differential voltage feedback

[zizi110]

Hi guys,
I have a full-bridge dc-ac converter that VDC= 24v. for having feedback voltage of inverter output ; my supervisor said that i can use this circuit ( below) but when i simulate it by Pspice ,the simulation result is not good .can anyone help please?

 

[FvM]

According to the circuit topology, all amplifiers need bipolar power supplies, e.g. +/- 15V.

 

[zizi110]

thanks FvM,
you are right , by doing so , the waveforms becomes as expected
but is there any way that i can use it in single supply mode?

 

[chuckey]

You establish a false "earth", by putting a divider across your 24V line, to give +12V with reference to the 24V negative terminal. This line must have a low impedance (big decoupling capacitor). Then your old +Vcc lines become +12V wrt the "earth" and the old DC 0V connections become -12V wrt the "earth". This does put an offset of +12V on all your signal lines which have to be sorted out.
Frank

 

[zizi110]

Hi chuckey ,
sorry i can't understand , did you mean i must connect vdc in this manner:

 

[FvM]

You can use a simplified circuit with single supply.

 

[zizi110]

thanks FvM,
i simulate it and it is ok, but why?
why the first stage is not necessary?

 

[FvM]

The first stage doesn't serve a meaningful purpose in your design. Would be different with non-inverted buffers or a differential stage as used in the classical three OP instrumentation amplifier, usefull if you want high input impedance.

I forgot to mention that + and - inputs have to be switched for the single OP circuit.

The original circuit lacks of a suitable bias. It could be fixed in a different way, but you need to connect the non-inverting OP inputs to a positive voltage instead of ground. But as said, this would be purposeless effort.