Small signal RF amplifier ideas?

[Bill Bowden]

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

 

[Phil Hobbs]

On 4/24/2013 8:53 PM, Bill Bowden wrote:

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

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.)

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[John Larkin]

On Wed, 24 Apr 2013 17:53:57 -0700 (PDT), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

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

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.



--

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

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