Small Signal Tuned Amplifiers

SMALL SIGNAL TUNED AMPLIFIERS

A number of tuned amplifier stages are connected together in cascade so that large voltage gain can be obtained. These tuned amplifiers are classified as

i. Single Tuned Amplifier

It uses only one parallel circuit as the load impedance in each stage.

All the tuned circuits are tuned to the same frequency.

ii. Double Tuned Amplifier

It uses two inductively coupled tuned circuits for each stage and all the tuned circuits are tuned to same frequency.

iii. Stagger Tuned Amplifier

In this method a number of single tuned stages are connected in cascade.

The successive tuned circuits are tuned to slightly different frequencies.

SINGLE TUNED AMPLIFIERS

These amplifiers use only one parallel circuit. With all tuned circuits tuned to the same frequency. Single tuned amplifiers are of two types.

i. Capacitance coupled amplifiers

ii. Inductance coupled amplifiers

i. Capacitance Coupled Single Tuned Amplifiers

The output across the tuned circuit is coupled to the next stage through the coupling capacitor. Hence it is called capacitively coupled tuned amplifiers.

Construction

Fig. 3.30 shows the circuit diagram of a capacitance coupled single tuned amplifier. The tuned circuit is formed by LC circuit. The capacitance and inductance values are selected such that the frequency to be amplified is equal to the resonant frequency.

The resistors R₁, R₂ and R_E are called biasing resistors since these provide the operating current and voltage to the transistors.

Operation

A radio frequency signal, which is to be amplified, is applied to the input of the amplifier.

The capacitance and inductance values are adjusted such that the resonant frequency is made equal to the frequency of input signal.

When frequency of the tuned circuit becomes same as that of the input signal, then resonance occurs. Hence large output signal appears across the output.

Thus the signal with frequency equal to the resonant frequency will be amplified and all the other frequencies will be rejected by the tuned circuit.

ii. Inductance Coupled Single Tuned Amplifiers

In single tuned amplifiers, LC circuit is used for frequency tuning.

If the output is taken across an inductor, then the tuned circuit is called inductively coupled tuned circuit. It is otherwise called as transformer coupled tuned circuit. Fig.3.31 shows the circuit diagram of an inductively coupled single tuned circuit.

Gain and Frequency Response

The voltage gain depends upon current gain (β), input impedance R_f and ac load resistance. The voltage gain is given by,

AC load resistance of parallel resonant circuit is given by

Substitute (2) in (1)

Thus voltage gain of a tuned amplifier is very high at resonant frequency and decreases as the frequency changes above or below the resonant frequency.

The plot of voltage gain versus frequency is called frequency response.

The bandwidth of a tuned amplifier is equal to difference in frequency between the points A and B. At resonant frequency, the voltage gain drops to 1/√2 of its maximum value.

Where

Q_O - Q-factor of tuned circuit

Drawbacks of Single Tuned Amplifier

The tuned amplifier should be highly frequency selective to select the desired carrier frequency selective to select the desired carrier frequency. For high selectivity, the Q factor should be high.

The high Q-factor leads to reduced bandwidth. This narrow bandwidth inturn result in poor reproduction of audio signal. This is the main drawback of single tuned amplifier.

Selectivity ↑ Q factor ↑ BW↓ Recovery of signal-poor