help with qrp class-A HF amplifier

[neazoi]

HEllo,
I have a good quality low distortion 100mW buffered HF oscillator and I would like to build a small qrp amplifier for it to output 1W or so.
To minimize harmonics without using an LPF I am trying to build this amp from a power transistor, lightly driven.
I have chosen the 2sc2166 and I have made a schematic for the amplifier. However the output waveform is more like a square wave.
Have you got any ideas of why this is happening and how to fix this?

I/O caps are 100nf, choke is 10T on FT37-43 TR is 2sc2166 and Rcb is a 500R pot.
No matter what values I change I always get a distorted output.

 

[erikl]

1. What about your input & output impedance matching?
2. Input overdrive?

For QRP 100mW should be enough, isn't it? ;-)

 

[BigBoss]

-You load you amplifier with 50 Ohm oscilloscope impedance and the amplifier cannot drive this impedance.Because optimum load impedance of this PA is probably very different than this
-Your PA has no impedance input/output matching circuit and there are so much matching losses.A PA is not designed and implemented in this way..
-Variable Pots are never used in RF circuits, never...
-PAs are very crcitical circuits and they should be designed and implemented very carefully.

 

[Audioguru]

Any untuned amplifier produces a squarewave on its output when its input signal level is too high.
The 'scope shows an output of 20Vp-p so I think there is no load. With no load then the gain is very high.

Why do people post schematics with no power supply voltage?? Is the supply 10V or 12V?

 

[neazoi]

Any untuned amplifier produces a squarewave on its output when its input signal level is too high.
The 'scope shows an output of 20Vp-p so I think there is no load. With no load then the gain is very high.

Why do people post schematics with no power supply voltage?? Is the supply 10V or 12V?

The VCC is 12v. The scope has 50R port so I guess there is an internal 50R in the scope. Since comms systems work in 50R I will have to cope with this impedance. I will try to drive this with less power (maybe a 50R variable attenuator at the input) to see how the final amplifier responds.

- - - Updated - - -

-You load you amplifier with 50 Ohm oscilloscope impedance and the amplifier cannot drive this impedance.Because optimum load impedance of this PA is probably very different than this
-Your PA has no impedance input/output matching circuit and there are so much matching losses.A PA is not designed and implemented in this way..
-Variable Pots are never used in RF circuits, never...
-PAs are very crcitical circuits and they should be designed and implemented very carefully.

I have tried to replace the inductor with a transformer like the ones shown in the final of this image http://qrp.gr/qrp/QRP3.png but it did not make any improvement.

- - - Updated - - -

1. What about your input & output impedance matching?
2. Input overdrive?

For QRP 100mW should be enough, isn't it? ;-)

I will try driving it with less input as said. 100mW is more like QRPP 1W or so seems more realistic to go on air.