I need advice on a problem I'm having

enter image description here

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 enter image description here

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.


1 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

  • \$\begingroup\$ Unfortunately the AC signal ruins the current source via that method, as well as any method of taking the output of the current source at the drain. I need a source follower current source I think, but I have no clue how to make one. \$\endgroup\$
    – coinmaster
    Apr 8, 2016 at 1:04
  • \$\begingroup\$ If the input signal varies from 0 to -300 volts, and you want a current as an output, then what do you mean by gain? How much current do you want when the input is -300 volts (I assume you want 0 current when the input is 0 volts). What is the load impedance? What is the adjustment range you need? You can see that your question is not well defined. Until it is, a suitable answer is not possible. \$\endgroup\$
    – Barry
    Apr 8, 2016 at 2:02
  • \$\begingroup\$ The input signal is only about 20v AC, the DC bias to the load is between 0 and 300v. The load impedance is about 300 ohms. I need to adjust the current into the load without altering the AC signal. \$\endgroup\$
    – coinmaster
    Apr 8, 2016 at 2:35
  • \$\begingroup\$ The load is a 6080 vacuum tube. The current range is between 0 and 100ma. \$\endgroup\$
    – coinmaster
    Apr 8, 2016 at 2:45

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