ajparent1/kb1gmx writes:
Paul,
For the case where the desired signal is audio and the DBM source is
a Common base amp is the desired case. For a mixer to RF then
other possible choice may have favor.
The OP was primarily addressing audio, such as a DBM being used for a direct conversion receiver (or maybe the product detector in a superhet). Though, I have little experience building direct conversion receivers, mostly superhets.
The KK7B form is Common Base with a emitter current of about .5 ma.
That will present a wide band load of about 50 ohms to the DBM and with
a 3300ohm collector resistor would net a voltage gain of about 66.
Jim Kortge, K8IQY, also used common base IF amps in his original famous 2N2/40 rig here:
On sheet 2, the Q5 CB amp matches the DBM to the XTAL filter impedance of 470 ohms, and the Q6 CB as an IF amplifier. Nice approach.
On my common base amps (for RF/IF), I also set Ie at .5mA to set Rin at 50 ohms and usually use a 1.2-1.5K collector load to limit gain to around 12dB for stability. Or, a toroid in the collector (like 10-12TP:3TS) if driving a 50 ohm DBM. I've also had good luck using the internal output transformer in a DBM as the emitter source for the CB amp by swapping the IF and RF ports on the DBM, such as the ADE series (the transformer is the RF port). Instead of grounding the bottom of that RF port transformer winding, that goes to ground through a resistor to set the emitter current. One of those ADE-1 ground pins is the bottom end of the RF port transformer.
the reverse isolation (S12) tends to be very good.
Isolation is certainly another advantage of incorporating an active stage following the mixer. It can tame an unruly leaking LO if that is a problem. On the ADE-1 (and most DBMs), the LO-IF isolation is 50-55dB and the LO-RF isolation around 60-65 dB, another advantage of using the RF port for the IF output.
Common emitter needs to run at much higher currents for a solid 50 ohms
in and the output must drive a 50 ohm load. The down side is with
resistive feedback you get noise. IF this is a DCRX or Image rejecting
DCRX that undesired added noise.
Or use a MiniCircuits 50 ohm MMIC if you have 50-100 mA to spare! :-)
A good discussion, Allison. A nice little tutorial and should give plenty of fodder to consider using either a common emitter or common base amplifier for matching to 50 ohms. There is no perfect solution, but lots of ways to skin a cat that works.
72, Paul NA5N
