0
\$\begingroup\$

I do not understand why do we need to perform small signal analysis in the first stage of an op-amp, that is the differential stage (2 mosfets with sources in common and a current generator under them). For example, if I consider the classic non inverting configuration of an op-amp with a constant generator (For esample, 5V), I don't see a sinusoidal signal between the 2 input terminals of the op-amp (the voltage between them is approximately 0)

\$\endgroup\$
  • \$\begingroup\$ What if the gain is (say) 3 instead of 1,000,000,000? \$\endgroup\$ – Andy aka May 20 '18 at 10:01
2
\$\begingroup\$

For example, if I consider the classic non inverting configuration of an op-amp with a constant generator (For esample, 5V) I don't see a sinusoidal signal between the 2 input terminals of the op-amp (the voltage between them is approximately 0)

You don't, if the amplifier is stable. If it's unstable, you'd certainly see something oscillating. If for instance you added a large capacitive load to the output, or a large capacitive stray to the inverting input, then that could send the amplifier unstable, even with a constant input.

How can you tell whether the amplifier will be stable or not? You do a small signal analysis of the amplifier, together with its load, external components and stray components.

Most of the time, we don't need to bother with this, as most op-amps are designed to be unity-gain stable. This means slow ones can usually be flung into a circuit in any configuration with impunity. However, use a fast one, or use one into a difficult load, and you have to know what you're doing, or follow advice in the application note, to get them to behave.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.