This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Basics of Frequency Response – Set 2”.

1. If the frequency response of the following circuit has a 3db roll-off at ω=10MHz, find the value of C if R=100. Ignore the parasitic capacitances of the MOSFET.

a) 3nF

b) 1nF

c) 5nF

d) 10nF

View Answer

Explanation: The transfer function of the above circuit is -g

_{m}*(R||1/Cs). The frequency response faces a 3db roll-off at ω=10MHz=1/RC. Thus, the magnitude of the C is 1nF.

2. If we have multiplied a signal with a co-sinusoid, we have translated the _____ of the signal to the frequency of the co-sinusoid.

a) Frequency Spectrum

b) Spectral Density

c) Phase Spectrum

d) Noise

View Answer

Explanation: When we multiply a signal with a co-sinusoid, we observe the that Fourier transform of the signal gets shifted left and right by an amount equal to the frequency of the co-sinusoid. The Fourier Transform is the spectral density of the signal, and not the Frequency spectrum. This is because the frequency spectrum represents the magnitude of each harmonic component, as derived from the Fourier Series, but the Fourier Transform represents the relative magnitude of the harmonic components and thus it is the spectral density. We note that the noise is present in the spectral density itself and the spectral density is a more fundamental parameter.

3. The mid band gain of the transistor relates to the frequency band where __________

a) All the capacitances connected to the terminals of the transistor are neglected

b) The parasitic capacitances are neglected

c) The bypass capacitors are neglected

d) Th coupling capacitors are neglected

View Answer

Explanation: The voltage gain of the transistor follows a constant value for a certain range of frequencies. The bandwidth of this range is referred to as the mid band and the voltage gain of the transistor for these frequencies is referred to as the mid band voltage gain. Note that the parasitic capacitors, as well as the bypass capacitors, are open in this situation. The coupling capacitors are shorted.

4. What is the maximum time taken for a simple RC circuit to reach steady state if the time constant is given to be 20nS? Assume that there is no initial condition & the circuit is going to charge itself.

a) 100nS

b) 80nS

c) 60 nS

d) 120nS

View Answer

Explanation: The maximum time for an RC circuit to reach steady state is 5 times the time constant of the circuit. Hence, the correct answer is 100 nS.

5. Which systems are called anticipatory systems?

a) Non Causal

b) Causal

c) Periodic

d) Non-Linear

View Answer

Explanation: Non-Linear systems can also be non-causal an anticipatory system is only those systems which are non-causal. The periodic classification is only valid for signals.

6. If the coefficients of a difference equation are a function of the dependent variable, the discrete time system is ____________

a) Non-linear

b) Linear

c) Cannot be determined until the equation is explicitly provided

d) Non-causal

View Answer

Explanation: Remembering the properties of linearity of a system, we note that the difference equation becomes a non-linear difference equation. Hence, the system is governed by a non-linear equation and the system is a non-linear system.

7. How many frequencies, in the complex domain, are present in the following signal?

exp(-5t) + exp(8t)sin(4t)

a) 0

b) 2

c) 1

d) 3

View Answer

Explanation: We note that in the complex domain, the real axis will represent the real frequencies and hence it will contain -5. For the second term, the frequency is 8±4j and hence we have 3 frequencies present in the given signal.

8. When a periodic signal is passed through a Linear Continuous Time system which offers a phase of α, the phase of the output of the system will be _______________

a) Sum of the initial phase and α

b) Difference of the initial phase and α

c) Product of the initial phase and α

d) Log of Sum of the initial phase and α

View Answer

Explanation: Typically, a periodic signal can be represented by an exponential signal with imaginary frequency and phase. The impulse response of the L.T.I. system can also be represented by its magnitude and phase. The mathematical formulations do show that the phase gets added linearly with the input signal, provided the system is linear.

9. For a C.T.L.T.I. system, the frequency response is given by the ratio of___________

a) Output phasor to Input phasor

b) Input phasor to Output phasor

c) Magnitude of output phasor

d) Phase of Output phasor

View Answer

Explanation: This comes from the basic definition of frequency response of a system. This allows us to observe the output of the system for any frequency because the frequency is the independent variable in the above equation.

10. For a Half Wave Symmetric Signal, what is the D.C. value?

a) 0

b) 1

c) Depends on the signal

d) Infinite

View Answer

Explanation: The Half Wave Symmetric Signal are those signals where the representation in the second half of a period is equal to the negative of the first half. Immediately we confirm that the D.C. value over an entire period will be 0.

11. A half wave rectified sinusoidal signal is__________

a) Odd

b) Even

c) Half Wave symmetric

d) Sum of Even & Odd Signals

View Answer

Explanation: We note that the half wave rectified sinusoidal signal is not a half-wave symmetric signal as the variation in it’s second half is not equal to the variation in the first half. Again, it’s neither an even nor an odd signal and this can be proved by folding the signal with respect to the vertical axis and checking whether at all it is an even or odd signal. Since it is neither an even nor an odd signal, it has to be a sum of even or odd signal according to the definition of a deterministic periodic signal.

**Sanfoundry Global Education & Learning Series – Microelectronics**.

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