J
Joel Kolstad
- Jan 1, 1970
- 0
Hi guys,
I was wondering if you might take a look at this schematic here:
http://oregonstate.edu/~kolstadj/amp.gif
It's the beginning of a class AB audio amplifier (ostensibly for around 50
watts), and I had a few questions...
-- Q2/Q3 and Q1/Q4 effectively make up 'compound' transistors, but this is a
technique that I hadn't seen until recently. Are there any rules of thumb
to choose the resistor sizes (R8 and R9)? I'm finding that making them
smaller can significnatly reduce the quiessent current through the power
transistors -- this occurs before a smaller resistors raises Vbe of Q3 or Q4
to more closely match the biasing set up by D4 and D2. Hmm... perhaps I
should choose R8/R9 to have the same current through them as D2/D4 do?
-- I did start out just using two transistors in a Darlington configuration
rather than the compound transistor approach shown. The drawback seemed to
be the additional ~0.7 volts lost in 'head room.' Does this configuration
have any additional benefits?
-- How would you go about deciding the quiessent current through the
R2/D4/D2/Q5/R3 leg? In classes I've taken on circuit design, usually it's
dictated by frequency response, which doesn't seem to be much of an issue in
an audio amplifier. I did discover some of the tradeoffs between that leg's
gain and the output swing -- as gain is reduced (R3 becomes closer to R2),
the negative-going output will saturate more quickly since there's a larger
voltage drop across R3 and therefore the base of Q4 approaches Vee sooner.
On the other hand, higher gains with smaller R3's make biasing Q5 more
sensitive.
-- Any of suggestions would be appreciated. Thanks!
---Joel Kolstad
I was wondering if you might take a look at this schematic here:
http://oregonstate.edu/~kolstadj/amp.gif
It's the beginning of a class AB audio amplifier (ostensibly for around 50
watts), and I had a few questions...
-- Q2/Q3 and Q1/Q4 effectively make up 'compound' transistors, but this is a
technique that I hadn't seen until recently. Are there any rules of thumb
to choose the resistor sizes (R8 and R9)? I'm finding that making them
smaller can significnatly reduce the quiessent current through the power
transistors -- this occurs before a smaller resistors raises Vbe of Q3 or Q4
to more closely match the biasing set up by D4 and D2. Hmm... perhaps I
should choose R8/R9 to have the same current through them as D2/D4 do?
-- I did start out just using two transistors in a Darlington configuration
rather than the compound transistor approach shown. The drawback seemed to
be the additional ~0.7 volts lost in 'head room.' Does this configuration
have any additional benefits?
-- How would you go about deciding the quiessent current through the
R2/D4/D2/Q5/R3 leg? In classes I've taken on circuit design, usually it's
dictated by frequency response, which doesn't seem to be much of an issue in
an audio amplifier. I did discover some of the tradeoffs between that leg's
gain and the output swing -- as gain is reduced (R3 becomes closer to R2),
the negative-going output will saturate more quickly since there's a larger
voltage drop across R3 and therefore the base of Q4 approaches Vee sooner.
On the other hand, higher gains with smaller R3's make biasing Q5 more
sensitive.
-- Any of suggestions would be appreciated. Thanks!
---Joel Kolstad