PIN diode RF switch circuit

- summary, overview and basic design considerations for a PIN diode RF switch circuit using a single PIN diode

This circuit shows a very simple but effective design for an RF switch using a single PIN diode. The RF switch circuit is simple to design and can be used in applications where the ultimate performance is not required.

The RF switch circuit shown only requires a few components and is very simple to implement. Nevertheless it is able to act as an RF switch for radio frequency or RF applications and is adequate for many applications.

RF switch circuit description

The RF switch circuit comprises a single PIN diode, an RF inductor or choke, a current limiting resistor and a DC block capacitor.

In operation, when a positive potential is applied to the control point current, this forward biases the diode and as a result the radio frequency signal is able to pass through the circuit. When a negative bias is applied to the circuit, the diode become reverse biased and is effectively switched off. Under these conditions the depletion layer in the diode becomes wide and does not allow signal to pass.

Simple PIN diode attenuator and switch

Simple PIN diode attenuator and switch

Although in theory any diode could be used in this position, PIN diodes have a number of advantages as switches. In the first place they are more linear than ordinary PN junction diodes. This means that in their action as a radio frequency switch they do not create as many spurious products. Secondly when reverse biased and switched off, the depletion layer is wider than with an ordinary diode and this provides for greater isolation when switching.

By varying the amount of bias on the PIN diode it is possible to vary the level of attenuation provided. In this way the circuit can be used as a very simple RF attenuator. Although the circuit can be used as an RF attenuator, more effective RF attenuator circuits are available for more demanding applications.

Circuit precautions

When using the RF switch, circuit layout is obviously important. Care should be taken to keep the input and output as separate as possible and then signal cannot leak around the circuit. Also leads should be kept as short as possible. In addition tot his the supply should be well regulated and decoupled to prevent the RF signal from being modulated by any hum or other products on the line.

By Ian Poole

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