Cute amplifier - bootstrapped

In the tube and early-transistor days, gain was expensive, so positive-feedback tricks, and things like transformer coupling, were more common.

e:

otstrapped and not the CE. So the CE + CC composite looks like the classic very high gain single pole amplifier inside the feedback loop formed by the resistor shunt feedback divider, everything is simplified. See the textboo k write-up here, section 1.17.2:

Yep, that's a classic way to raise a.c. input impedance.

ISTM there's really no distinction compared to describe it as bootstrapping the collector load. The collector load R2, being in parallel with the Q2 input impedance, bootstrapping R2 raises QA2's input impedance, that's true. But, the key effect in this circuit is raising the first stage gain, so it seems most appropriate to look at it from that perspective.

Either / or.

'T isn't needed when you do it right ;-) ...Jim Thompson

ote:

bootstrapped and not the CE. So the CE + CC composite looks like the classi c very high gain single pole amplifier inside the feedback loop formed by t he resistor shunt feedback divider, everything is simplified. See the textb ook write-up here, section 1.17.2:

Okay, look at it under a microscope if you prefer it that way.

edback

Yep. Jim William's description of the Philbrick varactor amp inspired R6 above. IRL Jim's throwing a handful of transistors at it makes more sense for an IC designer--less component-sensitive--but it's always fun to play.

As mentioned before, I'll likely use a dual op-amp instead, cascaded inverting stages, a.c.-coupled, d.c. offset. That's one op-amp, two caps, six resistors--boring, reliable, and repeatable.

positive-feedback

Yup, you can get a dual opamp for 8 cents. Takes the fun out of discrete design.

ote:

bootstrapped and not the CE. So the CE + CC composite looks like the classi c very high gain single pole amplifier inside the feedback loop formed by t he resistor shunt feedback divider, everything is simplified. See the textb ook write-up here, section 1.17.2:

=boo...

Q2's input resistance is gain(Q2) x (R3+R4) anyhow, which is on the order of half a megohm.

So, Q2's un-bootstrapped input impedance doesn't limit Q1's gain, R2 does. Boosting Q2's input impedance per se wouldn't help.

z.

I've seen that ages ago, a faded memory, I think likely from that same G.E. databook. Yours is much linear-er than the circuit I posted.

Hmm. Could even bootstrap the first collector load. Only drawback is possibly a limited output swing compared to the emitter-follower Q2 version...

I might goof around thinking about that.

James

That's not even remotely close. That is the two transistor job with superstablized Q-point self-bias and series feedback.

On a sunny day (Thu, 10 Jan 2013 17:31:00 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

mm, an ASCII magnifier?

superstablized Q-point self-bias and series feedback.

Gosh, really?

Just ASL each byte.

Yep- really- and that circuit has come up as a topic before. You couldn't get to square one figuring out the DC bias then IIRC.

output impedance.

The emitter bootstraps itself, if you use a current source load. (*) It's the collector of the follower that you have to bootstrap, to get rid of the Early effect and C_CB.

Cheers

Phil Hobbs

(*) Sprinkling simulated inductors made out of BFP640s is good medicine to stiffen current sources at high frequency. Cap from base to emitter (with a bead to keep it stable) and a resistor from base to collector.

to square one figuring out the DC bias then IIRC.

Thanks for not confusing us with any facts or schematics.

You can combine that with noise resistance matching too, which is a huge win.

Cheers

Phil Hobbs

bootstrapped and not the CE. So the CE + CC composite looks like the classic very high gain single pole amplifier inside the feedback loop formed by the resistor shunt feedback divider, everything is simplified. See the textbook write-up here, section 1.17.2:

Blech. That's why they quote PSRR as an equivalent input voltage, of course.

Cheers

Phil Hobbs

You're very welcome.

Also google "noiseless feedback amplifier"