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Voltage boosting an opamp
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dom0:

--- Quote from: David Hess on December 31, 2016, 12:29:57 am ---I would like to see the application note where it originated.  The oldest reference I have seen is in the Tektronix 7A29 vertical amplifier from 1979 where it was used with 741s.

--- End quote ---

I looked at the 7A29 and I believe you are referring to the current sources feeding the input limiting bridge, A2, the circuitry surrounding U100?

The idea of that circuit is imho a bit different from the power/voltage extenders, because it seems to be mostly used to work around the limited output swing of the 741, which wouldn't allow for the ~10-11 V or so at the node of U300 pin 3 if you add the output follower that it would need to achieve that. So you can't use a follower directly, so we either have to shift level from the OP's output to get to a follower or we use a common emitter amplifier. But that we can't control anyway with the OP, because it never reaches its rails. So shifting that through the power rail makes sense. RT109/R109/R107/R113 configure U111/Q114 for a gain of -0.75 or so for ~11 V at U100 pin 3, and U121/Q124 is just a straight inverter working off that node for -11 V at pin 13 of U100; diode bridge current is then set by the resistors internal to U100.

Hm, guess I do agree after all, same idea, just a bit of a different application.
Yansi:
Could you please post the part of Tek schematic you're talking about?

...meanwhile, I made the PCB. So let's clear the mess on my bench so I can solder it.

dom0:
[ Specified attachment is not available ]
Yansi:
Now if it would just work at least a bit how it was intended...

Time got spared, so I am working like no tomorrow.  Testing within a moment.

//Unfortunately, I could not find any brand name BCP53. Of course, I found those from ONsemi right after I soldered this chinese junk on the pcb.

Yansi:
So, taking exactly the schematic in post #38 and testing it on a prototype pcb, yields  very promising results:

Bias current through the final stage settled to 1.25mA (I'd say close enough to my design goal of 1mA).
Even though a glacier melts faster than MCP6001T moves its ass, I get 140kHz (-3dB) bandwidth, test from full swing. (test pcb supplied with +-20V and +5V for the opamp). Which I think is quite impressive, considering no tweaking was done whatsoever.

So I think it will be good enough for 100Hz :)

//EDIT: I will test the output short circuit current later. I will also change the supply voltages to the desired +12 and -55V to see what it'll do.

//EDIT: Great! current limits to about 12.5mA. I have used a smaller R7, so das stimmt.
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