Help with single chip transmitter for HF

[neazoi]

Hello,
I design a small transmitter for the 40m band and I have found this circuit http://www.qrp4u.de/docs/en/rf_ampl/ (the last circuit in the page). I have tested this amplifier and I am very satisfied. It requires 1-3mW to output 1-1.5W or so.
I would like to modify the circuit so that the "diode" transistor is replaced by an external diode and this transistor will be used as a crystal oscillator that will drive the amplifier (other 4 transistors).

Can a single crystal oscillator transistor provide 1-3mW or it definitely needs a buffer?

I have tried to replace the broadband input transformer with an identical transformer as the one in the output, but this time I connected the variable capacitor at the input winding to tune it to 7MHz. However I could not make it tune to 7MHz, I could only make it tune to the harmonic of 14MHz, so I guess more turns are needed?
But since this transformer is trifillar I fear that increasing the number of turns will affect the low impedance required by the transistors bases.

Any help is appreciated.

 

[betwixt]

You can use an 'external' (discrete) diode but make sure it is thermally bonded to the transistors. It sets their bias point and compensates for increased current (= higher temperature) by dropping the fixed bias voltage.

Generally, you should aim to keep crystal oscillator power as low as possible but you might be OK at 3mW. A buffer stage, even with zero gain is never a bad thing to have after an oscillator.

Brian.

 

[neazoi]

You can use an 'external' (discrete) diode but make sure it is thermally bonded to the transistors. It sets their bias point and compensates for increased current (= higher temperature) by dropping the fixed bias voltage.

Generally, you should aim to keep crystal oscillator power as low as possible but you might be OK at 3mW. A buffer stage, even with zero gain is never a bad thing to have after an oscillator.

Brian.

This is exactly like this circuit **broken link removed** which uses a discrete diode for the bias. Thermal coupling is good but I have not seen any problems even wthout thermal coupling of the diode (practically speaking). I have even managed to blow up a chip by overdriving it (still at 12v) so I do not think the internal thermal coupled "diode" is of very much use, practically speaking.

What I was thinking (for simplicity), was to build something like this **broken link removed** but with the driver transistor to be used as an oscillator, not a buffer. It couples this transistor using the same input transformer (but currently I have only managed to make it resinate at 20m not 40. But I am not sure if I will be able to provide 2-3mW out of a single XO to drive the amplifier.

Any thoughts/ideas/schematics?