What's this synthesising you talk of ?
AIUI, the OP intended to *voltage drive* the cable, not 'match' it. Note his
concerns about shorting the output. Not that 'matching' audio to 50 ohms makes any
sense in the first place.
I know that I often don't write clearly, but I thought I made this
clean in my OP. I mentioned "series matching resistors of 50 ohms"
which also help prevent damage with an output short.
Obviously I didn't explain my problem very well as so many people
don't seem to understand what my concern is. If the physics of
dissipating a watt on a small PCB cause a large temperature rise my
customer will have to deal with providing adequate cooling. But I am
responsible for adding the circuit and making it drive the cable
correctly.
The opamps I have found that will drive this line (100 mA) have a very
high output short current, typically much higher than this level. The
largest output resistor I can use is 50 ohms when driving a 50 ohm
load to 10 Vpp with these supply levels. This limits the output
current to 200 mA. But the power into this resistor with shorted
output is a full watt. The power in the driver in the two cases does
not differ much, 0.6 Watts into 50 ohms and 0.7 watts with shorted
output (and 50 ohm series resistor, of course). The series resistor
has to be *very* large to handle a full watt in the shorted output
case. I found a 2512 part rated at 1.5 watts, but it is hard to get.
I picked an 0805 part which is 0.25 Watts and I can use four of them
and still be smaller than a 2512. But this is HUGE on this small
board. I have four outputs and I was incorrectly thinking that I
needed four on each input. They, along with the other passives on the
board, uses up so much space on the back side (as well as around the
chips on the front side) that there is no room left for the vias
required.
I am well aware of the physics of heat generation and removal. The
current solution is not workable. One alternative circuit which would
meet my requirements (perhaps) is to use positive feedback to sense
the current in the output series resistor to, in effect, multiply the
series resistor value. This reduces the power in the resistor while
raising the power in the amp unless the supply voltage is reduced.
Using a factor of 5, the power in the now 10 ohm output resistor is
only 50 mW. Even shorted, the power is only 200 mW, so now a single
0805 part will do the job.
I could use a switcher to supply a lower voltage and reduce the opamp
power level, but I am not eager to do that, even if I could make it
fit.
So I am looking for a part better matched to the application. I think
I mention an audio part that was a perfect fit in every regard except
for the output current limitation of around 60 mA. But this is not
spec'ed at +-12 V supplies, so I will be simulating this morning to
see what it does with this power level.
Thanks for all the comments. Explaining it has really made me think
about it clearly.
