Depletion mode B1

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Since I had a bunch of DN2540s left over from another project (Laser diode drivers, if you must know), I've been looking for a good project for them.

So I decided to try a version of the B1 follower using them. It's very similar to Nelson's version.. I only changed component values to suit parts I had on hand. I wanted to build a single channel mock up first, and if it behaved reasonably, then make a proper 2 channel version. I'll probably try it out as a headphone amp.

i567m.jpg


I tests out pretty well. The simulation shows rising response around 1 MHz, but in real life, it starts to fall off around 150 kHz. Also, the sim says it has some 50 mA bias, but as built, it has 100 mA. I see 2V across R1. So much for simulation.

The response is flat from 10 Hz (as low as my sig gen goes) up to about 150 kHz. I'm not going to listen to it until I have two channels built properly, but looks very promising. I'll be making some caps larger, and adding a pot at the input.
 
I have a theoretical question about the output impedance: In a circuit such as the B1, or the version I posted above, what is the relationship between the output impedance, and the bias current? Or stated otherwise, in such a circuit, how do I compute the output impedance?
 
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