Varactor Diode Circuits

The varactor diode can be used in a variety of circuits from oscillators to filters. Knowing how to apply the control voltage is a key issue.

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There are many aspects to using varactor diodes in RF electronic circuits. The configuration to the varactor circuits can affect their operation – some small variations can lead to major differences in performance.

In view of the fact that RF circuits are not always easy to optimise, it is necessary to ensure the varactor circuits utilise the best methods of driving varactor diodes as well as the most successful basic circuits.

Driving varactor diodes

The varactor diode requires the reverse bias to be applied across the diode in a way that does not affect the operation of the tuned circuit of which it is part. Care must be taken to isolate the bias voltage from the tuning circuit so that the RF performance is not impaired.

The typical voltage controlled oscillator circuit shown below shows the way in which the varactor diode is often included within the circuit.

Typically the cathode is earthed or run at the DC common potential. The other end can then have the bias potential applied. The bias circuitry needs to be isolated for RF signals from the tuned circuit to prevent any degradation of the performance.

In this circuit, two diodes are used. One is taken directly to earth, but the second one is taken to the DC earth voltage with the DC current path through the inductor. Having two diodes used in this manner enables the variations caused by the RF oscillation itself to be better balanced out. The circuit overcomes the problem of the RF modulating the tuning voltage. The effect is cancelled out - as the RF voltage rises, the capacitance on one diode will increase and the other decrease. The back-to-back configuration also halves the capacitance of the single diode as the capacitances from the two diodes are placed in series with each other. It should also be remembered that the series resistance will be doubled and this will affect the Q.

It is possible to replace D1 with a single capacitor should this be required. If this is not done, then the control voltage would have a DC path to earth through the inductor and the circuit would not work.

The control voltage needs to be applied to the junction of the two diodes. Either a resistor or an inductor can be used for this as the diodes operate under reverse bias and present a high DC resistance.

Inductors can operate well under some situations as they provide a low resistance path for the bias. However they can introduce spurious inductance and under some circumstances they may cause spurious oscillations to occur when used in an oscillator. Resistors may also be used. The resistance must be high enough to isolate the bias circuitry from the tuned circuit without lowering the Q. They must also be low enough to control the bias on the diode against the effects of the RF passing through the diode. A value of 10 kΩ is often a good starting point.

When designing a circuit using varactor diodes, care must be taken to ensure that the diodes do not become forward biased. Sometimes, especially when using low levels of reverse bias, the signal in the RF section of the circuit may be sufficient over some sections of the cycle to overcome the bias and drive the diode into forward conduction. This leads to the generation of spurious signals and other nasty unwanted effects.