DOUBLE TUNED AMPLIFIERS







DOUBLE TUNED AMPLIFIERS

The disadvantage of POTENTIAL INSTABILITY in single tuned amplifiers can be overcome in Double tuned amplifiers.

Double tuned amplifiers consists of Inductively coupled two tuned circuits. One L1, C1 and the other L2, C2. In the Collector terminals.

A change in the coupling of the two tuned circuits results in change in the shape of the Frequency response curve.

By proper adjustment of the coupling between the two coils of the two tuned circuits, the required results(High selectivity, high Voltage gain and required bandwidth) may be obtained.


CIRCUIT DIAGRAM OF DOUBLE TUNED AMPLIFIERS


double tuned amplifier circuit




CIRCUIT OPERATION OF DOUBLE TUNED AMPLIFIERS


The high Frequency signal to be amplified is applied to the input terminal of the amplifier. The resonant Frequency of TUNED CIRCUIT connected in the Collector circuit is made equal to signal Frequency by varying the value of C1.
Now the tuned circuit L1, C1 offers very high Impedence to input signal Frequency and therefore, large output is developed across it. The output from the tuned circuit L1,C1 is transferred to the second tuned circuit L2, C2 through Mutual Induction. Hence the Frequency response in Double Tuned amplifier depends on the Magnetic Coupling of L1 and L2.



LIMITATIONS OF SINGLE TUNED AMPLIFIERS

OR
ADVANTAGES OF DOUBLE TUNED AMPLIFIERS


Tuned amplifiers are usually used in Radio Frequency stage of wireless communication systems, where such circuits are assigned the work of selecting the desired carrier frequency and of amplifying the permitted pass band around the selected carrier frequency.

ie; The Tuned amplifiers should be highly Selective. But high selectivity requires a Tuned circuit with a high Quality Factor.
A high Quality Factor (Q Factor) circuit will give a high Voltage gain, but at the same time, it will give much reduced bandwidth because the bandwidth is Inversely Proportional to Q-Factor. It means a tuned amplifier with reduced bandwidth may not be able to amplify equally the complete band of signals and results in Poor Reproduction of input signal. This is called POTENTIAL INSTABILITY in tuned amplifiers.



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