Tuned Amplifiers

TUNED AMPLIFIERS

An amplifier which amplifies a specific frequency is called tuned voltage amplifier or tuned amplifier.

Need for Tuned Amplifiers

In situations, when the receiver is required to pick up and amplify the desired radio frequency signal and reject the other frequencies, tuned amplifiers are used.

For example, in radio and television stations, for transmitting at a particular radio frequency, we are using tuned amplifier.

The use of tuned circuit makes the selection and amplification of a particular desired radio frequency possible.

Thus the tuned amplifiers should do the following functions.

i. Selection of a desired radio frequency signal

ii. Amplification of the selected signal to a suitable voltage level.

Types of Tuned Amplifiers

The tuned amplifiers are classified as

i. Small signal tuned amplifiers

ii. Large signal tuned amplifiers

Small Signal Tuned Amplifier

This is used for amplifying small signals at radio frequencies.

The small signals have very small power and. They are operated in class A amplifiers to minimize distortion.

Large Signal Tuned Amplifiers

These amplifiers amplify the large signals at radio frequencies. The large signals have large amount of power, hence they are operated in class AB, B or C amplifiers.

The circuit efficiency will be high and it eliminates harmonic distortion.

PARALLEL RESONANT CIRCUIT

The parallel resonant circuit consists of an inductor (L) and a capacitor (C) connected in parallel to each other Fig. 3.26 shows the parallel resonant circuit connected across ac voltage source. R denotes the coil resistance, in the order of few ohms.

Let the frequency of the ac supply source is varied, then the circuit will have different impedance at different frequencies.

When the frequency is increased, the inductive reactance (X_L) is also increased and the capacitive reactance (X_C) is decreased.

At a certain frequency, the inductive reactance is equal to the capacitive reactance. This frequency is called resonance frequency, denoted by f_o.

At resonance, the impedance of the circuit becomes maximum and the current is minimum.

At resonance,

Resonant frequency, f_o is expressed in Hertz.

Resonance Curve

The response of tuned amplifier is maximum at resonance frequency. The tuned amplifier is designed to reject all frequencies below the lower cut off frequency f_L and above the upper cut-off frequency f_H

At frequency

f = f_o, tuned circuit - resistive load (reactance = 0 i.e., V & I are in phase)

f > f_o, Circuit becomes capacitive (current leads the applied voltage)

f < f_o, Circuit becomes inductive (current lags the applied voltage)

Impedance

Fig. 3.28 shows the variation of circuit impedance with the change in frequency of applied voltage. The impedance value is maximum at resonance.

When the frequency is varied above or below resonance, the impedance value is decreased.

The gain is directly proportional to the value of its load impedance. Thus tuned amplifiers are used for amplifying narrow band of frequencies.

The impedance value is maximum at resonance, and is given by

Z = L/CR

From Fig. 3.28, (b) the current is minimum at resonant frequency.

When the frequency is varied above or below the resonance, the current increases rapidly.

Bandwidth

Due to the change in impedance value, the tuned circuit has the ability to discriminate between the resonance frequency and frequencies, other than resonant circuit is expressed in terms of bandwidth.

Bandwidth of a tuned circuit is defined as the band of frequencies between the points in the resonance curve. When the impedance drops to 1/√2 or 0.707 of its maximum value at resonance.

The points A and B are called as half-power points as the power at these points is half the power developed at resonance.

f₁ - lower half-power frequency

f₂ - upper half-power frequency

Q-Factor

For better selectively, the resonance curve should be very sharp, i.e., impedance should be decreased rapidly if the frequency is varied above and below the resonant frequency.

Selectivity or sharpness of a resonant circuit is defined as the ratio of the bandwidth of the circuit to its resonant frequency. It is otherwise represented using quality factor or Q-factor.

Sharpness = Bandwidth/Resonant Frequency

Where Q_O is Q factor, defined as the ratio of inductive reactance at resonance to the circuit resistance.

Q_O depends upon the coil resistance R.

The circuit with smaller R, has higher Q_O.

Table 3.1. Summary on relation between Q and BW