Small signal RF amplifier ideas?

Some ideas I hear about RF amps are to decouple each stage with a series resistor and a cap across the stage. The resistor is chosen to drop a small voltage and the cap to have a low reactance. Next, use a low noise device in the first stage and also separate the stages across the board to avoid positive feedback. Another trick is to align any tuned inductors at 90 degree angles to minimize feedback. Also use double sided PB board to provide a good ground plane. Another trick is to shift the phase 180 degrees with a unity gain stage to further reduce feedback if necessary depending on layout. Is there some limit to the overall gain of a small signal RF amp, or does it depend on the noise generated by the circuit itself? I don't have much experience with low noise devices. What devices are recommended (transistors, op- amps)?

-Bill

Reply to
Bill Bowden
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One commonly-heard metric is that an amp or filter will have a hard time if you expect more than about 60 dB isolation per inch of case length.

Supply coupling is one really serious source of instability. Lots of folks don't realize what a few nanohenries of ground inductance does to destroy the effectiveness of decoupling networks, or that a a few femtofarads of distributed capacitance between stages can cause instability. (A 40-dB, 50 ohm amp with 10 fF of coupling between its input and output can easily oscillate anywhere above about 3 GHz, for instance.)

You can make grounding systems with inductances down in the hundreds of picohenries, but that doesn't happen by accident, and it's really hard to go lower than that.

So if you try putting a nanohenry or two in series with all your decoupling caps, you'll get a general feeling for what decoupling can really do.

Cheers

Phil Hobbs (Who has been getting his face rubbed in some of these issues recently.)

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

You can get opamps that have decent gains over 1 GHz.

But the real steal these days is InP or SiGe MMICS. These are 3-terminal wideband amplifiers that are easy to use, very stable, and cheap.

Look at the MiniCircuits ERA series, like ERA5 maybe. There are tons of similar parts around, with gains like 10 to 20 dB and bandwidths from 2 to 10 GHz.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    
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John Larkin

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