TUNED VOLTAGE AMPLIFIER

The amplifier which amplify selected frequency and specific frequency is known as tuned voltage amplifier. This amplifier is used in all types of receiver. The selected frequency is resonance frequency of tuned circuit which is used in collector circuit of the amplifier.

Selectively

The ability of amplifier to select the desired frequency or signal is measured in term of selectivity.

Resonance circuit

The phenomena by which in an ac circuit at a particular frequency, inductive reactance become equal to capacities reactance is called resonance.

Types of tuned voltage amplifier

Single tuned voltage amplifier
Double tuned voltage amplifier

1. Single tuned voltage amplifier

A single tuned amplifier circuit shown in figure. A parallel resonance circuit is connected in the collector. Usually capacitor used in the resonance circuit is variable. The value of L and C are so selected that its resonant frequency is equal to the signal frequency to be amplified .

Operation:-

Number of high frequency signals are fed at the input of the amplifier. Depending upon the resonant frequency of the parallel tuning circuit a particular frequency signal is selected. The tuning circuit offers very high impedance to this signal frequency and amplifier it. This amplifier signal is taken at the output. All other frequency signal are rejected by the tuning circuit. A particular frequency can be selected by adjusting the value of C in the tuning circuit.

Bandwidth:-

The range or band of frequency over which the gain of the amplifier is equal to or more than 70.7% of maximum value of voltage gain is called bandwidth.

Limitation of single tuned voltage amplifier

As per the discussion so far, we have seen that tuned voltage amplifier are generally used in radio frequency stage of communication system. Here, the main function of these amplifier is to select the desired carrier frequency and to amplify the allowed bandwidth around this selected carrier frequency. To obtain higher gain and improved discriminating property, a tuned circuit with higher Q-factor is employed.

2. Double-Tuned Voltage Amplifier

To obtain high selectivity, higher gain and required band-width, double tuned voltage amplifier are used.

Circuit analysis:-

A double tuned voltage amplifier circuit shown in figure. In the circuit arrangement, it may be noted that all other circuit components are the same except collector circuit. This circuit contains two tuned circuit ; one in the collector and the other in the output. The two coils L 1 and L 2 are magnetically coupled and are usually placed over **fer-rite rod.

Operation:-

When a signal containing many frequency is applied at the input, only that frequency is selected which correspond to the resonant frequency of tuned circuit, rejecting all other frequency. The tuned circuit offer very high impedance to this signal frequency. Consequently, Amplified output appears across the tuned circuit L 1 C 1. The output from the tuned circuit is transferred to the second tuned circuit L2C2 through mutual induction. The frequency response of a double tuned circuit depends upon the magnetic coupling of L 1 and L 2.

Frequency response:-

The frequency response of a double tuned amplifier depends upon the degree of magnetic coupling [also called coefficient of coupling] between the two coils L 1 and L 2. In fact through this coupling, the load resistance is transferred to the collector and effects the frequency response curve.

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