Got asked this question in an interview, didn't have a good answer.

[vk6kro]

The unusual thing about the 741 was that it had a capacitor in it (across R7 and R8).

Before this chip, opamps had to have an external compensating capacitor and the quality of this capacitor (or lack of quality) affected the stability of many circuits.

It limits the high frequency gain of the chip so that internal phase shifts do not turn negative feedback into positive feedback.

It is still very rare to see capacitors in integrated circuits.

 

[berkeman]

It is still very rare to see capacitors in integrated circuits.

And costly at that size.

We have capacitors in our analog ASICS very often now, but not that large.

 

[Jiggy-Ninja]

That's one half of a simple current mirror. The BJT's with the shorted CB are acting like diodes. This branch sets the current for the mirror and it's partner follows it.

Is there a specific reason it's done that way? Couldn't they just make it a normal diode and save from having to make one P/N region?

EDIT: So the answer is the cap? 30 pF is considered large for an IC?

 

[uart]

Is there a specific reason it's done that way? Couldn't they just make it a normal diode and save from having to make one P/N region?

The idea is that the two transistors are very closely matched, basically side by side on the same die with identical doping profiles.

The fact that it's a transistor, even with the CB junction externally shorted, still ensures that when operating it has an excess minority carrier concentration profile that closely matches that of it's partner.

 

[berkeman]

EDIT: So the answer is the cap? 30 pF is considered large for an IC?

Yes. That's my answer, and I'm sticking to it!

 

[berkeman]

It's kind of ironic that such a low-tech component like a capacitor can be the largest area component on an IC. We run into this often now when we need to provide on-IC high-frequency decoupling of digital IC cores. All that area...

 

[uart]

Hi Berkeman. Many years ago as a young engineer I remember using some active filter IC's that were described as "hybrid" IC's. Basically what they had is the Si wafer containing the op-amps along with some laser trimmed resistors and capacitors (not on the Si wafer) with the whole thing encapsulated into an 14 pin DIP package.

Obviously these were more expensive to produce than monolithic IC's, but that's where I first learned about the relative difficulty of getting capacitors (and at the time high tolerance resistors, though I don't know if this is still as bigger problem) onto a monolithic chip.

 

[carlgrace]

Is there a specific reason it's done that way? Couldn't they just make it a normal diode and save from having to make one P/N region?

EDIT: So the answer is the cap? 30 pF is considered large for an IC?

30 pF is enormous for a signal-path cap. To give a flavor of how expensive it would be, in current technology precision cap density is on the order of 1 fF/um^2. And, as berkeman said, large caps are used for power supply bypassing but these are not precision caps (and can often go under wiring and such).

 

[KingNothing]

If a capacitor is the feedback element, what's the difference between a current-feedback op-amp and a voltage-feedback op-amp?

 

[carlgrace]

If a capacitor is the feedback element, what's the difference between a current-feedback op-amp and a voltage-feedback op-amp?

I'm not being glib, but the difference is what is being fed back, a voltage or a current. The vast majority of op amps you will encounter in the wild are voltage feedback.

And this compensation cap is not the feedback that is referred to when an op amp is described as voltage feedback or current feedback.

 

[KingNothing]

I'm not being glib, but the difference is what is being fed back, a voltage or a current. The vast majority of op amps you will encounter in the wild are voltage feedback.

And this compensation cap is not the feedback that is referred to when an op amp is described as voltage feedback or current feedback.

Well, can you explain what is then? Are there two different things in the circuit you would describe as "feedback"?

 

[Studiot]

Seems like a good time for the tale...

Well you guys seem to have sussed most of the tale yourselves.

There is one point still worth making. Berkeman posted the 741 'schematic', although it is only diagramatic since it should be realized that componeents in monolithic chips have different geometries from discrete ones and in some cases, eg multiple emitter transistors are only available in monolithic circuits. That was a digression, the point is that other op amps of the 741 generation which were not internally compensated had a better available frequency response. I think this was hinted at, in one post back along.

Sorry this tale was all business, not one of my more spicy ones.

go well

 

[Studiot]

Well, can you explain what is then? Are there two different things in the circuit you would describe as "feedback"?

There are a bunch of emitter resistors also providing (local) feedback.

The point about the capacitor is that the feedback is frequency dependent, and its value is such that it reduces the gain to unity before the phase variation can change the feedback to positive. That is what is meant by 'unconditonally stable' and what 'compensation' does.

 

[carlgrace]

Well, can you explain what is then? Are there two different things in the circuit you would describe as "feedback"?

In any analog circuit there are a lot of feedback loops. Usually you attempt to ignore as many as possible and focus on on the ones more germane to the question at hand. The "main" feedback loop in an op-amp circuit is the negative feedback network you insert between the output and the input to get the behavior you desire.

The compensation network is a (usually) distinct network that keeps the op-amp stable at different closed-loop gain settings. Most op-amps you work with are internally stabilized for any gain, so you don't really have to worry about it. So you can abstract it away.

And to answer your original question, a voltage feedback op-amp is sensitive to voltage at its input, and a current feedback op amp is sensitive to current. The voltage feedback opamp has a constant gain-bandwidth product that is set by the large compensation cap (30 pF in the case of the 741 we are discussing). Current feedback op amps have a low impedance input and are not limited by the dominant pole imposed by compensation.