Parallel Transistors
I have seen several different pictures and schematics of some small signal transmitters that are using parallel NPNs to get 500-900mW of power. I keep running into these examples and they make me wonder.
I decided to try to put it into a simulator to see what it looks like.
Modeling:
This circuit is running in class C. The transistor base needs to be in 1.5-2 voltage
range which is between +10 to +15 dBm range with +20 dBm looking like the upper limit
as it starts getting current hungry in a hurry. I played with some different combinations
of resistors in the voltage divider and adding some DC bias to the base. The DC bias
could make it move into a different class beyond class C but the circuit got extremely
current hungry which makes sense. (Class C is fine for a CW rig with some good low
pass filtering.)
Plan:
I think that I will draw up this circuit in TinyCad and then move it into FreePCB
to eventually make printed circuit board. Once the board is etched and the parts are
stuffed I will measure and document it. I will then try to remodel this in software
(either in the Java based simulator or LTSpice) to see how the physical model and
software model compare.
Summary:
In the end this is a gimmick design using small signal parts like a PN2222 (2N2222A)
and/or a 2N3904 to push out some real power. It likely would be more productive and
stable to move this to a push-pull design and evaluate it based upon if it needs to
be in class AB or C depending it's intended use. The other option beyond a push-pull
design is to use a part that is somewhere between the "small signal" parts and the
"high power" parts as a driver or pre-driver, or even a "final" depending on the desired
power level.
With all of that being said... very high fun factor in the design and the software
modeling tonight.
73 de NG0R
I decided to try to put it into a simulator to see what it looks like.
Modeling:
This circuit is running in class C. The transistor base needs to be in 1.5-2 voltage
range which is between +10 to +15 dBm range with +20 dBm looking like the upper limit
as it starts getting current hungry in a hurry. I played with some different combinations
of resistors in the voltage divider and adding some DC bias to the base. The DC bias
could make it move into a different class beyond class C but the circuit got extremely
current hungry which makes sense. (Class C is fine for a CW rig with some good low
pass filtering.)
Plan:
I think that I will draw up this circuit in TinyCad and then move it into FreePCB
to eventually make printed circuit board. Once the board is etched and the parts are
stuffed I will measure and document it. I will then try to remodel this in software
(either in the Java based simulator or LTSpice) to see how the physical model and
software model compare.
Summary:
In the end this is a gimmick design using small signal parts like a PN2222 (2N2222A)
and/or a 2N3904 to push out some real power. It likely would be more productive and
stable to move this to a push-pull design and evaluate it based upon if it needs to
be in class AB or C depending it's intended use. The other option beyond a push-pull
design is to use a part that is somewhere between the "small signal" parts and the
"high power" parts as a driver or pre-driver, or even a "final" depending on the desired
power level.
With all of that being said... very high fun factor in the design and the software
modeling tonight.
73 de NG0R