# Electronic Devices and Circuits Questions and Answers – Second Harmonic Distortion

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Second Harmonic Distortion”.

1. What is the cause of harmonic distortion in a power amplifier?
a) Exact replication of output signal
b) Perfectly synced harmonics
c) Non-linearity of transistors
d) Linearity of transistors

Explanation: Harmonic distortion in a power amplifier is caused due to the non-linearity of transistor devices. Transistors are the active elements in the circuit and are used for amplification. However, they do not equally amplify each point of the input wave form which then creates distortion.

2. What is the second harmonics of a wave form with fundamental frequency of 50Hz?
a) 50Hz
b) 150Hz
c) 25Hz
d) 100Hz

Explanation: The harmonics of a wave are the integer multiples of the fundamental frequency. Hence, the second harmonics of a wave with fundamental frequency 50Hz is 100Hz.
Second harmonics = 2 × fundamental frequency = 2 × 50Hz = 100Hz

3. The second order harmonics is the most prominent even harmonic.
a) True
b) False

Explanation: All audio amplifiers produce a certain level of distortion usually in the form of even harmonics. Out of all the even harmonics, the most prominent one is the second order harmonics. Second order harmonic distortion is the amount of 2nd order harmonic content present in the output signal with respect to the fundamental frequency.

4. What is the total harmonic distortion if the amplitude of first harmonic distortion is 20dB and second harmonic distortion 25dB?
a) 1.2
b) 1.52
c) 1.25
d) 1.5

Explanation: The total harmonic distortion is calculated by the formula given below. Given the first harmonic distortion V1 = 20dB and V2 = 25dB:
THD = (V22 + V32 +…+ Vn2)1/2 / V1 = (V22)1/2 / V1 = V2 / V1 = 25 / 20 = 1.25

5. What is the signal to noise ratio (SNR) of a second harmonic wave if the input signal amplitude is 20dB and the output signal amplitude is 25dB?
a) 0.8
b) 0.5
c) 1.25
d) 1.5

Explanation: To calculate the SNR (signal to noise ratio) we take the ratio of the input signal amplitude to the output signal amplitude.
SNR = input wave / output wave = 20dB / 25dB = 0.8
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6. What reduces the second harmonic distortion?
a) Amplifiers
b) Demodulators
c) Modulators
d) Transistor switch arrays

Explanation: The second harmonic distortion can be reduced by transistor switch arrays. The switch arrays are implemented in a pseudo-differential configuration. This design of the transistor switch arrays as a side effect reduces the second harmonic distortion.

7. What is the unit of second harmonic distortion?
a) Ampere
b) dBc
c) Volts
d) Watts

Explanation: The second order harmonic distortion is the ratio of second-order harmonic to the input signal or the carrier signal. It is measured as dBc. dBc is the power ratio of a signal to a carrier signal, expressed in decibels.

8. Which of the following configuration classes has the lowest second order harmonics?
a) Class A
b) Class AB
c) Class B
d) Class C

Explanation: Class A configuration has the highest linearity due to which is produces the least amount of distortion, which in turn produces even lower second order harmonic distortion. It is then followed by Class AB, Class B and Class C.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.