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| HV amplifier: putting a common-source (emitter) amplifier on opamp outputs |
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| Berni:
The easiest way of doing this is using a high voltage opamp like the LTC6090 to drive a complementary transistor follower stage to give the output the required oomph. The datasheet for the LTC6090 even has a example schematic for that on page 20: https://www.analog.com/media/en/technical-documentation/data-sheets/6090fe.pdf You can probably also use a pair of BJT transistor in there too if you wanted, but for continuous operation you might need some very large transistors to have them survive all of the heat generated inside of them. Perhaps running multiple of these in parallel might be a good idea if the transistors get too hot. But for pulsed operation you can push a ton of power trough just a single transistor. Indeed follower configurations are inherently more stable because they have a built in negative feedback loop. If the load on a follower suddenly disappears this causes the output to jerk upwards but in doing so also rapidly reduces the voltage on the base/gate and so the transistor shuts itself off without the opamp controlling it having to do a single thing. This makes such follower stages very stable, especially when driving weird loads that are very inductive, capacitive or non linear. But on the down side is that they don't provide any voltage gain so you need to provide them with a high voltage low current input signal. This can be done with a high voltage opamp like i suggested above, or using a low voltage opamp with an output transistor as you have shown, but use that transistor to only boost up the voltage, but let the follower stage after it actually drive the full output current. Oh and physical layout of these sort of amplifiers can also be quite critical or you can accidentally end up with an oscillator instead. |
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