Brought a NXP BLT50 power transistor long time ago, today try to build a quick test circuit to see how it feels like. If this transistor works well, it would be put into the 27MHz remote control transmitter power amplifier stage. To maximize the voltage gain and reduce distortion, the transistor is biased as class A at 75mA, and supply voltage 8.4V, it dissipate about 0.6W of heat, for a SOT223 package, it doesn't became too hot. At lower quiescent current, the maximum output power would be reduced.
The circuit is simple, no matching circuit, a small emitter resistor to provide a little bit thermal stability, the circuit is simulated before put into board, to reduce cut and try time.
I didn't want to spent a few days to wait for the PCB back from factory, so I use part of the board from 433MHz receiver project. The big inductor is the collector loading inductor, about 2uH inductance.
In 27MHz, 0dbm input, the amplifier produce an nice output spectrum. with -30dbc higher harmonics and 23dbm output power, which equal 200mW.
What is the range of this 200mW transmitter if it is put in the remote control transmitter?
May be I should go for a field test.
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