BJT Transistor Mixer / Multiplier

- overview of the essential details of the BJT, bipolar junction transistor mixer or multiplier showing configurations and circuits.

Simple bipolar transistor mixers, BJT mixers tend to be found in low cost receivers, such as portable radios that use discrete components, although BJT mixers are found in many RF receiver ICs as well. Here these BJT mixers provide the required performance, giving conversion gain, although the linearity is poor. However for the applications where basic BJT mixer may be used the performance is normally adequate and the low cost is a major requirement.

Basic BJT transistor mixers

It is possible utilise a variety of different circuit configurations for a transistor mixer. Possibly the most obvious method is to apply both signals to the base of the transistor.

Basic transistor mixer circuit

Basic bipolar transistor mixer circuit

In this circuit the capacitor on the output is required to remove any of the high frequency LO and RF signals. Typically this circuit will be used to convert signals from a high frequency down to a much lower frequency. Ideally the capacitor on the output should appear as a short circuit to provide the optimum performance. It may be realised as a tuned circuit, tuned to the require output frequency - it will then appear as a short circuit to the unwanted LO and RF signals.

Additionally the tuned circuits on the input prevent the LO and RF signals coupling in to the opposite sources.

A more common circuit for a transistor mixer applies the local oscillator to the base and the RF input to the emitter of the transistor.

Basic bipolar transistor mixer circuit with signal applied to different transistor electrodes

Basic bipolar transistor mixer circuit
- signal applied to different transistor electrodes

It is this transistor mixer circuit that forms the basis of many circuits within transistor radios using discrete transistors. Often a self oscillating mixer is used, where a single transistor circuit based around this configuration acts as an oscillator and mixer.

Self oscillating bipolar transistor mixer circuit

Self oscillating bipolar transistor mixer circuit
as used in a typical discrete component transistor radio

Other transistor mixer configurations

There are many other transistor mixer configurations. One type of transistor mixer that is able to offer very high levels of performance is the Gilbert cell. This can be made using either bipolar or field effect transistors. However it is widely used within RF integrated circuits where the individual components will be closely matched because of their physical proximity on the IC chip providing the same fabrications conditions. This form of transistor moxer is detailed on a further page of this RF mixer tutorial.

By Ian Poole

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