Transistor input Z and BPF termination

Hi Ivan,

Yes, you're right. I'm trying to use 2n3904 in small signal buffer/drivers and PN2222A in final so that the circuit can easily be replicated. RF transistors like 2n3866 are getting rare. But I've to use several stages as the gain is quite low but the circuit can be copied easily and cheaply. I was also able to get 700mW at 96MHz from two parallel PN2222As.

I've removed the final tuned circuit as it was oscillating on some other frequency. Output is 4mW only after 3 stages of amplification including the common base stage. You can tell if the filter is matched or not by listening to the audio quality.

Will this circuit work just as fine with the 7th harmonic, or 9th?

Hi,
my first answer was bound to your first schematic. That amplifiier is common emitter. Adding the first stage using common base improves the circuit behaviour and filter matching considerably. Using transistors with fT>1GHz still seems more promising to me - more gain, less tweaking.

BR from Ivan

Del

Del

Hi Ivan, spice and real life experiment showed that 2n3904 works with about 6dB gain at 100MHz. Maybe it's transition frequency is 350MHz?

Hi dare,
the theory around BETA looks nice. But
- I expect that using a transistor at 1/3 ot its transit frequency will cause great problems;
- you neglected the phase shift on the input capacitance. Filters require a pure resistive load on both ends.

BR from Ivan

Can this common emitter amplifier properly terminate a 50ohm 100MHz band pass filter? If my calculations are correct then base Zin is 6ohm and input 47ohm series resistor + 6ohm = 53ohm

The BPF should see 53ohm right? I tried common base but it added more parts and gain is less.

Plz advice.