Transistor Tuned Amplifier Interview Question Answer

Important Questions of Tuned Amplifier

In this post, some of the important questions of transistor tuned amplifier is given. It includes function of tuned amplifier, disadvantages of audio amplifier, tuned amplifier for radio frequency, effect of frequency on amplification, resonance conditions in the parallel tuned circuit, parallel resonance curve, band of frequency for tuned amplifier, applications of tuned amplifier and quality factor.

Function of tuned amplifier

Describe the function of the tuned amplifier.

Tuned amplifier

• The function of the tuned amplifier is to amplify selected or narrow band of frequencies for amplification rather than wide band of frequencies as that of audio amplifiers.

Describe the disadvantages of audio amplifiers.

Disadvantages of audio amplifiers

• Less efficient particular at radio frequency
• The gain of the audio amplifier is independent on signal frequency due to resistive load.

Why the tuned amplifiers are used for radio frequencies?

Tuned amplifier for radio frequency

• The function of the tuned amplifier to select particular band of frequency. 
• As the radio frequency is single, it is amplified by the tuned amplifier.

Why the tuned amplifiers are not used for audio frequencies?

Tuned amplifier for audio frequency

• The function of the tuned amplifier to select particular band of frequency. 
• As the audio frequency is mixture of frequencies from 20 Hz to 20 kHz and not single, they are not used for tuned amplifier.

Whether the resistor is used as load in the tuned amplifiers?

• No, the LC parallel tuned circuit is used as load in the tuned amplifiers.

Describe the effect of frequency on the amplification in the tuned amplifier.

Effect of frequency on amplification

• The tuned circuit offers high impedance at resonance frequency whereas it offers low impedance at all other frequencies. 
• If the input signal has same frequency as that of resonance frequency, large amplification occurs due to high impedance of the LC tuned circuit at resonance frequency. 
• Therefore the tuned amplifier amplifies the signals of resonance frequency.

Sharp Parallel Resonance Curve

Describe the condition for resonance in the parallel tuned circuit.

• The resonance in the parallel tuned circuit occurs when the net reactive component of the circuit current becomes zero.

Describe the effect of resistance of coil and frequency on sharpness of Parallel resonance curve.

Sharp Parallel resonance curve

• The sharpness of the Parallel resonance curve depends upon resistance of the coil. 
• Higher the resistance of coil, less sharp resonance curve.
• The impedance of the resonance curve falls rapidly as the frequency varied from the resonance frequency in the sharp parallel resonance curve.

Tuned Amplifier

How the tuned amplifiers select narrow or selected band of frequency?

Band of frequency for tuned amplifier

• The collector of the transistor amplifier in the tuned amplifier consists of parallel LC tuned circuit. 
• The parallel tuned circuit offers very high impedance at resonance frequency and very small impedance at off resonance frequencies. 
• If the signal has same frequency as that of resonant frequency, large amplification occurs due to high impedance of the parallel tuned circuit at resonant frequency. 
• The amplifier selects only those bands of frequencies which lie in the range of the resonant frequency while it rejects all other range of input frequencies.

Applications of Tuned Amplifiers

Give reason : The tuned amplifiers are not used for low frequency applications.

Application of tuned amplifiers

• The resonant frequency of the parallel tuned amplifier is given by

f_r   =  1 / 2π√ ( LC )

• The rating of the capacitor and inductor increases at low frequency therefore the tuned amplifiers become bulky and costly at low frequency.
•  The low frequencies are the mixture of audio frequencies in the range of 20 Hz to 20 kHz and it is not single. 
• As the tuned amplifiers amplify only single frequency, it is not useful for amplification of low frequency.

Q factor & Bandwidth

What is relation between quality factor and bandwidth in the amplifier?

Quality factor

• The quality factor Q of the amplifier is given by

Q  = f_r / BW

Where f_r   = Resonant frequency

BW  = Bandwidth

• Higher the bandwidth of the amplifier, lower the Q factor and vice versa.

Describe the importance of the value of the Q factor in the tuned amplifier.

OR

Describe the importance of the sharpness of the frequency response of the tuned amplifier.

Importance of Q factor

• The bandwidth of the tuned amplifier depends upon the Q factor of the parallel LC tuned circuit. 
• Greater the value of Q factor, lesser is the bandwidth of the amplifier and vice versa. 
• Therefore higher value of Q factor must be necessary in the tuned amplifier in order to select narrow band of frequencies. 
• The sharpness of the resonance curve and therefore its selectivity depends upon the Q factor in the tuned circuit. 
• Higher the value of quality factor, better selectivity

Which parameters greatly affect the voltage gain of the tuned amplifiers?

• The voltage gain of the tuned amplifier is given by 

A_V  = β × ( Z_C / Z_in )

Where  β = Current amplification factor

Z_C   =  Collector load or impedance of parallel tuned circuit

• Therefore the current amplification factor and collector load greatly affects the voltage gain of the tuned amplifier.

State the application of the Double tuned amplifier.

Application of Double tuned amplifier

• The Double tuned amplifier is used for coupling various circuits of the radio and television receivers.

On which parameter the frequency response of the Double tuned amplifier depends?

• Mutual coupling between two parallel LC circuit

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