J
Joel Kolstad
- Jan 1, 1970
- 0
So here's the current schematic: http://oregonstate.edu/~kolstadj/amp3.gif .
I replaced the 'resistive' current source with a 'real' one and added a
potentiometer for tweaking the bias. (Somehow I prefer two diodes and a pot
to a Vbe mutiplier.)
It's certainly getting a lot better, I think. Remaining questions:
-- The voltage gain seems a little difficult to predict correctly. The
current source should have an output impedance of gm*Ro*R2, and with
gm*Ro=Va/Vt, and the data sheet saying a 2N3906 has an Early voltage (Va) of
40V, I get 40/26m*56=86k. This would predict, then, a gain of ~86k/100 (R3
being the 100 ohm resistor), or 860. SPICE simulations show a gain of ~325.
Not that I'm too wirred -- any gain over about 10 is fine with me.
-- Biasing Q5 correctly is annoying, involving tweaking the exact valve of
R7. Even with a potentiometer, I doubt this is a reliable way to build the
actual circuit? (I am planning to physically build this thing shortly.) I
could set up a current mirror off of Q6 to bias Q5, but I'm thinking that
now's the time to replace the input with a differential pair so that the
feedback will automatically bias Q5 appropriately, and it's only a small
number of additional transistors as well.
-- I still don't see where The Art of Electronics gets their '20%' number at
the bottom of page 93 from.
Thanks for all the help so far!
---Joel Kolstad
I replaced the 'resistive' current source with a 'real' one and added a
potentiometer for tweaking the bias. (Somehow I prefer two diodes and a pot
to a Vbe mutiplier.)
It's certainly getting a lot better, I think. Remaining questions:
-- The voltage gain seems a little difficult to predict correctly. The
current source should have an output impedance of gm*Ro*R2, and with
gm*Ro=Va/Vt, and the data sheet saying a 2N3906 has an Early voltage (Va) of
40V, I get 40/26m*56=86k. This would predict, then, a gain of ~86k/100 (R3
being the 100 ohm resistor), or 860. SPICE simulations show a gain of ~325.
Not that I'm too wirred -- any gain over about 10 is fine with me.
-- Biasing Q5 correctly is annoying, involving tweaking the exact valve of
R7. Even with a potentiometer, I doubt this is a reliable way to build the
actual circuit? (I am planning to physically build this thing shortly.) I
could set up a current mirror off of Q6 to bias Q5, but I'm thinking that
now's the time to replace the input with a differential pair so that the
feedback will automatically bias Q5 appropriately, and it's only a small
number of additional transistors as well.
-- I still don't see where The Art of Electronics gets their '20%' number at
the bottom of page 93 from.
Thanks for all the help so far!
---Joel Kolstad