A simple RF switch using bipolar transistors

A simple RF switch using bipolar transistors

I am currently working on a project that requires me to switch signals in the range of 1-30MHz. I could probably get away with using regular relays, but they’d be rather noisy as I have about 10 signals that need to be switched at the same time. Reed relays would be more quiet, but they can be rather expensive and often have only one contact. I also wanted to avoid any special ICs such as off-the-shelf RF switches or PIN-diodes since these can be rather hard to get. During my research, I came across this article on gusbertianalog.com where the author looked into using regular BJTs as RF switches.

I did some measurements using my favorite small signal transistor, the BC547C. The original circuit can be seen below.

SPST RF switch

What I found out is that R3 can be omitted, which will just increase the attenuation by around 0.3 dB.
The base current should be adjusted for the desired load current, similar how you would design a PIN-diode based switch. With R3 removed, this circuit works up to ~40mA load current, after that the signal will be distorted. It should be noted that the base resistor should NOT be connected to ground when the switch is desired to turn off but rather left open. Otherwise, base current will flow if the RF source voltage is high enough.

A SPDT switch is also possible by adding another transistor.

SPDT RF switch

You could also add more transistors like this to create an RF (de)multiplexer.

Stay tuned to find out about the project I need this circuit for!

Leave a Reply

Your email address will not be published. Required fields are marked *