Opamp with transistor

[SneaKSz]

Hello all,

I'm not getting the principle of the following schematic :

**broken link removed**

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This a voltage follower , the voltage on the not inverting input will be the same as the voltage on the inverting input.

if V inv = 0V , V not inv will be ~ 5V and will be higher than Vinv => Vout = max and V not inv ~0.2V. If V inv = 1V , Vout will be low and V not inv will be high. e Now the output of the opamp will be high and V not inv will be low again.

My question is , when I simulate this it doesnt oscillate , but when I try to undestand it it does!

Someone can help me?

Kind regards

 

[MikeMl]

It is a voltage follower with a transistor booster. Since the transistor is inverting, the normal feedback inputs to the opamp have to be swapped. It is only conditionally stable (or not) because the loop gain of combination is higher than the originally internally-compensated opamp.

The simplistic circuit you posted does oscillate in simulation. Look at the first circuit below. Once perturbed, it never stabilizes. The degree of oscillation depends on the Gain-Band-width Product of the Opamp and the gain/band-width of the transistor. It is worse with slower opamps.

A practical circuit requires some additional compensation, as shown in the second sim.

 

[SneaKSz]

Thanks for the info mike