can I use any OP AMP as buffer

Question

I am still studing OP-AMPS and I was playing around with voltage follower/buffer configuration, the very basic diagram connecting the inverting input to the output directly and put some voltage on the non-inverting input

I supplied the op-amp with 11.1 volts

what I have notice that it was working as expected with TL092 and LM741, the output voltage was following the input voltage but when I tried LM393 or LM339 it didn't work as expected actually the output voltage totally became irrelevant for example if I apply 5 volts on the input, the output measures .9 volts or something around that value

if I apply vcc on the input I get just 1.308 volts

can someone explain why is this happening?

Answer 1

As TEMLIB pointed out, a comparator is not an op-amp, even if it happens to be drawn as a triangle with with 2 inputs and one output.

A typical design for a comparator is an open drain comparator, which can be modeled like this:

^{simulate this circuit – Schematic created using CircuitLab}

Here, OA1 is an ideal op-amp. If you connect Vout to V-, you won't get a voltage follower.

Additionally, even if you stick to the world of "true op-amps", there are some restrictions on what op-amps you can use and how you use it. For example, some op-amps are not unity gain stable. Since a voltage follower has by definition a gain of one, trying to build a voltage follower with one of these will always be unstable, and you won't get the desired voltage follower behavior.

You could start with an advertised unity gain stable op-amp and drive it unstable by what you connect the output to and what input waveform you give to the op-amp. It is also possible to do the reverse and add components to stabilize an unstable configuration. There's a wide variety of literature available online on op amp stability.

Answer 2

No - you cannot use any opamp as a buffer. This is because some opamps are not frequency-compensated for closed-loop operation down to unity-gain. For those opamps the data sheet says: "Stable only for gain values larger than...".

The frequency response of these opamps has a second pole within the active region (open-loop gain Aol>0 dB) and the slope of the magnitude function at the cross-over frequency (for Aol=0dB) is considerably larger than 20dB/dec.

This property does not allow stable unity-gain operation.