Eeyore a écrit :

Jim Thompson wrote:

wrote:
"Winfield Hill" wrote in message
Eeyore wrote...
In response to various posts please find attached a mic amp
which has seen volume production in commercial equipment.

Feel free to ask any questions / criticise whatever.

Content-Type: image/jpeg; "mic amp.jpg"
It's nice to see the diff-pair currents going directly to
the output opamp, with it's 6.8k resistors. But why'd ya
steal away loop gain so aggressively by choosing 680-ohms?
Now theres no resistors, the diff voltage input to the op amp is always
zero, so theres no diff voltage accross the 680r so they dont have an effect
on
diff voltage.
Sno-o-o-orkle! Does not!

...Jim Thompson

Do you have some bizarre new theory of op-amp operation ?


Your feedback theory is bizarre.

For a TL07x family device Avol @ 1 kHz for example is 4000.

Yep.

For a 1 volt rms output the differential input voltage at the op-amp inputs will
be 1/4000 volts = 250uV.

Yes, and don't forget that this make your output error 250uV*11=2.75mV,
which is 11 times worse than necessary and that you'll obtain with a
current source load.
I said you that was simple. But you'll probably find this difficult as well.

Oh yes, I know. Just posturing.

Maybe you should reread about loop gain, and the difference between CFB
and VFB.


That means 250uV/2x680R of signal current is 'wasted'
= 184nA.

Graham



--
Thanks,
Fred.