Follower current source?

Question

I need advice on a problem I'm having

I need a follower to transmit an AC signal into a low impedance load which is the cathode of a triode but I also need it to be an adjustable current source to the load.

Unfortunately normal current sources have gain, I need a unity gain current source so the AC signal is not amplified.

The ac signal voltage is about 20v and the DC voltage levels are from 0 to -300v and I have access to 300v apex opamps if needed but I'm a novice at circuit design so I'm not sure how I would design it.

Any ideas?

Edit: Sorry I think people are getting confused.

This is a more accurate depiction of what I want to do

Ignore the opamp.

The 1uf capacitor carries the audio signal.

The cathode presents an ultra low impedance so I need the source follower for a buffer.

I want the audio signal to pass through the follower untouched but I also want the follower to provide a constant current to the load.

Imagine the 150k resistor as the current source bias. The problem is figuring out how to replace that fixed resistor with a variable current source bias.

Answer 1

Maybe a current mirror, depending on how much current you need. A current gain of 1 is pretty reasonable if you just get two of the same FET to use in the mirror; although depending on tolerance it could vary between .5 and 2-ish. You could use power MOSFET's in the mirror. current-mirror-wiki

Example schematic below: It is for positive voltage, but topology should work if you change out PMOS and VDD/GND for GND/VSS EDIT: I am trying to interpret your request. But if you are trying to control the current using some 0-20V source, or a knob, or a combination of the two, this may work for you? Also assuming you want the 300V AC signal to be the supply to the load. Because yes, in this case when the AC sine voltage decreases to 0V you will not have any voltage across R3. But in this case you need to use an AC-DC converter.

^{simulate this circuit – Schematic created using CircuitLab}