Tuned amplifiers

What is double tuned amplifier?

Double tuned amplifier is one (or) more stages with each stage using coupled circuits having different frequencies of resonance. The two resonant circuits are normally inductively coupled. The tuning is done in both secondary and primary circuits. Following are some of the advantages of double tuned amplifiers:

a)  Large gain bandwidth : For simplicity assume the stages are non-interacting (Generally this is the case as these stages are inductively coupled) then the overall voltage Gain = (Av)^n/(1+w^2)^(1/2) and bandwidth = B.W*(2^(1/n)-1)^(1/2) (since for n Non-interacting stages with each stage having lower and upper cut-off frequencies as fl, fh , the upper and lower cut-off frequencies are given as follows  fh” = fh*(2^(1/n)-1)^(1/2) , fl” = fl/(2^(1/n)-1)^(1/2) and fh”>>fl” ).

b)      Large 3 db bandwidths.

c)      Provides a frequency response having flatter sides.

d)     Sensitivity can be increased due to increased overall gain. Here sensitivity refers to the ability to receive weak signals.

e)      Selectivity can be increased. This is due to decrease in quality factor due to increase in bandwidth. Sensitivity is inversely proportional to quality factor. Selectivity refers to the ability to discriminate the signals in adjacent band.

Quality factor of a double tuned amplifier is given as

                                            Qeff = fr*(2^(1/n)-2)^(1/2)/(B.W)

Double tuned amplifier are used in many applications, one of which is IF amplifier in analog communication receiver.                                       

What is staged tuned amplifier?

In stagger tuned amplifiers one or more tuned amplifier stages are cascaded with each stage having different tuned frequencies, i.e. a number of tuned circuits with active elements operated in union. The name stagger tuned amplifier comes from the fact that the resonant frequencies of different tuned circuits are displaced or staggered. They are characterized by increased bandwidth and reduced selectivity.

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