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I would like to switch between two drains resistors in a high voltage (300 VDC) audio LND150 gain stage. I opted for a BJT instead of a MOSFET, because the MOSFET parasitic capacitance was too high and thus reduced the bandwith of the stage. Here is the idea: enter image description here

It is basic but it should work.

The problem is the switching transient, it will induce noise at the output of the circuit.

I would like to "soft switch" the BJT to limit switching noise. I thought to ramp on and off the base current of the BJT in about 200 or 300 ms but I don't find how to implement it with discrete components.

Could you suggest a circuit? Or if you have an other idea to switch the drain resistor, keeping in mind that V+ is about 300 VDC?

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You can place a capacitor across R6, maybe with a small series resistance, to avoid current spikes. Here's a mockup in LTspice:

mockup

A larger cap will mean a larger transition, but also a longer delay. Play around, see what fits. This is one (cheap) solution.

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