I am already worried ;-)
Maybe an op amp is not the best solution. BTW, I only need to match
one frequency at a time. Not a range, as I understand this is not
practical. The aim is to maximize power transfer.
Disregarding the 450 ohm figure, what is the recommended technique and
signal source for matching a signal to a reactive load? More
specifically, one that has both resistance and capacitance in
parallel.
Steve Bower
It would help, as others have suggested, if you could state your
problem in terms like "I want to deliver xxx watts of power to a load
that looks like yyy ohms in parallel with zzz pF. The waveform from
the source is a square wave at 100kHz, and I (do/do not) care that the
voltage across the load looks pretty much like a square wave. As a
source, I have a power op amp that's rated to deliver up to qqq volts
peak, at up to rrr amps." If you can put the problem in terms like
those, I suspect we'll be able to give a lot more specific answers.
Another couple examples of common situations where you do NOT want to
maximize "power transfer":
--When you plug a load into a standard wall outlet, the source
resistance of the power supplied through that outlet is generally
under an ohm. You'd theoretically get maximum "power transfer" if you
put a load on it equal to the source resistance. But you'd drop the
voltage to half the open-circuit voltage for an instant before the
circuit breaker tripped.
--An audio power amplifier is typically designed to deliver its power
into loads around 4 to 8 ohms, even though its output impedance is
almost always a small fraction of an ohm. If the amplifier didn't
have built-in current overload protection, you could get a lot more
power out of it if you put a much lower load resistance on the output--
but only for a short time till the output stage went into melt-down.
You may save yourself a lot of trouble if you put your circuit into a
program like the free LTSpice. You'll be able to check current and
voltage waveforms in an instant, and try all sorts of different
matching networks till you get something you're happy with. You can
even check power dissipations--in the load, in the op amp.
Cheers,
Tom