PIN diode AM Modulator-PIN diode Amplitude Modulator circuit
This PIN diode application note covers PIN diode AM modulator.This PIN diode based modulator circuit is used for amplitude modulation.
PIN diode has many applications. Refer PIN diode basics and applications and PIN diode equivalent circuit and RF Switch application note for more information.
PIN diode is also used in high frequency amplitude modulator circuits at UHF, VHF and microwave frequencies. As mentioned PIN diodes are special silicon junction diode designed to operate above 100MHz.
When PIN diode is forward biased, is acts as variable resistor. Resistance depends on magnitude of current flowing through diode. A high current results into low resistance and low current results into high resistance.
This property is utilized in AM modulator circuit. In this circuit, modulating signal is used to vary forward bias current through the PIN diode. This results into generation of amplitude modulated (AM) signal.
As shown in the figure, positive going modulating signal reduces bias on the PIN diodes. This will cause the resistance to increase. This results into reduction in amplitude of carrier across load.
A negative going modulating signal will add up with forward bias. This will cause resistance of diodes to decrease. This results into increase in amplitude of carrier across load.
Variation of above circuit is also used as AM modulator in which diodes are arranged in the form of PI network. It consists of 3 diodes unlike two diodes mentioned in the figure above.
PIN diode circuit reduces the amplitude as it acts as variable attenuator. In order to compensate power loss, amplifier devices are used to bring the AM signal to sufficient power level.
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