EEVblog Electronics Community Forum

Electronics => Projects, Designs, and Technical Stuff => Topic started by: petersanch on June 15, 2021, 10:10:26 am

Title: Analog Switch to Control Low Voltage AC?
Post by: petersanch on June 15, 2021, 10:10:26 am
I have 2 audio amplifiers with output voltages between +10V and -10V. I want to make a digitally controlled switch to control which amplifier is driving the speaker.

The clear choice is a relay for this but I want to do this using FETs if possible to take up little space on the pcb and be inexpensive. I also want the amplifier output to remain as unattenuated as possible.
I found this circuit that uses a p channel and n channel to make an analog switch
(https://www.eevblog.com/forum/projects/analog-switch-to-control-low-voltage-ac/?action=dlattach;attach=1227869;image)
I can see this working for positive voltages, but will this be able to control the negative voltages from the amplifiers? Is there a better option for this?



Cheers!
Title: Re: Analog Switch to Control Low Voltage AC?
Post by: Kleinstein on June 15, 2021, 12:18:38 pm
The CMOS switches are usually relatively high in resistance (e.g. 50 Ohms range) this is not really practical for a low impedance (e.g. 4 or 8 Ohms) load.
The more practical way is to to have only 1 amplifier and do the switching at the amplifiers input side. Than CMOS switches are a very pratical solution.

If really lower resistance FET swtiching is needed, it would be more the Photo-mos / electronic relais with MOSFET output. Some are relatively low impedance, maybe around 1 Ohms. If needed one can use 2 MOSFETs and a photovoltaic optocoupler to drive the gates. It may not be smaller of lower cost than a relais.
Title: Re: Analog Switch to Control Low Voltage AC?
Post by: David Hess on June 15, 2021, 05:21:23 pm
Almost all discrete power MOSFETs include the body diode between their drain and source, so they would need to be used in pairs for a total of 4 power MOSFETs per switch.

Solid state relays which use a pair of power MOSFETs in series to isolate their body diodes and have floating drive could work, and you can make one using a photovoltaic optocoupler and a pair of power MOSFETs.  4 MOSFETs are not required because the gate drive signal is isolated.  This method also does not require supply voltages greater than the voltages to be switched.