Mixer circuits are used to change from one frequency to another and these rely on the non-linear property of a circuit. Mixers produce a range of output frequencies and the required output is chosen by means of a tuned circuit or filter. The circuit in Fig 3.31 is the typical application of a junction FET as a mixer.
Fig 3.31. Typical junction FET mixer circuit
The local oscillator is fed in on the source while the RF signal is fed in, often by a tuned circuit, to the gate. These signals mix and the wanted mixing product is picked out by the tuned circuit on the drain. The advantage of using a transistor or FET as a mixer is that it can provide gain at the same time.
The dual-gate MOSFET is also now very common as a mixer and a typical circuit is given in Chapter 5 (Fig 5.4).
