Electronics > Projects, Designs, and Technical Stuff
Voltage boosting an opamp
David Hess:
--- Quote from: Yansi on December 31, 2016, 12:39:57 pm ---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.
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The cascodes are added to the current source and output transistor to allow high voltage operation with fast transistors and eliminate the effect of Miller capacitance which would otherwise limit high frequency performance. They could be added to Q3 and Q4 for the same reason but with such a slow error amplifier, they will not be necessary.
In your circuit, I would add small value emitter resistors to Q1 and Q2 to better control the idle current of the common base input stage whether 2 or 4 diodes are used.
Yansi:
Sure, that is also one of the clever methods of improvement some members suggested me when I was designing the regulated high voltage power supply - ie not to drive the high voltage power mosfet gate directly from an opamp, but to use a cascode with a smaller suitable low voltage BJT.
Yeah, shall add small emitter resistors there to be on the safe side.
Zero999:
--- Quote from: Marco on December 31, 2016, 01:38:31 pm ---
--- Quote from: Yansi on December 31, 2016, 12:39:57 pm ---Marco: Current limiting of what will I get?
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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.
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Very good, except it doesn't have any voltage gain!
Increase the values of R4 & R3.
Yes it does. It's just the common base stage doesn't. The preceding common emitter stage does have plenty of gain.
The circuit only needs to be able to provide 1mA so all the resistor values can be higher.
As the output voltage swing is +5V & -50V, the bandwidth is low and linearity isn't important, then the gain could even be asymmetrical, with more gain on the negative side than the positive.
Zero999:
The power supply doesn't have to be +/-55V. +15V & -15V will do.
f5r5e5d:
why complicate the output?
you really only need SE Class A with 2 Q and simple bias:
the Q1 LED biased ccs on the negative rail
the Q2 folded cascode on the positive end, which is just another simply biased from Vsource(s) ccs and a R to the op amp output that "steals" some current
more "interesting" is how to compensate which really, really needs a load Z spec - with the cascaded gains of op amp and any V boost stage there will always be some C_load that will be unstable
my sim compensation is totally eyeball guesstimation and a few repeated sims looking at overshoot, 1p to 1n Cload - with a load spec simplified loop equations more optimum compensation could be found
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