Cute amplifier - bootstrapped

Commonly done in class-B output stages to get closer to rail-to-rail behavior. ...Jim Thompson

That's another advantage.

Bootstrapping rocks. This thing improves the constant-current-ness of a linear ramp generator at higher speeds....

Phil does some uber-bootstrapped stuff with jfet front ends. He sort of bootstraps the entire planet, or the entire universe, all around a BF862.

Except for the Early effect in Q1.

Cheers

Phil Hobbs

Bootstraps are fun.

I'm doing one right now for a surface voltage tool for a semiconductor equipment maker. Even with vaguely realistic strays, SPICE says that the bootstrap gain is between 0.9995 and 0.9998 over the frequency band of interest. I actually get to build this one, so we'll see.

Doesn't fix the noise due to the bootstrapped capacitance differentiating the BF862's voltage noise, but since that's about 0.8 nV in 1 Hz, going lower is hard unless the external capacitance is large.

My best ones so far use series-shunt feedback around the BF862, to get a lower source impedance without increasing the noise, and then bootstrap the whole works. You can also put the bootstrap inside the series-shunt loop, which has some minor noise advantage (since the bootstrap device's noise is inside the loop) but makes the compensation really squirrelly at high frequency.

Cheers

Phil Hobbs

You need to turn that around, it is the emitter follower that is being boot strapped and not the CE. So the CE + CC composite looks like the classic ve ry high gain single pole amplifier inside the feedback loop formed by the r esistor shunt feedback divider, everything is simplified. See the textbook write-up here, section 1.17.2:

Except for the shunt-feedback >:-}

The whole thing is a bit crappy.

Here's how to do it...

Or broken out for analysis, from a previous post...

How did that eventually work out?

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:

The output impedance of the follower is probably a few tens of ohms. For small signals it doesn't even notice the extra loading on its emitter.

You can bootstrap a follower, in fact you have to if you want the bootstrap itself to work properly, but that involves doing something with its collector.

Cheers

Phil Hobbs

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:

Huh? You bootstrap the follower to increase its input impedance, not lower its output impedance.

I didn't used it, actually.

I did use this one

which is simpler but doesn't have adjustable slope. One could include R28 or R36 to add a little 2nd order curvature correction to improve linearity.

The follower's input impedance is limited by its beta and Early voltage at low frequency, and its C_cb at high frequency. Beta you can't do too much about, but Early and C_cb you can fix by bootstrapping its collector. You can't improve the SNR, but you can make the frequency response and Zin much prettier.

Cheers

Phil Hobbs

I remember that trick of yours. Fun.

One of the neatest I've seen was a VLF JFET amplifier. It used transformers to bootstrap its input capacitance to nada.

James

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:

We bootstrapped a unity-gain 5-volt chopamp to let it follow +-40 volt swings. That was a bear to get working correctly and reliably. The PSRR of the amp was negative (ie, it had gain from power rail to output) at some frequencies.

I made a cascode version, but that was getting silly. A dual op-amp makes more sense at that point with fewer parts.

Talking about silly, a little positive feedback really makes it sing...

| |>. | | | | | R4 470R | | | | | +---------------' | | | | R6 3r3 | | | | | === | | | '-----R5------' 220k

G= 1050 @ 40KHz.

There actually is some negative feedback via R5--that stabilizes the gain a little bit.

That's plenty cute--a three-transistor inverting amp, with feedback, then an emitter follower, plus the bias stuff. But is there a bootstrapped collector load in the signal chain? I didn't see it.

Okay, those are fine points, but in this case the designer is trying to eliminate the shunt effect of the emitter follower input bias resistor network, and the bootstrapping succeeds well there.

eliminate the shunt effect of the emitter follower input bias resistor network, and the bootstrapping succeeds well there.

That's sort of a strange way to look at it. The common-emitter stage wouldn't do much without those resistors, so I'd identify them as the collector load of the first stage rather than an input network for the second stage.

Cheers

Phil Hobbs

Not to thread-drift or anything (who, me?) but similar toplogies are, if possible, even more ancient than JT. This is the once-famous "GE circuit", widely used for phono and tape preamps.

I wouldn't be surprised if there were a tube ancestor.