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I'm reading an article about how a rail-to-rail opamp works. The materials from Maxim and Analog Device all mentioned that, in a conventional operational amplifier, the output stage, in its essence, is a pair of NPN/PNP forming an emitter follower.

Push-Pull Output

One can instantly recognize that the principle of operation is similar to a push-pull output stage found in discrete Class-AB amplifiers, the output stage is used as a buffer for current gain. The input is at the center of the diodes, and since it's an emitter follower, the output is a replica of the input. This arrangement comes with a Vbe drop, with means the output can never get to the power rails.

Common-Emitter Rail-to-Rail Amplifier

The articles continue to explain that in order to overcome this limitation, in a rail-to-rail opamp, the output stage is a pair of PNP/NPN transistors forming an common-emitter amplifier. And in this arrangement, the maximum output swing is only limited by Vce_sat, and is able to get very close to the rail.

But I have never seen this configuration before (The only time I've seen it before, is in The Art of Electronics as a bad example of a miswired Class-B amplifier - once an input is applied to the bases, both transistors are turned on and they blow up...).

How does this output stage supposed to work? How do I apply an input? Are there some practical examples of this circuit with discrete transistors?

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  • \$\begingroup\$ R2R Outputs use CMOS Op Amps.. \$\endgroup\$ Commented Dec 24, 2019 at 17:44
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    \$\begingroup\$ Look up Edward M. Cherry. In particular, his articles in the early 1980's (say 1982 to 1984?) in the journal of the audio engineering society and read about this topic. One in particular you may enjoy is "Feedback, Sensitivity, and Stability of Audio Power Amplifiers." It does, memory serving, do justice to at least the important part of your question. I don't have time, right now. So for now, you'll just have to accept the references and my assurance that it covers the topic you are asking about. \$\endgroup\$
    – jonk
    Commented Dec 25, 2019 at 8:23

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The reason one doesn't find "rail-to-rail" complementary collectors drivers is that its hard to balance two current sources without accurate sensing using current mirrors.

Unlike emitter followers that buffer base impedance Zout = Rbe /hFE, they are current sources which by definition are high impedance which is not what you need for a voltage source rail to rail driver. Using negative feedback is less stable and chances for shoot-thru are too difficult to prevent.

Most R2R drivers are CMOS but some are high current with NPN low side In complmentary drivers, it is ok to have one current sink, or source if shunted by a complementary low Rce against a low Pch RdsOn resistance, as per below.

e.g. LMV32x X=1,2,4 2.7~5.5V 160mA enter image description here

e.g. PA50 100Apk 40A power Amp 400W

enter image description here

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The ordinary CMOS stage exploits the same idea where a PMOS and NMOS FETs are connected with their drains as a voltage output. We can see it also in the internal structure of a CFA (current feedback amplifier)... and, of course, in every differential stage with a current-mirror dynamic load.

Although this connection is incorrect ("fighting" current sources), it ensures an extremely high gain. We can think of it as of a "dynamic voltage divider" which two resistances vigorously change in opposite directions (like the partial resistances of a potentiometer when we move the slider). So it needs a negative feedback to keep the output voltage somewhere between the two supply rails. We can see the effect of applying a NFB if simply connect the output of a CMOS stage to its input.

In digital CMOS gates it is not necessary.

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Here's an example of an op-amp (the LT1218) with R-R output (and, incidentally, input) that consists entirely of bipolar elements:

enter image description here

The two resistors on Q18/Q19 will be very different from each other to get the output stage close to balance with similar currents from Q16/Q17.

Of course no amplifier is actually R-R output, particularly when there is a load attached. They can only drive so close to the rails, particularly when sinking or sourcing current to the opposite rail. Here are the specs for the above op-amp- with 2.5mA source/sink they can get to within 300mV of the negative rail and 500mV of the positive rail- PNP transistors on a chip tend to be rather inferior to NPNs:

enter image description here

The input stage consists of two different input stages that cross over at some point, yielding a bit of weird and sometimes unpleasant behavior that does not exist in non-R-R input opamps such as shift in Vos and bias current shifts. In this case, to minimize the effect the amplifiers need to be individually trimmed by the manufacturer (which suggest the question of how well CMRR behaves over supply voltage and temperature). The input CM range using this scheme (there are others) does include both rails, however.

These designs are more complex and finicky than more straightforward designs such as the LM741. For example, I've observed low amplitude oscillation in an otherwise very stable amplifier using LMV32x chips when the output is within a few hundred mV of one of the rails. Not to suggest the LM741 is a good op-amp in 2024 (or even 2014), it isn't, but similar effort put into non-RRIO op-amps has resulted in much better parts.

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    \$\begingroup\$ "when the output is within a few hundred mV of one of the rails" Yes, I have also witnessed unstable behaviour of various op-amps under similar conditions. The problem can be triggered or made worse when the inputs are also close to either of the supply rails, and/or the current in/out of the output pin is heavy. It seems many of these op-amps with R-R output (and input) are not sufficiently characterised for the extremes of the operating ranges, eg: most phase-shift data is still only offered under "ideal test cases", where the output voltage remains well away from the supply rails. \$\endgroup\$ Commented May 19 at 9:00
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A good example of an output stage capable of swinging from rail-to-rail was written by Dennis Monticelli, back in 1986, citation below. I have pasted screenshots of the relevant sections here.

Author: MONTICELLI, Dennis
Title: A Quad CMOS Single-Supply Op Amp with Rail-to-Rail Output Swing
Publication: IEEE, JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-21, NO. 6, DECEMBER 1986

This particular circuit was explored further by Horowitz & Hill, refer:-
The Art of Electronics, the X-chapters,
4xp11_RR_op-amps

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