In my schematics GaAs SPDT switch is used to feed two ICs in turn from single signal source. Switching between two signal destinations is made by applying opposite voltages to two SPDT control pins - A and B. According to datasheet voltages on control pins are always opposite - and it is obviously from typical reflective-short GaAs SPDT schematics (picture below).

But sometimes I need to feed two destinations of signal simultaneously. Could I use reflictive-open GaAs SPDT and apply equal ON-voltage to control pins (V1 and V2 at picture) to split input signal between outputs? In reflective-open SPDT Q3 and Q4 don't exist, so what could possibly go wrong?

If such using of SPDT is forbidden, how to solve this problem?

GaAs SPDT Schematics


1 Answer 1


Could I use reflictive-open GaAs SPDT and apply equal ON-voltage to control pins (V1 and V2 at picture)

No you can't, look in the schematic at how the transistors are connected, if you make the pass transistors (Q1 and Q2) conductive (which is what you want) you will at the same time also make Q3 and Q4 conductive which will short the signal to ground.

You can solve this by using 2 SPDT switches from which you only use on switch. You then ground the other contact of the switch.

But a GaAs SPDT is generally only used in RF applications and in RF applications you don't just split a signal like that because you will not have a 50 ohms (or whatever value) characteristic impedance anymore.

So if this is for RF, you must use a proper splitter. If you don't your signal will be deformed/distorted/reflected/etc. Real RF engineers will point and laugh at you so don't ! Use a proper splitter !

  • \$\begingroup\$ In reflective-open SPDT transistors Q3 and Q4 don't exist, so signal will not be shorted to ground. But you are right about impedance distortion - it is a huge problem and I don't see how to resolve this problem using only one SPDT. Mu application is RF and actually is more complex then I described. There are two RF signal sources and two destinations. I see perfect solution in using DPDT diversity switch, but I cant find DPDT for my frequency range, so I gonna make own DPDT from four SPDT switches. \$\endgroup\$
    – user125603
    Oct 6, 2016 at 7:48

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