Composite amps

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plifiers"

It?s interesting to try using still floating power supply op amp.

Dmitriy P.

fig.3 Output of bootstrap amplifier

Using the photocurrent twice is fun, I agree. I sometimes use a cascode with two collector resistors in series, with a diode clamp on the higher-value one. You digitize both, and use the lower-gain one when the high-gain one rails.

That's handy when you have a relatively slow pulsed application where the signal varies over a wideish range and you don't want to have to do range switching. I last used it a couple of years ago, in the follow-on version of my blood spot detector for hen's eggs.

That feedbeside thing is disappointing IME--it settles OK to 1% or so, but then it's all whoopdedoos. Good for scopes, which is where it was originally used.

Yup. You do have to watch out for the CMR though--TIAs with bootstrapped power supplies can lead to oscillations. CMR is input-referred, and in a TIA the noise gain gets pretty high at high frequency, just where the CMR is tanking.

It's usually the negative CMR that's the problem.

Cheers

Phil Hobbs

Another dinosaur with capacitors between the base and the emitter:

A Low-Noise Precision Op Amp National Semiconductor Linear Brief 52 Robert A. Pease 1980

I don?t understand why HP shunted the transition at the second tran sistor with a collector current 5 times the input stage...

It's sort of fun--it's a Darlington at DC and not at AC.

The Darlington topology is primarily intended to reduce the input bias current. The 10 uF caps get rid of the gross AC voltage noise of the driver pair Q1A / Q1B, and as a bonus they eliminate the transfer function problems of the Darlington.

Cheers

Phil Hobbs

Isn't a classic differential amp configuration a composite amp?

No, because there's no overall feedback from output to input.

Cheers

Phil Hobbs