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Voltage boosting an opamp

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Yansi:
Marco: Current limiting of what will I get?  The current through the common base stage is limited by the 10k  resistor. Moving the base up a diode or two won't change much, nor will it add current limit to the output stage. Or have I missed something?


David Hess: The analog scopes I know and I have worked with, were mostly fully discrete and used cascode stages, using a small signal NPN on the bottom, and a high voltage one on top of it. I have never looked through the Z input stages, but have some schematics still printed out here, so will take a look.

Here's what I have come up with. Added current limiting for the output stage. Little biasing for the common base stage to minimize the emitor voltage "wiggle" between the Q1 and Q2 opening.  Also changed the feedback network by adding the 100ohm series resistor on the output - it should make it stable into capacitive load, while maintaining precise DC voltage accross load. (that also is a response for f5r5e5d about the load spec) What do you think about it?


Marco:

--- Quote from: Yansi on December 31, 2016, 12:39:57 pm ---Marco: Current limiting of what will I get?

--- End quote ---

Output current. As I said, the transistor stage is a V to I converter. If you add emitter resistors it's a pretty linear V to I converter in fact, so with a limited voltage swing you get a limited output current swing. In the case of my pic you get about 10 mA per Volt.

Yansi:
In that case the current limit is NOT because of adding the diode bias.  The output current is limited due to adding emitter resistors for the output transistors - which of course can be done also in my schematic, even in the cropped original one from the application note.
But you haven't said anything about emitter resistors, did you? That's why I haven't understood you.

But thanks for the tip, I can eliminate out the Q5 and Q6 from my last schematic and limit the output current by choosing a suitable resistors in the rest of the schematic.

EDIT: Doh! Silly me! I read wrong. U said give all the transistors emitter resistors... then you were of course right!

Brutte:

--- Quote from: Yansi on December 29, 2016, 09:25:27 pm ---The problem is, for +5-50V output I'd need an opamp designed for at least 60V supply operation.

--- End quote ---
with single power supply.
The voltage sink/source always has only two terminals. So to build 4-quadrant one you either need a +5V:-50V push-pull tied to global GND or you can use two push-pulls not referenced to GND (floating, supplied from separate DC source). The advantage of the latter is that the voltage effectively doubles but the supply must not be GND referenced (your op-amp power supply must not be earth'ed). Then a raw LM224 would do.
If #1 outputs 0V and #2 outputs 30V then you get +30V and when #1 outputs 30V and #2 outputs 0V then you get -30V. Just add a fixed -25V offset between GNDs and instead -30:+30 you get -55V:+5V

The 35V limit only applies to Vcc-Vss

Yansi:

--- Quote from: Brutte on December 31, 2016, 02:08:51 pm ---
--- Quote from: Yansi on December 29, 2016, 09:25:27 pm ---The problem is, for +5-50V output I'd need an opamp designed for at least 60V supply operation.

--- End quote ---
with single power supply.

--- End quote ---

Single or not, +5-55V is the same 60V accross opamp as +-30V or whatever else combination.

Floating load is not an option, I need the output referenced to ground. Floating the power supply is also not an option, because the design as a whole will need at least two more floating supplies for some other circuitry, really not a big fan of making stuff with too much complex power supplies.

I do not understand the rest of your comment, as all circuits posted above will work in 4 quadrants, both polarities of voltage and current.

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