# Transformers Questions and Answers – Audio Frequency Transformer

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This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Audio Frequency Transformer”.

1. Audio frequency transformers are used ________________
a) Input stage of audio frequency electronic amplifier
b) Intermediate stage of audio frequency electronic amplifier
c) Output stage of audio frequency electronic amplifier
d) Anywhere in the amplifier circuitry

Explanation: Audio frequency transformers are used at the output stage of audio frequency electronic amplifier for matching the load to the output impedance of the power amplifier stage. Here the load is fixed but the frequency is variable over a band.

2. Which of the following is the most desirable frequency response for audio frequency transformers?
a) Rising
b) Flat-topped
c) Decreasing
d) Depends on the application

Explanation: The response being the ratio V2/V1. A flat frequency response over the frequency band of interest is most desirable in these cases. The corresponding phase angle (angle of V2 w.r.t. V1) is called phase response. A small angle is acceptable.

3. Stray capacitance in the IF region acts as __________
a) closed circuit
b) open circuit
c) first close then open circuit
d) first open then close circuit

Explanation: Shunting effect of transformer windings stray capacitance CS. In the intermediate frequency (IF) range the shunt branch acts like an open circuit and series impedance drop is also negligibly small such that V2/V1 remains fixed (flat response).

4. Magnetizing susceptance is low in __________________
a) IF region
b) LF region
c) HF region
d) In all regions

Explanation: In the LF (low frequency) region the magnetizing susceptance is low and draws a large current with a consequent large voltage drop in (r1 + jωL1). As a result, V2/V1 drops sharply to zero as Bm = 0.

5. Stray capacitance susceptance has a strong shunting effect in _________________
a) IF region
b) LF region
c) HF region
d) In all regions

Explanation: In the HF (high frequency) region BS = 1/ω CS (stray capacitance susceptance) has a strong shunting effect and V2/V1 drops off, can be shown by the complete frequency response of a transformer on logarithmic frequency scale as decreasing.

6. How low reactance condition is achieved in potential transformer?
a) By using minimum number of turns
b) By varying thickness of wire
c) By interlacing both windings on same limb
d) By other methods

Explanation: Low reactance is achieved by interlacing primary and secondary both on core limb. High magnetizing reactance according to physics, requires minimum iron path and high permeability steel. Low loss requires low-loss steel and very thin laminations.

7. Which of the following is most important way to reduce errors in PT?
a) Low magnetizing current
b) High magnetizing reactance
c) Low loss
d) High burden

Explanation: There are various ways to minimize the errors in potential transformer, like low leakage reactance, low loss and high magnetizing reactance, etc. But the most important thing for low PT errors is to make the burden (Zb) as high as feasible.

8. Potential transformers are ____________
a) step up transformers
b) step down transformers
c) can be of any type according to the application
d) step up and step down transformer

Explanation: Potential transformer or voltage transformer are used in electrical power system for stepping down the system voltage to a different safe value which can be fed to low ratings meters and relays.

Sanfoundry Global Education & Learning Series – Transformers.