Another member mailed me with a question about using the current mirror to set the DC offset in this amp.
Obviously people are getting confused - usuall, simple explanations don't use the mirror's degeneration or take it into account but just the load and that seems to be easy to mix up.
The point is that Vbe of both BJTs is (ideally) equal but the current here is not set only by Vbe because the voltage drop on the degeneration resistor also has to be added in the equation for calculation of current. We can manipulate that voltage drop (by changing the values of emiter resistors) on both sides of the mirror.
The current in first branch will be determined by JFETs (i.e. their Vgs) but change of the value of voltage drop in emitter circuit of that branch of mirror will be reflected in the other branch beacuse the Ohm's law won't have it any other way.
If it still isn't clear, you'll just have to make your own tests or trust me that it works...
Obviously people are getting confused - usuall, simple explanations don't use the mirror's degeneration or take it into account but just the load and that seems to be easy to mix up.
The point is that Vbe of both BJTs is (ideally) equal but the current here is not set only by Vbe because the voltage drop on the degeneration resistor also has to be added in the equation for calculation of current. We can manipulate that voltage drop (by changing the values of emiter resistors) on both sides of the mirror.
The current in first branch will be determined by JFETs (i.e. their Vgs) but change of the value of voltage drop in emitter circuit of that branch of mirror will be reflected in the other branch beacuse the Ohm's law won't have it any other way.
If it still isn't clear, you'll just have to make your own tests or trust me that it works...
Thank you guys, I failed to recognise the Wilson current mirror here. I did eventually figure out how it worked but would have been a whole quicker had I made the connection in the first place.
Ian
Ian
Thank you Kasey - always nice to be missed 🙂
Actually I haven't so much been away as separated from my projects due to everything being in storage until I finally move house in the next month or so. I regularly peruse this forum but can't actually build anything right now.
Actually I haven't so much been away as separated from my projects due to everything being in storage until I finally move house in the next month or so. I regularly peruse this forum but can't actually build anything right now.
Will be a bit longer before I can come back on this,
Been away for a few days and caught a bug that I can't get rid of.
My mind is all mixed up. Judging by the typing mistakes, I will be a while before I can think clearly.
Been away for a few days and caught a bug that I can't get rid of.
My mind is all mixed up. Judging by the typing mistakes, I will be a while before I can think clearly.
Oh boy....
No Vogon poetry from me. Just a small quote about classic - Henryk Sienkiewicz, In Desert and Wilderness.
No Vogon poetry from me. Just a small quote about classic - Henryk Sienkiewicz, In Desert and Wilderness.
Actually, it was a prose literature quote. I had to resort to that kind of answer - i know that you know what caps in feedback loop do, and even if you didn't, it's written in post #1 (third paragraph from the bottom, last sentence).
You mean more output pairs, I suppose.
200W at 2R means about 14A_peak per channel and for that we'll need 4 output pairs and maybe a follower stage in between (building and testing a prototype is needed to know for sure).
Of course, massive upgrade of the PSU is inevitable.
200W at 2R means about 14A_peak per channel and for that we'll need 4 output pairs and maybe a follower stage in between (building and testing a prototype is needed to know for sure).
Of course, massive upgrade of the PSU is inevitable.