I had an idea of a project to build which translate these respective voltages -3 0 and +3 volts into whatever the two variable power supply are delivering. See in the figure below the architecture I am building:

system architecture

First approach: My first approach was to use a push-pull circuit but it turned out a push pull acts just as a follower base input (but highly amplifies the current which is not what I was looking for)

Current state: I am looking on internet if there is something that might help but I haven't found anything for the moment. I want to build a transistor-based solution (PNP and NPN), since operational amplifiers have a minimum rail to rail supply voltage to be applied to work.

Even if I use it and build a zero comparator circuit applying the +VDAC 2 and -VDAC 1, I will have to respect the minimum rail-to-rail voltage of the op-amp to work and so I want be able to deliver on the output values below the min value to power the op-amp (e.g. 2V) when varying the power supply voltages.

I am also quite concerned about the output impedance of an operational amplifier. I saw most of them have an output impedance going from 50 to 200. I haven't defined yet with which loads I will be testing this system so you can give an approach for any output impedance values.

  • 1
    \$\begingroup\$ "... which translate these respective voltages -3 0 and +3 volts into whatever the two variable power supply are delivering." Sorry, but that's not clear at all. The block diagram isn't either. 1. Are there only three possible input states, -3, 0 and +3 V? 2. Do you want that VDAC1 = -2 V and VDAC2 = +2 V should result in -2, 0 and +2 V? 3. What happens if the VDACs are asymmetrical? 4. What are the maximum output voltages required? 5. What is the maximum current output required? 6. Where is the output power coming from? Good designs require good specifications! \$\endgroup\$
    – Transistor
    Commented Feb 17 at 16:52
  • \$\begingroup\$ use two relays to switch the two power supply voltages, one controlled by the +3 V and the other by the -3 V \$\endgroup\$
    – jsotola
    Commented Feb 17 at 18:10
  • 2
    \$\begingroup\$ When you say -3V;0V;+3V, what outputs would you expect for inputs which are, say, +2V, or -1.5V? \$\endgroup\$ Commented Feb 17 at 18:41
  • \$\begingroup\$ AOP is not a common English abbreviation, presumably, of operational amplifier, although it makes perfect sense in French, I admit :) Just say op-amp instead of AOP. I've edited the question to fix that. \$\endgroup\$ Commented Feb 17 at 19:37
  • \$\begingroup\$ I would expect both negative and positive voltages on input. The positive voltage would lead to +VDAC2 in the output whereas the negative voltage put on input would lead the output to -VDAC1 and the output should remain at zero when the input is put on zero volts. \$\endgroup\$
    – Nedou
    Commented Feb 17 at 20:19

1 Answer 1


You said you're translating three discrete voltages, not a voltage range. So all you need is a mux and some comparators:


simulate this circuit – Schematic created using CircuitLab

For a linear mapping of a voltage range, you'd be talking about a two-slope map. 0V to 3V will be mapped with the slope of VDAC1/3V. -3V to 0V will be mapped with the slope of VDAC2/3V. With multiplying DACs you can feed the 0..3V range to a DAC and apply constant gain, same with the 0..-3V range, then switch between them based on a single comparator output.

If the DACs only provide a voltage output, you'll need one or two voltage multipliers, and for that the most linear way is to use a PWM modulator. Convert 0..3V into a 0..100% PWM1 signal, feed it to a switch that alternates between 0V and VDAC1. Convert 0..-3V into a 0..100% PWM2 signal, feed it to a switch that alternates between 0V and VDAC2. When input is >0V, 1st switch output goes to the output filter, otherwise the 2nd switch output goes to the output filter.


simulate this circuit

An alternative implementation can use just one PWM block:


simulate this circuit

Avoiding two muxes in series for analog signals may improve performance:


simulate this circuit

The bipolar power supply to the entire circuit must be encompassing the maximum range of VDAC1 and VDAC2. Let's say VDAC1 and VDAC2 can be anywhere between +10V and -10V. Then the supply voltages will need to be say +/-12V for the muxes and the output amplifier.

  • \$\begingroup\$ Maybe you meant analog switches instead of mux ? Because muxes generate ttl outputs however as you can see there are two power supplies with different polarities +VCC and -VEE \$\endgroup\$
    – Nedou
    Commented Feb 17 at 20:23
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    \$\begingroup\$ @Nedou Kuba is referring to analog muxes, not digital ones. \$\endgroup\$
    – Hearth
    Commented Feb 20 at 15:07

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