Confusion related to Single and Dual Supply OP-AMPS

[Johnny101]

Hi guys,

I have made a few general observations related to the usage of single supply and dual supply op-amps and wanted to know if I am moving in the right direction. Please correct me where wrong.

- If input signal is positive and has no -ve part then single and dual supply op amps are analogous?
- If signal is single ended (having both -ve and +ve part) and we have a single supply op amp - just bias the op amp at half the supply?
- If signal is differential (having both -ve and +ve part) and we have a single supply op amp what do we do now?
Could we add an offset to the input signal before giving it into the op-amp?
- If we have a dual supply op-amp it doesn't matter what type the input is?

Thanks in advance.

 

[Harald Kapp]

- If input signal is positive and has no -ve part then single and dual supply op amps are analogous?

Not necessarily, e.g. when the opamp is used in an inverting configuration. When input signal and/or ouput signal come near to 0V, you will also need an opamp that can deal with these voltages, see rail-to-rail opamp.

- If signal is single ended (having both -ve and +ve part) and we have a single supply op amp - just bias the op amp at half the supply?

This is possible where you can decouple the input signal e.g. by a capacitor such that the input signal sits atop the bias signal. this effectively converts the bipolar input signal to a unipolar input signal for the opamp. The decoupling capacitor forms a high pass filter, however. So this methos is only suitable for AC signals.

- If signal is differential (having both -ve and +ve part) and we have a single supply op amp what do we do now?

You need an opamp circuit with diffferential inputs to handle the input signal at all. Difference amplifiers or instrumentation amplifiers can do this (look up these terms). You can also bias the amplifier (or, in the cas eof an instrumentation amplifier provide the necessary offset) to shift the output signal into the unipolar output range.

Could we add an offset to the input signal before giving it into the op-amp?

There are several ways to add an offset to an opamp circuit.

If we have a dual supply op-amp it doesn't matter what type the input is?

It always matters. You'll have to take into account the voltage range, the frequency range, impedance of the source etc.

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