Hello all, I'm trying to put together a 50 MHz preamp for a
transverter project I'm building and I can't seem to get much gain
out of two separate designs I've tried. I built AA3SJ's amp that's
on the front end of his transverter (http://www.qsl.net/aa3sj/Pages/
50MHz-Xverter.html)
and I even tried Hamtronics' design (http://www.hamtronics.com/pdf/
Manuals/LNK%20Preamp.pdf)

Does anyone have a proven design for a VHF preamp using BF998's or any
help? Thanks.

Curt, K8AI

On Feb 11, 5:38 am, wrote:
I can't seem to get much gain
out of two separate designs I've tried. I built AA3SJ's amp that's
on the front end of his transverter (http://www.qsl.net/aa3sj/Pages/50MHz-Xverter.html)

That design looks OK to me, though I haven't tried calculating the
resonant frequency of the tuned circuits. Do you get nice resonant
peaks in gain when varying input and output caps ? The 2 pF output
coupling capacitor is intended for a very high impedance load (the 2nd
BF998/mixer) so may not work for a 50 ohm load if that is what you
need. I would probably try a one or two turn link on L2 for output
coupling to 50 ohms.

and I even tried Hamtronics' design (http://www.hamtronics.com/pdf/Manuals/LNK%20Preamp.pdf

Very similar design except it seems the output is untuned and very
heavily loaded (by the large coupling capacitor C6), and has a 47 ohm
resistor in the output for stabilization.

The BF998 has good gain past 1 GHz. For stability, it is probably
necessary to design this like a microwave circuit: very short leads,
use only chip capacitors for bypassing (and chip resistors, at least
AA3SJ's R3 and Hamtronics' R4), double sided circuit board with full
ground plane, ground the source lead of the package within 0.1" of the
body of the BF998, either directlly to the ground plane or to a top-
side pad which is grounded with at least two wires or eyelets or
plated through holes to the ground plane, within 0.2" of the BF998.

I assume you haven't got gates 1 & 2 reversed ? (input to gate 1, 3 or
4V or so bias to gate 2)

Other things to check:
-is the gate 2 bias voltage correct (roughly 3-4 V) ?
-is the drain current on the order of 10 mA ? (that is where the BF998
noise figure is specified)
-are the inductors used exactly as specified ?(absolutely must be
either toroids or in shield cans to avoid feedback, I think)
-is the amplifier oscillating ? (look for RF voltage on the output or
hold an antenna connected to a frequency counter or spectrum analyzer
up to the coils, or look to see if the supply current jumps when you
tune the input or output.)

Good luck. I'd be interested to hear what works as I've just acquired
some BF998s to play with !

73,
Steve VE3SMA

Thanks for the help. Well, I can see now on the spectrum analyzer that
it's oscillating. I also found that the input wasn't tuned right. I
now get around 20 dB or so
of gain but I can't seem to get rid of the oscillations. I put a slug
tuned inductor (.3uH
7-1/2 turns) on the output in place of the toroidal inductor (9t on a
T30-6) but tuning it doesn't do anything.

In this version of the Hamtronics amp, I'm using 1206 size R's and C's
and the BF998
is a SOT style package. Everything is close to the device. I tried
putting a shield made of sheet brass straddling the G1 lead, going
across the width of the board, I tried changing the R on the drain
lead and I put a SMT ferrite bead on the drain lead but it's still
taking off. I built this amp on a single sided board - could that be
my problem? Should I have built this on a double-sided board with
vias?

In the meantime, I'll try to build one on a double sided board and
drill some holes
and wire the grounds to the bottom side.

Thanks much for the help. If I get it working, I'll post some pics and
a schematic.

Curt, K8AI