FET RF Mixer

FETs provide an excellent basis for RF mixers - they can be used as active or passive mixers & in a variety of ways.

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FETs are able to provide an excellent base component for the design and construction of RF mixers.

Not only can single gate FETs be used as RF mixers, but dual gate FET mixers are also widely used and can offer a simple circuit whilst providing good performance.

Dependent upon the type of FET mixer used, these circuits 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.

FET RF mixer types

There are two broad categories for FET RF mixers, the distinction being made according to the way the mixer operates.:

Passive FET mixer: In this form of RF mixer, the FET is used purely as a switch and it operates in a similar manner to that of a diode mixer. In this mode the mixer would not provide any gain.
Active FET mixer: In this form of FET mixer, the circuit is operated as a transconductance mixer. In this mode a FET RF mixer can provide some gain.

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.

Basic passive / switching FET RF mixer concept

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 RF Mixer

Active FET RF 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 RF 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.

Basic active transconductance FET RF mixer — Basic transconductance FET mixer

The diagram above shows the simplest form of a FET based transconductance mixer. The filter on the output removes the LO and RF input signals. This ensures 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.

Basic active transconductance dual gate FET RF mixer — Dual gate FET mixer

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 and this results in higher levels of intermodulation and other spurious signals.

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

The dual gat MOSFET mixer circuit shown above is typical of the format that is widely used in many domestic and professional equipment. It is possible for very low noise FETs to be used to enable very high levels of performance to be achieved.

Field effect transistors are able to provide excellent performance in many mixer circuits. Used in active or passive modes they can give a good account of themselves, often being used in circuits where high levels of performance can be achieved. Although in the circuits above they have not been shown to be used in double balanced mixer circuits where the local oscillator and RF input are balanced out, they can be used in this mode when the FETs are used as switches in what is termed their passive mode of RF mixer operation.