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A double-tuned amplifier is a tuned amplifier with transformer coupling between the amplifier stages in which the inductances of both the primary and secondary windings are tuned separately with a capacitor across each. The scheme results in a wider bandwidth and steeper skirts than a single tuned circuit would achieve. There is a critical value of transformer coupling coefficient at which the frequency response of the amplifier is maximally flat in the passband and the gain is maximum at the resonant frequency. 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. Cascading multiple stages of double-tuned amplifiers results in a reduction of the bandwidth of the overall amplifier. Two stages of double-tuned amplifier have 80% of the bandwidth of a single stage. An alternative to double tuning that avoids this loss of bandwidth is staggered tuning. Stagger-tuned amplifiers can be designed to a prescribed bandwidth that is greater than the bandwidth of any single stage. However, staggered tuning requires more stages and has lower gain than double tuning. == Typical circuit == The circuit shown consists of two stages of amplifier in common emitter topology. The bias resistors all serve their usual functions. The input of the first stage is coupled in the conventional way with a series capacitor to avoid affecting the bias. However, the collector load consists of a transformer which serves as the inter-stage coupling instead of capacitors. The windings of the transformer have inductance. Capacitors placed across the transformer windings form resonant circuits which provide the tuning of the amplifier. A further detail that may be seen in this kind of amplifier is the presence of taps on the transformer windings. These are used for the input and output connections of the transformer rather than the top of the windings. This is done for impedance matching purposes; bipolar junction transistor amplifiers (the kind shown in the circuit) have a quite high output impedance and a quite low input impedance. This problem can be avoided by using MOSFETs which have a very high input impedance.〔Bhargava ''et al.'', pp. 382–383〕 The capacitors connected between the bottom of the transformer secondary windings and ground do not form part of the tuning. Rather, their purpose is to decouple the transistor bias resistors from the AC circuit. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Double-tuned amplifier」の詳細全文を読む スポンサード リンク
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