varicap

The varicap or varactor diode is used in many appliances.

The second varicap effectively replaces the blocking capacitor in the first circuit.

Experimentation with the varicap effect need not remain in the realms of advanced electronics or solid state physics labs.

The tank circuit (LC), connected to the collector via a capacitor, contains a pair of varicap diodes.

Because a dc voltage must be applied reverse bias across the varicap to alter its capacitance, this must be blocked from entering the tuned circuit.

In these applications, SAW filters are almost always used with a phase locked loop synthesized local oscillator, or a varicap driven oscillator.

A second circuit using two back-to-back (cathode to cathode), series-connected varicap diodes (shown lower-left in the image) is another common configuration.

Indirect FM employs a varicap diode to impose a phase shift (which is voltage-controlled) in a tuned circuit that is fed with a plain carrier.

Generally the use of a varicap diode in a circuit requires connecting it to a tuned circuit, usually in parallel with any existing capacitance or inductance.

Many of the diodes in this family exhibit a change in capacitance with reverse bias and can thus be used by experimenters as makeshift varicap diodes.

The varicap was developed by the Pacific Semiconductor subsidiary of the Ramo Wooldridge Corporation who received a patent for the device in June 1961.

If the capacitor is a varicap diode, then the 'moving the plates' can be done simply by applying time-varying DC voltage to the varicap diode.

Special types of varicap diode exhibiting an abrupt change in capacitance can often be found in consumer equipment such as television tuners, which are used to switch radio frequency signal paths.

This happens because when the capacitance of a charged capacitor is reduced, the voltage across it is increased which, in turn further reduces the capacitance if it is a varicap.

Before the development of the varicap, motor driven variable capacitors or saturable-core reactors were used as electrically controllable reactances in the VCOs and filters of equipment like World War II German spectrum analyzers.

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode whose capacitance varies as a function of the voltage applied across its terminals.

In some applications, such as harmonic multiplication, a large signal amplitude ac voltage is applied across a varicap to deliberately vary the capacitance at signal rate and generate higher harmonics which are filtered off and used further down the signal chain.

Since no significant dc current flows in the varicap, the value of the resistor connecting its cathode back to the dc control voltage can be somewhere in the range of 22KΩ to 150KΩ and the blocking capacitor somewhere in the range of 5-100nF.

In the consumer AM/FM tuner depicted at the right, a single dual-package varicap diode adjusts both the passband of the tank circuit (the main station selector), and the local oscillator with a single varicap for each.

When designing tuning circuits with varicaps it is usually good practice to maintain the ac component of voltage across the varicap at a minimal level, usually less than 100mV peak to peak, to prevent this changing the capacitance of the diode too much and thus distorting the signal and adding harmonics to it.