Double-tuned amplifier

The scheme results in a wider bandwidth and steeper skirts than a single tuned circuit would achieve.

Designs frequently use a coupling greater than this (over-coupling) in order to achieve an even wider bandwidth at the expense of a small loss of gain in the centre of the passband.

Capacitors placed across the transformer windings form resonant circuits which provide the tuning of the amplifier.

[1] The capacitors connected between the bottom of the transformer secondary windings and ground do not form part of the tuning.

[2] Tuning both sides of the transformer forms a pair of coupled resonators which is the source of the increased bandwidth.

The gain of the amplifier is a function of the coupling coefficient, k, which is related to the mutual inductance, M, and the primary and secondary winding inductances, Lp and Ls respectively, by There is a critical value of coupling at which the gain of the amplifier is a maximum at resonance.

[5] Like synchronous tuning, adding more stages of double-tuned amplifiers has the effect of reducing the bandwidth.