What are these nodes?

AC analysis relies on linearity, so all signals are noninteracting.

Cheers

Phil Hobbs

AC analysis is indeed "small signal". That doesn't negate the fact that the analysis finds a frequency misbehavior.

And I don't have a clue what your "non-interacting" means. Sounds like BS to me >:-}

I'll run a transient analysis and prove it to you.

Sorry "prove" is the wrong word. Some on this group think their opinion outweighs science. ...Jim Thompson

I lurk here mostly, but I don't understand the big deal over this circuit. Isn't it just a poor man's TIA? Input current against R5 creates current in the output -Iin*R5/R4 so (neglecting the small current contribution from Iin) Vo/Iin = -(R5*(R3+R4))/R4?

Depends on what you mean by "AC Analysis". Most people are interested in the overall transfer function of an amplifier, so the left side of C2 isn't ground, it's the input.

The transfer function is OUT/INPUT, which isn't useful if you define the INPUT node to be ground.

Yes, exactly! A breath of fresh air from someone who knows what they are doing, instead of BS'ing!

The big deal is that the "bootstrap" is not broadband. It is actually a "poor man's gyrator", as I've demonstrated. I'm guessing that the component values were chosen specifically to enhance behavior at

So, as a plain vanilla amplifier, it's performance is terrible... unless driven from a high impedance source... then it's pretty much useless. ...Jim Thompson

James assumed that the signal source was a voltage, whose source impedance, including the coupling cap C2, was far lower than the 220K bias resistor, so the thing runs open-loop with a voltage gain of hundreds. The whole point of bootstrapping was to increase the overall voltage gain. The feedback is just to set the DC operating point, not to limit the gain or to make a TIA. As James said, if AC feedback were a concern, add a bypass cap in the feedback path so that it isn't.

The true poor man's TIA is a single resistor to ground.

Which is what Bitrex said, and I missed emphasizing, input "current".

As a voltage amplifier it's a differentiator. ...Jim Thompson

I agree that running a simulation doesn't prove a whole lot by itself, but then I don't have a dog in this fight.

"Non-interacting" means what it says--in a perfectly linear system, the response to any combination of stimuli, applied wherever you like, equals the sum of the responses to each stimulus alone.

I do build a whole lot of bootstraps, though.

Cheers

Phil Hobbs

PhD-ish for the superposition theorem ?:-)

Whoopee Doo! I build thousands of cascoded mirrors.

...Jim Thompson

You really are pretty torqued about this, aren't you?

Cheers

Phil Hobbs

Almost missed that jewel. Are you saying opinion outweighs simulation?

Just your nose ?:-)

Perhaps! I'm thinking of writing a book, "America's Worst Engineering" and quoting from this public forum... citing names along with the examples :-)

The "bootstrap", as presented, works only with a capacitive sensor and is crap otherwise.

When I posted my misgivings, I was dumped on, rather than _anyone_ presenting a proper analysis. ...Jim Thompson

No, just that simulation isn't reality. You know perfectly well the level of effort required to make a decent set of models for nontrivial devices--it sure doesn't happen by accident. All those models have to be validated by experiment (physical and numerical), and they only work over a limited set of input parameters. For instance, zenering a BE junction will reduce beta, but good luck getting that into a SPICE model.

You know, Jim, the last time I checked, Usenet was a public forum. If you want your tiffs to be private, you might consider conducting them over the phone.

Cheers

Phil Hobbs

Getting a bit off-track there aren't we?

Are you claiming that my simulation isn't reality? That there's something in this simple circuit that is black magic, such that jerks can claim a performance not demonstrated by a simulation?

I've just been vindicated by a Marshall Leach paper.

...Jim Thompson

Here's some, IMHO, relevant info the OP wrote, which seems to go agree with your last line.

The input impedance doesn't matter--the input source is a low-impedance highly capacitive transducer. ____________________________________________________________ It has nothing to do with gyrators, feedback, etc. Those all totally miss the boat. ______________________________________________________________________ As far as the original problem which inspired this--which is entirely irrelevant to the thread, and which was never going to use this ckt anyhow--all I wanted was a spike out when the transducer got pinged. ____________________________________________________________________ That wasn't my assumption; I assumed a capacitive, low-impedance source. ___________________________________________________________________ freq as spec'd in the drawings and all the results--40KHz. Source impedance WAG'd @ 10nF, e.s.r. unknown (believed < 100r).

I looked at about seven 40khz transducers they were all ~2000pf.

I so noted that when flipper was pontificating.

Except it doesn't. I've plainly shown that the "bootstrap" is a gyrator-created inductance.

Then someone threw up another style of gyrator that wasn't as good.

Fit's with my resonance" comments, doesn't it?

And I agreed. Capacitor with signal as a current source... since vindicated by Leach's paper.

I guess. I'm not sure. I have some 30 year old ones in my parts bin from a motion detector project. ...Jim Thompson

Isn't the thevein equivalent of what those nodes are connected to a short circuit?

I guess you might say that. What are they "shorted" TO?

In an AC analysis, all _FIXED_ voltage sources are presumed ZERO. ...Jim Thompson

If you stick thevein in then it is clearly shorted to gnd so it must be equivalent to zero