Varactor diode basics

This page describes varactor diode basics and varactor diode applications.The link to varactor diode calculator,PIN diode,Tunnel diode and GUNN diode basics and applications are also mentioned.

Varactor diode is one of the many microwave semiconductor devices in use today.

They are manufactured with gallium arsenide. Figure depicts symbol of varactor diode and also typical manufacturing package. This diode is used as variable capacitor and as variable reactor in microwave circuits.

varactor diode

Varactor diode is special type of PN junction diode, in which PN junction capacitance is controlled using reverse bias voltage. When the diode is forward biased, current will flow through the diode. When the diode is reverse biased, charges in the P and N semiconductors are drawn away from the PN junction interface and hence forms the high resistance depletion zone. The equation of the varactor capacitance proportional to the reverse bias voltage is outlined below.

Cj = CK/(Vb - V)m
Cj is the diode capacitance
C is the diode capacitance when the device is unbiased i.e. when V is zero.
V is the applied reverse voltage
Vb is barrier voltage at junction
m is the constant and depends of the material
K is also constant often taken as value of 1

The equivalent circuit of the varactor diode is mentioned in the figure along with the symbol. From the circuit maximum operating frequency of the varactor diode depends on the series resistance and diode capacitance and it is mentioned in the equation below.
F = 1 / 2*pi*Rs*Cj

Quality factor of the varactor diode is mentioned in the equation below.
Q = F/f , where F is the cutoff frequency and f is the operating frequency.

Varactor Diode Applications

Following are the varactor diode appplications:
•   It is used in variable resonant tank LC circuit. Here C part is varied using varactor diode.
•   AFC(Automatic Frequency Control) where in varactor diode is used to set LO signal.
•   Varactor is used as frequency modulator.
•  It is used as frequency multiplier in microwave receiver LO.
•   It is used as RF phase shifter.

Application Note-Varactor diode used as Frequency multiplier

Refer application note which explains varactor diode used as frequency multiplier and as tuner.

Varactor diode calculator

Refer varactor diode calculator page. This calculator calculates varactor diode capacitance, cut off frequency and quality factor.

Microwave Diode related links

Types and basics of microstrip line
Tunnel diode basics and applications
PIN diode basics and applications
GUNN diode basics and applications
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rf measurements tutorial
waveguide basics tutorial

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