FET mixer

- overview of the essentials of the FET mixer in both passive and active roles with circuits.

FET mixers are often used in radio frequency, RF applications. FET mixers can be provide advantages over other forms of mixer and as such they are used in a number of different radio frequency applications.

Dependent upon the type of FET mixer used, they can offer gain, improved noise figure performance and lower spurious signal levels.

While other types of mixer, especially diode mixers can be bought as component blocks, FET mixers are rarely obtainable in this form and are generally built within the overall circuit being designed.

Types of FET mixer

FET mixers can be broadly categories into two main areas according to the way in which they operate:

• Passive FET mixer:   In this form of mixer, the FET is used as a switch and it operates in a similar manner to that of a diode mixer.
• Active FET mixer:   In this form of FET mixer, the circuit is operated as a transconductance mixer.

Passive or switching FET mixer

When used in the passive mode, a FET mixer effectively acts a switch. In this role the drain-source resistance behaves as a voltage variable resistor. The resistance of the channel is set by the gate-source voltage.

When used as a switch, the FET is biased with the drain and source at zero volts DC. The gate is then biased between 0v and pinch-off. Set at this mid-point, it allows the local oscillator to act as a switching signal, switching the FET mixer element on and off.

The choice of gate resistor value is important. At low frequencies FETs have a high impedance as the small level of capacitance has comparatively little effect and the very high DC resistance of the gate dominates. At higher frequencies the gate capacitance has an increasing effect. Typically a gate resistance value of 200 - 300 ohms may be used.

Active FET Mixer

Active FET mixers are what are known as transconductance mixers. In this type of mixer the local oscillator, LO signal is used to vary the transconductance of the FET. These FET mixers have the advantage that they can provide gain and they have lower noise figure levels when compared to passive designs.

There are a number of circuit topologies that can be used for FET mixers. The simplest FET mixer circuit is to apply both signals to the gate of the device. However in this realisation of a FET mixer, some method of providing a diplexer at the input is required to isolate the LO and RF sources from each other.

The diagram above shows the simplest form of a FET based transconductance mixer. The capacitor on the output providing a short circuit to the LO and RF input signals is provided to ensure that the value for the drain source voltage Vds is not moved significantly from its DC bias point as a result of the local oscillator signal. This maximises the conversion gain. In practice a parallel tuned circuit tuned to the output frequency will give an effective short circuit

A superior circuit can be realised by using two FETs with their channels in series. The two gates can then be used, one for the local oscillator signal and the other for the RF input. This provides isolation between the two input signals.

The ideal way of realising this circuit is to use a dual gate FET such as a dual gate MOSFET.

The advantage of active FET mixers is that they provide a conversion gain rather than a conversion loss. The disadvantage is that their linearity is not as high as that of a passive or switching mixer circuit.

A practical implementation of a FET mixer circuit as might be used in a radio receiver or other application is given below:

Dual gate MOSFET mixer

Field effect transistors are widely used within RF mixer circuits. FET mixers are able to provide excellent performance, although the correct form of FET mixer or any RF mixer should be chosen for any given application.

By Ian Poole

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