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natumbri 22nd April 2008 11:32 AM

Extra current for Chipamp
2 Attachment(s)

I'm new to ChipAmps and electronics generally.

I was wondering if using output transistors to increase the output current of a chipamp is a good idea.

I saw the idea in a patent Google Patents.

The gist of the patent is set out in Fig 4 in the patent.
I also recall seeing a similar idea, but done without the resistor on the output of the chip amp. Something like the attached picture - but I'm not sure that I've got it quite right.

Would someone mind explaining how these circuits work and offering a view about whether they are a good idea or not?


ttan98 22nd April 2008 12:00 PM

no need new thread, see this thread, design is similar,

under high powered LM3886TF

KSTR 22nd April 2008 12:10 PM

USPTO surprising me once again.... granting a patent for long established circuits, the well known current dumper.... that circuit can be found in a lot of opamp datasheets.


WRT your attached circuit:
It won't work, sorry. You are effectivly shorting the output to the opamp supplies. What you probably had in mind was using common emmiter outputs instead of common collecter (emitter followers). Then the idea looks better, but you have to change a few things. No cap, swap inputs of opamp and open the connection from opamp to output, instead use load to GND, use degenation on the emitters. The load current reflected on the supply pins then gives drops on R3/R4 which turn on the transitors accordingly. Basicly the transistors + diodes (one only, not two) + resistors form current mirrors with gain. This type of circuit needs quite a bit of refinement (not to forget compensation) to really work.

- Klaus

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