Electronics > RF, Microwave, Ham Radio

Common gate wideband RF amplifiers

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G0HZU:
Based on my s-parameter measurements here's a plot of GMAX and K for a single CPH3910 at 10Vds and 15mA. You can see that K dips below 1 even with a directly grounded gate pin. If the inductance of the gate connection goes up to just 1nH see the second plot for what happens to K up at UHF. I think great care is needed with this device to manage stability. It looks like this device can oscillate up above 2GHz.

Gerhard_dk4xp:
Now that you have measured the s-parameters, would you share them?
It would remove the need for me to do that over the holidays.

I found that an array of 16 ON CPH3910 oscillates happily at 650 MHz,
working common source into a bipolar cascode. And every gate has a
different AC voltage, even though they are connected by relatively short
traces; 2.4 GHz scope including active probe
(16 pcs. in par for voltage noise reasons)
The parallel lines are an efficient  coupler. It took a ferrite bead in each
gate and a 0603 cap at each drain to kill that. ~2nF in total until it began
to impair my 2 MHz baseband bandwidth. (cascode is low impedance)
I admit it could take less, but I was fed up with oscillations.

BTW NSVJ3910 is quite repeatable. Cannot be compared to IF3601 etc.


G0HZU:
Hi Gerhard
I only have CPH3910 s-parameters in common gate so far. If you let me know the operating point of your CHP3910 in common source I can measure a set of  s-parameters for you. Note that my test setup only supports up to about 15mA current. I should be able to do up to 20V at 12mA or maybe 15V 15mA but that is about the limit. I do measure the dc operating point very accurately, usually to <0.5% using a pair of DMMs. What is the BJT you use in the cascode and what is the dc operating point?

Gerhard_dk4xp:
45 mA/ 3V for 16 FETs. The Cascode is folded, with BC860.
I will probably unfold it since I need more than 2+2 Li batteries anyway.
Bandwidth is only 1Hz to 2 MHz but the 3910  is much faster.

Don't measure it when you don't need it yourself. My ZVB8 has built-in
bias-tees, so it is no effort with the test circuit.

73, Gerhard

G0HZU:
OK thanks. 45mA is way too much for my test fixture so probably best you measure it with the ZVB8.

I had a go at doing a practical LNA design using the CPH3910 s-parameters today. The aim was to make a low noise preamp for the 88-108MHz band. See below for the simulation based on the s-parameters at 10Vds and 10mA. The result was quite good in terms of passband gain and port match across 88-108MHz. I'd expect it to achieve a noise figure of about 3dB but with an L match at the input this could probably achieve 2dB noise figure with a bit of input mismatch.

I'd expect the OIP3 to be better than +20dBm so this would be a cheap LNA for the 88-108MHz FM band with 13dB gain and fairly good noise and signal handling performance from just 10mA.

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