Electronics > Projects, Designs, and Technical Stuff
Voltage boosting an opamp
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f5r5e5d:
spec the load, the supplies you expect to work from and I could sim up something


--- Quote ---  ...The CCS makes it really high gain stage...
--- End quote ---
not really, the common emitter output from the complementary Q collectors is just as high gain in your earlier circuit too


but I won't build/debug/compensate when there is a $5 solution - I expect to spend to achieve even hobby goals and as a comfortably retired US EE that threshold is a lot more than minimum wage

Marco:

--- Quote from: Yansi on December 30, 2016, 04:25:26 pm --- The common base gain stage works more than good enough, however for AC signals, I'd be very afraid of significant crossover distortion.

--- End quote ---

Why not just bias the whole thing to have quiescent current then? (Needs some more resistors and the first stage bases can no longer be connected to ground.)
David Hess:

--- Quote from: Yansi on December 31, 2016, 02:05:47 am ---I have tried the single ended class A with CCS few times. Didn't work as expected, stability problems. The CCS makes it really high gain stage, the opamp didn't like it. Still, class A with CCS (standard "discrete VAS" ) doesn't solve much, if both voltage polarities are required (how will you interface the opamp output to the class A transistor thats near the V+ or V- rail respectively)

Can you show a schematic, how would you do it?
--- End quote ---

Take a look as how analog oscilloscope z-axis amplifiers work.  They are discrete and very fast but if the single transistor error amplifier was replaced with an operational amplifier, then they would look like this.  One side of a horizontal amplifier usually works the same way.

They both generally use a constant current load and shunt feedback to make an inverting transimpedance (current to voltage) amplifier.  Tailoring the output voltage range requires a level shift to drive the output transistor but that will not be required if the output transistor is in range of the operational amplifier.
Yansi:
evb149:  I know these supply voltage bootstrapping apnotes, but I don't like the idea do to the possible latchup problems (supply voltage sequencing) and other limitations.

Yes, the schematic was cropped from that one (SNOA600B). Sure, can add other biased stages, but there is a lot of trannies extra, that I think the DC amplifier won't need.  Would rather do what Marco suggests and that is to bias the complementary pair with the common base. I think for the simpliest, a pair of diodes in between the bases, biased with a pair of resistors to V+ and V-. Like this, please see the attachment.

I will response to the rest after a lunch.



Marco:
If you use a string of 4 diodes instead and give all the transistors emitter resistors the biasing is more stable and you get current limiting for free (the transistor stage is actually a voltage to current amplifier, with those resistors the opamp swing limits the current swing).
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