Ans. Generally, the slew rate (SR) is specified for unity gain and is measured by applying a step input (dc) voltage.
17. Why slew rate is caused?
Ans. The slew rate SR is caused due to current limiting and the saturation of internal stages of an op-amp when a high-frequency, large-amplitude signal is applied.