Introduction to Typical FET Switch Circuits

1. Single-Pole Single-Throw (SPST) Switch

Structure:

A single FET (e.g., MOSFET, GaAs FET, or GaN HEMT) acts as an ON/OFF switch.

Series Configuration: FET is placed in series with the signal path (high isolation when OFF).

Shunt Configuration: FET is grounded when OFF (low insertion loss when ON).

Applications:

Basic signal gating in RF systems.

T/R (Transmit/Receive) switching in simple radios.

2. Single-Pole Double-Throw (SPDT) Switch

Structure:

Uses two FETs (one series + one shunt or two series FETs).

When one path is ON (low impedance), the other is OFF (high impedance).

Working Principle:

Control voltage (V_ctrl) toggles between FETs to route signals to Port 1 or Port 2.

Isolation resistors may be added to improve port-to-port isolation.

Applications:

Antenna switching in mobile devices.

RF front-end multiplexing in wireless systems.

3. Series-Shunt FET Switch (Reflective-Type)

Structure:

Combines series FET (for signal blocking) and shunt FET (for grounding reflections).

Provides high isolation in OFF state.

Advantages:

Broadband performance (DC to mmWave).

Improved power handling due to distributed RF voltage division.

Applications:

High-frequency radar systems.

Test equipment (e.g., signal generators, VNAs).

4. Absorptive (Terminated) Switch

Structure:

Uses matched termination resistors (50Ω) at unused ports.

Unlike reflective switches, it absorbs (rather than reflects) unused signals.

Advantages:

Minimizes VSWR (Voltage Standing Wave Ratio) issues.

Reduces intermodulation distortion in multi-tone systems.

Applications:

Precision test & measurement instruments.

Multi-antenna beamforming systems.

5. Multi-Throw Switches (SP4T, SP8T, etc.)

Structure:

Extends SPDT concept with multiple FET branches.

Uses binary or decoder logic for path selection.

Applications:

Frequency-agile radios.

Satellite communication routing.

Key Design Considerations

Insertion Loss: Minimized by using low-R_on FETs.

Isolation: Improved with cascaded FETs or λ/4 resonators.

Power Handling: Enhanced via stacked FETs (GaN preferred for high power).

Linearity: Critical for avoiding intermodulation in broadband systems.