line driver with automatic bias

Ban · 2010-08-28T21:17:43+00:00

features AB/operation of the output transistors with a bias current of 1.5mA(set by 15k). The complementary pairs should have a similar beta. To have asymmetrical clipping at around +/-11.5V a zener in the positive supply ofthe opamp overcomes the bootstrapping action of the 100u capacitor.Keeping the output transistors conducting reduces distortion by a wholemagnitude. There is no current limit, so an 100R output resistor protectsthe transistors. Temperature compensation is intrinsic to the circuit, nothermal runaway should occur. .--------o------------o--o | | | +15V | .-. | | | | | | | |15k | z '-' | 2.7V A | | | .-----o | | | | |/ | --- PNP |--------| NPN | --- >| |> | |100u | | ___ | | | ___ | ___ o--|___|-o---)--)-----)---|___|----o-|___|-o in 10k | | | | 10k | out | | | | | | | | | |/ | | | NPN |--------| PNP | || |

Aug 28, 2010 49 Replies

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dagmargoodboat 15 years ago

That's a bunch of mismatched Vbe's and indeterminate outputs, not a current mirror. You could as well call it a band-gap, but it isn't. A current mirror uses matched transistors to produce a scaled copy of an input current.

This would be my 1rst-cut at a VLV-type circuit--sans labels (as per Tim Williams' standard :-) .

--o--------o-------o-- | | | .-. | | | | | | | | | | '-' | | | | |/ o--------)-----| | | |>. | | | V | | --- | .-. | | | | | |/ | | ---o----)------| '-' | | |> | | |< | o-------- '--| | | |\ | .-. | | | | | V | | | --- '-' | | | | | | | | |< | o-----| | | |\ | .-. | | | | | | | | | | '-' | | | | -----o--------o-------o---

(created by AACircuit v1.28.4 beta 13/12/04

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I added diodes and emitter ballast resistors to keep the output transistors throttled. Idle current decreases with temperature. VLV's slewing diode is left out, for simplicity.

-- Cheers, James Arthur

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dagmargoodboat 15 years ago

of 1.5mA

Yep. I skipped over the beta-bias part. I was too busy turning the PNPs right-side up...

...and looking at that base-to-base thing.

-- Cheers, James Arthur

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Martin Riddle 15 years ago

Reversing the Collector and Emitter will guarantee that it will not or may not always work.

Cheers

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Tauno Voipio 15 years ago

Reversing emitter and collector for a small signal silicon transistor will still work, but:

- the 'collector' to base junction will zener at 6 to 9 V, - the beta may be less than 1, - the saturation voltage is tiny.

Tauno Voipio

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JosephKK 15 years ago

Looks kinda like a lame rebuild of an LH0002C

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JosephKK 15 years ago

I wish it were that good, the PNPs are upsidedown.

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JosephKK 15 years ago

Build it _exactly_ as drawn, measure it and then tell us about it.

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JosephKK 15 years ago

That was uncharastically kind. The circuit is fubar. It may even work but it is evil and wrong.

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Jim Thompson 15 years ago

Sonuvagun! I usually catch hell for being blunt.

Burr-Brown used to do something like that to compensate input stage bias current in their bipolar OpAmps. ...Jim Thompson

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JosephKK 15 years ago

I had alread seen plenty of that when they still published decent equivalent schematics. Early bias cancellation was elegant.

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