On Nov 2, 10:01 pm, "numeric" wrote:
Looking for a transistor suggestion, for an RF receive pre-amplifier to
cover 10Hz to about 60Mhz. So far, I have found a pair of 2N5109B bipolar
transistors, which work ok in a grounded base push-pull design. These
transistors provide a gain of about 12db over an input voltage range of 1
microvolt or less to over 4 volts. These transistors were designed many
years ago; hopefully better transistors now exist which can provide superior
linearity and better isolation form input to output. In addition, a spice
model must be available.

Thanks.

Some of the best transistors for that sort of application are those
designed for CATV amplifier service, where low distortion and low
noise are important. Careful and creative circuit design can help a
lot, too.

Your 4V input suggests you get 16V out at a gain of 12dB. Is that all
at 50 ohms, and RMS? If so, that's about five watts output.

Have you measured IIP3 for your circuit? If so, what sort of
performance do you get? What sort of noise figure? Can you share
more details about the circuit?

Do you really mean input-to-output isolation, or output-to-input
isolation? Normally, I'd consider +12dB in the forward direction to
be poor "isolation" but in a direction I didn't really want isolation
anyway--I want gain that way. You should be able in a push-pull
circuit to add parts in a cross-coupling manner to lower the output-to-
input isolation considerably, if that's what you are after. You can
also run each side cascode (or in your case more literally, grounded-
base into grounded-base...), and that can help the linearity and noise
(as the input stage will run at lower voltage drop and thus run
cooler), as well as the isolation.

I asked here not all that long ago about ideas for a low-gain, low-
noise preamp with high IIP3 (and of course even higher OIP3) -- my
mention of +55dBm as an IIP3 goal met with some disbelief (and one
very respected suggestion for a way it could be done with some parts
no longer readily available), but my thought is that it should be
possible, given a fairly high power dissipation. Some of the modern
op amps are getting close to being able to do it, at least through HF
(30MHz); not to 5 watts output though. Not sure why you need that in
a receiver. ;-)

Cheers,
Tom