I have spent some time the last few days researching and redesigning the
preamp on Larry's - W7IUV website. (see
http://home.earthlink.net/~w7iuv/w7iuv/flag.htm) It uses the 2N5109
transistor to provide the gain.
I changed the circuit from a single ended design to a push - pull
arrangement using the same topology only different input and output
transformers for differential input (balanced). The transformers are wound
1:1 for the moment to provide a ~ 50 ohm input for testing purposes. The
final design will have a 16:1 input transformer to be used on a topband
pennant antenna array I am building this fall.
After building it using the circuit as presented, I placed the arrangement
into a test fixture using an HP-8690B sweep generator and an HP-8552/8553
spectrum analyser. I am fairly happy with the design. Roughly 20 db of
gain from 4 to 30 mhz. It starts to roll off after 30 mhz and below 4 mhz.
I am puzzled why the coupling caps and the emitter bypass caps were chosen
to be .01uF. On topband, the .01uF has a reactance of almost 9 ohms. I've
changed the coupling caps from .01uF to .1uF but left the emitter bypass
caps thinking that it was intended to make the preamp somewhat deaf on the
AM broadcast band for obvious reasons (IMD).
At 1.8 Mhz, the preamp provides about 15db of gain. Distortion products
begin to develop at about 0 dbm (1 mw) input with about +15dbm output.
Hooking the preamp to my xmit antenna, which is about 2dbi gain on topband
and provides S-9 +40 on the AM band, I see that the amplifier is able to
take the heat from the AM band without introducing IMD on topband. I am
tempted to swap out the .01uF emitter bypass caps for .05uF or even .1uF and
just run it full wide band. Just temping in some .1uF caps shows that I
will end up picking up the additional 5db of gain on topband. My gut tells
me that hooked to the pennant, the amp should still perform IMD free.
Has anyone had experience with this amp design and did they also see the
gain roll-off below 4mHz?
Thanks for the input
73s
Ford-N0FP
ford@cmgate.com
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