Electronic – Why the circuit behaves in this way

I've seen "bootstrapping" used in a few more books/articles for this technique:

• Small Signal Audio Design, 2nd ed., by Douglas Self pp. 136-137; shows both an opamp-based boostrap and a BJT-based one. EDIT to add: As it turns out, there some free excerpts from the first edition of the book published in EE Times; the relevant circuits [aimed at JFET input C/V tweaking] are published in an article called "Op amps in small-signal audio design - Part 2: Distortion in bipolar and JFET input op-amps. Rail bootstrapping to reduce CM distortion".

• Analog-to-Digital Conversion, 2nd ed., by Marcel J.M. Pelgrom, pp. 210-211 uses "boostrapping" for an NMOS transistor operating above the rails. There are more CMOS books that use this term for the same purpose, e.g. Uyemura's CMOS Logic Circuit Design p. 319.

• "Advanced techniques tackle advanced op amps' extremely low distortion" by J. Graeme, also reproduced in The EDN Designer's Companion, Here p. 213 the bootstrap is done for measurement purposes.

So I guess that clinches "(rail/[power-]supply) bootstrapping" as a relatively common name for this, even though not every source utters it when discussing such circuits... And the purpose of the rail bootstrap may not always be a voltage swing increase.

If someone finds another (relatively common) term, please contribute another answer.

EDIT: As interesting titbids (found thanks to a rather vague mention on a forum), I found two UK patents on this idea:

• "Zero input capacitance amplifier" (1988) by Bernard / ABI Systems ; uses the 3-opamp version. Title is self-explanatory.
• "Interface circuits" (1980) by Mohapatra / Sandman. Rather uninformative title, if you ask me; shows several variants, some with two BJTs and some with opamps all-around. The stated goal is to increase input impedance and CMRR.

Neither of these UK patents uses the term "bootstrap" though... as far as I can tell, anyway; old UK patents are not OCR'd, so search doesn't work in those PDFs.

There's also an output voltage boost circuit based on this idea in Linear appnote 67 in the article titled Extending Op Amp Supplies to Get More Output Voltage by Dale Eagar (starts on p. 58 in this long document). This one uses MOSFETs on the opamp rails. But it never uses the term "bootstrap". So yeah, "bootstrap" is a common enough term for this, but not universally used... The preferred term at Linear for this circuit idea is extended supply mode, which (if you google it) is found exclusively in their documents... so this alternative term appears to have less traction across the industry as whole.

Your test setup is basically a follower, which tends to reduce the effects of gain spreads. While this is good in production, it's not what's wanted here. In your job, the idle current is known, so you can set up a common emitter stage to pull the idle current. Also, the \$V_{CE}\$ the discrete op amp provides to the transistors can be determined. So you are setting up your test transistor under the same DC conditions that it will see in the discrete op amp. Now use a standard common emitter circuit with NO negative feedback. This implies using a fully bypassed emitter resistor. Now you will see more variability when you feed in your AC from your signal generator. Picking a pair will now be clear.