Cute amplifier - bootstrapped

If I'm remembering right, I first saw that in Microwave Journal, late '80's. Neat stuff. I never used it, but I found the VLF thing later (Burhan, Radio Electronics (of all places)) and was reminded of it.

I suppose even Google Groups has to be "good" for something.

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It's been with great enjoyment, that I've been reading this thread. (Thanks all.) But I lack a complete understanding at times. I think I've used bootstrapping only twice. Both driving down capacitance. In A photodiode and a driven shield in a short cable.

So my simple minded definition of bootstrapping is, using positive feedback in just the right amount. (Too much and you get excessive gain.) Is that correct? Can you add more?

Thanks again, (It's great having a somewhat civil SED.) George H.

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Bootstrapping to me means using a circuit's output to power or drive one of its earlier stages. That is, to pull yourself up by your bootstraps.

Here, by using the output to drive the 'high side' of R2, the voltage seen across R2 is kept roughly the same. That, in turn, keeps i(R2) fairly constant.

The total effect is that Q1's collector sees a current that doesn't change much with a.c. changes in Vc(Q1). From Q1's perspective, R2 is effectively greatly increased, which increases the stage gain enormously.

I get g ~=23 @ 40KHz without C1, and g ~=230 with C1=22nF, a ten-fold improvement.

Also, R2's supply can exceed Vcc, such that Q2's emitter can swing within millivolts of the positive rail.

The real interest for me was the technique of making a resistor into a pseudo constant-current source, simply. John did it already way back with his ramp circuit; this was just another example.

current

I didn't see any collector bootstraps either. I do see emitter bootstrap.

?-)

Shunting the base has the same effect as boot-strapping the collector... reduces the "Early" effect. ...Jim Thompson

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OK, Thanks James. I was rereading AoE on bootstrapping... similar type 'tricks'.

George H.

I pondered that schematic for some time before realizing what it was.

It's some amateur's attempt at making a band-pass filter (quasi-gyrator), but it has horrible input impedance...

Looking like 10nF in series with 16K at low frequencies,

and -223nF at 194KHz ;-)

The shunt feedback directly to the base, driven thru the 10nF capacitor was the clue.

...Jim Thompson

Yep- the input is garbage.

And thanks for not belittling us with your insightful analysis of the biasing circuit...where is that belittling insight anyway?

With bigger caps, it's a perfectly reasonable audio amp stage. The bootstrap increases the gain about 20 dB. The biasing is fine.

circuit...where is that belittling insight anyway?

James' circuit works fine. What do you find wrong with it?

A voltage gain of 300ish from 3.3 volts is cool.

One EROT is that a common-emitter amp has a voltage gain of 40 times the DC drop in the collector resistor. So with a 3.3 volt supply, gain is ballpark 40. Bootstrapping increases that by about 10:1.

Transistor circuits are fun. Too bad we use them so seldom nowadays.

Okay, great, you have taken a fundamental misunderstanding of the original circuit operation to new heights :-)

How so?

It's no worse than a shunt current source[1],

Incidentally, this was drawn in 2010, so, if it were worth copyrighting, and doesn't appear earlier elsewhere, I won.

Tim

circuit operation to new heights :-)

I wish you would try to make sense once in a while. I also wish you would contribute something besides whining, but that's even more unrealistic.

That last circuit is a simplification of James's bootstrap amp. It eliminates a few parts and pushes the gain close to 60 dB.

Playing with circuits is a fun and valuable sport. Your turn.

Tim Williams a écrit :

Nah, I win. I wrote about that CS back in 2005 in a thread current source opportunely named... 'Low current source'

And it's not as useless as you might think. That one transistor CS is very simple good trick, albeit not very accurate...

This is a nice current source

Pretty low TC, not bad compliance, and makes light for free. One might get away without the base resistor.