Two feedback paths (again)

In the South, we have y'all as the nongendered second person plural. You Guys seems to work out west. Strangely, "Ladies" does too; pro quarterbacks have been known to use the term in a huddle.

What I do is take the DC feedback from the very output but take the AC feedback from somewhere sooner in the circuit.

This works fine, but probably a bit of resistance in series with C3 would improve the load step response.

For best load transient response, you've got to manage the crossover frequency.

A huge polymer aluminum output cap is good, too.

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Sounds good. One more stage and you'd have a good old fashioned phase shift oscillator. With just two, it looks something like a Sallen-Key filter -- feedback delayed by another RC. The feedback phase shift isn't

In reality, a couple microseconds around the loop can fill in the last bit of angle required to make an oscillator. Effectively, as the filter's Q approaches infinity, that little extra bit will push it "just past infinity", loop gain goes >1, and it oscillates.

Real numbers are modular at infinity (well, in electronics they are), so "greater than positive infinity" is the same as "greater than negative infinity". So if you drew an RLC equivalent circuit, the loss would be a negative resistance. By extension, a negative resistance amplifier necessarily has a negative Q.

Seconded.

Incidentally, this is also handy with op-amps (notoriously bad at driving capacitance or capacitive cables), shunt regulators (like TL431) and LDOs (i.e., an upside-down TL431) -- the difference with LDOs being, they don't give you the internal FB node, so you're screwed and have to make do with ESR.

Tim

Load current requirements? Noise level target? ...Jim Thompson

Almost forgot... did you try a BOOTSTRAP ?>:-} ...Jim Thompson

Good idea.

Rout(DC) = re + Rf/(beta + 1)

The 're' term is why I asked about load current.

...Jim Thompson

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Hi Jim, Well no real 'target' spec's. I just scribbled down the circuit taking the feedback from the output... then tried it the next day.

The circuit in use (with feed back after the pass transistor) drives a 75 ohm heater from ~30V supply, so ~350 mA. The noise is in the

George H.

"No!!!!!", Flails arms over head and runs screaming from room.

George H.

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OK I hadn't thought that far ahead. re ~25mV/Ic ~ so most of the time less than an ohm.

George H.

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to inverting input Hey I just stuck in a big ass cap in the feed back path. (The 20uF as shown above.) The output low pass has a tau of 0.5k ohm*100uF = 500ms. and a high pass feed back of R/4 (2.5kohm) * 20uF = 500ms.

This seems to work. Though I have to do some 'real' work today and can't test it totally.

George H.

Why the need for low noise to a HEATER? Surely its mass would low-pass any "power" noise?

Or down to basics... what are you trying to accomplish? ...Jim Thompson

You've discovered "lead", to compensate for a capacitive load ;-)

What OpAmp and transistors are you using? ...Jim Thompson

No, it can be a good idea in voltage references! :)

You can often use the (boosted) output to source the drive to the reference. So it is almost completely immune to supply variations.

Grin... yeah, Well that's a long story. But the heater is down the bottom of a LN2 probe that is used to measure johnson noise of resistors as a function of temperature. Depending on the resistors used there would be some capacitive pickup from the lines feeding the heater. (they shared a common cable.) So... cap multiplier to the rescue!

Anyway I might have a need in the future for a nice low noise power supply. (And just thinking of ways to improve what I've already done.)

George H.

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The opamp is an LT1013 (input voltage can go down near the negative power rail) and the transistors are TIP31C's... Both just pieces I have from other projects.

George H.

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Oh sorry I thought Jim was making a joke. So feed the reference from the output voltage.... Thanks I'll remember that too. In the current circuit I don't think it's noise on the supply rail that is the limiting noise source.... not really sure what it is... (the circuit works so I moved on. :^)

George H.

I was.

Did that around 1970, built with a uA741 chip and a zener. It's in the TOW missile. Only trick is to make sure it self starts... some OpAmp architectures won't.

...Jim Thompson

You have to make sure the circuit starts up, though.

Cheers

Phil Hobbs

George, Your numbers are confusing. What is the voltage across the heater? What is the supply voltage? What is the reference voltage? ...Jim Thompson