Cute amplifier - bootstrapped

circuit operation to new heights :-)

I already told you how the circuit works, and in the simplest possible terms. Your comprehension is not my problem. And you keep referring to it as James's circuit when he stated he found it on the web.

More elaborate...

but works nicely. Been selling for about 50 years now >:-} ...Jim Thompson

circuit operation to new heights :-)

Was there ever any doubt? It's pretty simple.

late

later

ten-fold

No Bout Adout it, this is an interesting circuit. Of course the much higher voltage gain will aggravate the Miller capacitance issue hugely and may also aggravate Early voltage issues as well.

It is an engineering tradeoff.

?-)

It's an engineering crock. I guess everyone missed my comments in...

Subject: Re: Cute amplifier - bootstrapped Date: Sun, 13 Jan 2013 13:48:15 -0700 Message-ID:

It confused (even :) me for awhile, until it registered that, bootstrap or not, the base of Q1 is a virtual ground (or summing node, take your druthers). ...Jim Thompson

You're still confused. It's not a summing node if the generator impedance is low. Do the math.

Bootstrapping increases the forward voltage gain by about 10:1, more in my PNP variant. Why is that a "crock"? Because you don't understand it?

The primary intention of R1 is to provides the biasing point.

While it will reduce input impedance I wouldn't call it a virtual ground or an intended signal summing point unless driven from a high impedance source.

It does that. What else does it do?

You would be wrong.

Might I suggest you simulate it, both from a low-Z and a hi-Z source?

No hand-waving. Prove your position >:-} ...Jim Thompson

On the face it may be more elaborate, but isn't rail to rail.

For me, the primary purpose of bootstrapping is to ensure near rail to rail by providing a more positive supply to the base drive of an output device. All the other characteristics through bootstrapping, of the amplifier circuit presented by the OP, will actually degrade performance.

Did you miss, "Been selling for about 50 years now"? It predates CMOS being even considered for analog.

Or raise the input impedance (most common application).

Yup. It's a hacker's circuit. ...Jim Thompson

It's more rail-to-rail than the OP circuit.

Like I say, anyone who thinks the OP circuit is marvy, just simulate it, then come back and tell me about it. ...Jim Thompson

Hi Jim, I'm pretty much a newb, when it comes to transistor level design. But the original circuit looks very similar to Figure 2.66 in AoE (2nd ed.) "Bootstrapping driver-stage collector load resistor in power amp."

So is it the bootstrapping you don't like or the DC biasing scheme? (Maybe I can try to simulate it at home tonight.) A hint might help me find the issue when simming it.

George H. (Does simming have two m's or one? :^)

It misses the 'w' :-)

Man, somebody sure mangled that. Here's the original post again:

| |>. | | | | | R4 470R | === | | | | === '-----R5------' 220k

No, that's not the purpose. The purpose of the bootstrap is to increase voltage gain. The input impedance doesn't matter--the input source is a low-impedance highly capacitive transducer. This is not a general-purpose generic IC that needs to do everything.

The IC designer has luxuries the discrete designer doesn't. Jim's got a sweet little part there, but that circuit has 13 (matched) transistors, 3 caps and 10 resistors--not a fair comparison to a littl' ol' two-transistor circuit.

Bootstrapping here increases the voltage gain and output swing--it's cute. All two-transistor gain-of-220 amplifiers have their limitations, naturally.

The bootstrapping is fine, in and of itself.

The feedback scheme and input circuit (and the "short" feedback loop) make for a very un-useful frequency response. ...Jim Thompson

Try my PNP version! Less parts, more gain. It simmed at 57 dB.

I don't know why Jim is so hung up on the "summing point" thing. All amplifiers have an input impedance. The nfb of the biasing resistor reduces the input impedance down to something in the 2K ballpark.

You sort of left that part out of your original post ;-)

Is the "transducer" actually C2? Grounding the input, it is certainly stable, with ~100° of phase margin, but barely shows 20dB of loop gain.

As a voltage amplifier it looks like a low-Q bandpass.

Awwwwh! Shucks! >:-}

Naturally. ...Jim Thompson

-R5------' 220k

True. My purpose was to explore the bootstrap, not an optimum amplifier.

Yes.

LTSpice says g=220 @ 40kHz.

Jim

Aww, if only discrete transistors were matched and free...

I get that too, IF I drive the input with a VAC source and look at the output.

If I do a loop gain and phase analysis with the input grounded, the loop gain is ~20dB.

I have hundreds of DIL's with 5 matched NPN's ;-) ...Jim Thompson