How RF Switch Works | RF Switch Equivalent Circuits PIN diode,FET

This page describes RF Switch working operation. It mentions how RF switch works along with its equivalent circuits. The PIN diode SPDT RF switch and FET based RF switch use cases or examples are explained.

PIN diode based SPDT RF Switch

PIN diode SPDT switch

• PIN diode is semiconductor device which functions as variable resistor at RF & microwave frequencies.
• The PIN diode resistance is less than 1 Ohm in ON State condition. It is more than 10 K-Ohm in OFF state condition. It depends on amount of current flow through the PIN diode. It is a current controlled device. Hence the resistance depends on forward biased DC current only.
• PIN diode can be used for switching based on ON/OFF of the control current through it.
• PIN diode can control large amplitude RF signals with the help of much lower level of DC excitation.
• PIN diode resistance in forward bias condition is inversely proportional to total forward bias current. This makes PIN diode perfect to achieve excellent isolation at higher frequencies.
• The figure-1 depicts PIN diode based SPDT RF Switch.

FET based RF Switch

FET SPDT switch

• To switch RF from common port to port-2, Q1/Q4 are reversed biased. Due to this, the channel between the drain (D) and source (S) terminals are pinched off. At the same time, Q2/Q3 are forward biased such that low channel resistance exists between source (S) and drain (D) terminals.
• The FET transistors Q1 & Q3 act as series devices to switch RF to ON and OFF condition, For better isolation FET transistors Q3 & Q4 are used to shunt RF to ground which leaks to OFF port of the switch.
• More FETs are added in the series on each of the arm of RF Switch, in order to increase isolation and linearity.
• The figure-2 depicts FET based SPDT RF Switch.


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