level shifting stage for op-amp

It's hardly specific to an op-amp.

Graham

Because a level needs to be shifted? Seriously.

Thomas

I think it is because the output voltage swing from the front end stage needs to be near one of the supply rails, to maximize the common mode input voltage range (the output signal from this stage can not overlap the input voltage). But the output stage is normally a follower that needs a very large swing capability to drive it. So the level shifter converts the small swing near one rail to a nearly rail-to-rail swing to drive the output stage.

You may be able to tell that I don't design opamps.

The level shifter in OP-AMPs is specific to each OP-AMP. It's possible to design an OP-AMP that would not require one, although I have not designed enough of them to know how frequent they are.

Anyways, what happens is that at one point, the DC voltage of the OP-AMP may be high due to the math behind all of the stages hooked together (pretend it is 2.5V with Vdd=3V.) In this case, the input may be an AC signal centered at 1.5 volts, swinging in both directions by about 1.5V. But if towards the output of the amp the DC level moves from 1.5 v to 2.5V, then Vout high will be slamming into the Vdd rail, while Vout low will never go below 1V. To fix this problem - when it occurs - a level shifter is used in OP-AMPs.

I hope this conceptually makes sense. It's lacking detail, but the idea is there.

John P> > why is a level shifting stage required in an op-amp? cheers.

Level shift is often loosely defined. I define a level shifter as a circuit that has no gain, i.e. just a DC shift. So is the level shift to which you refer the VBE multiplier in the output stage? That is, to bias a push-pull buffer?

Visualize a simple CMOS opamp. Input is a source coupled pair feeding a current mirror. The second gain stage is a common source amplifer loaded by a current source. I don't consider any level shifting in such a circuit.

Now there are designs where the output stage current source would be replace by another common source amplifier. In this case, the AC signal at the gate of the common source amplifier driven directly from the current mirror (i.e. first gain stage output) needs to be level shifted to an AC signal at the opposite power supply rail to drive the other common source amplifier.