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walid

Ft of the transistor

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Next I increased the feedback cap to 22pF. Now the drive level was near 2.6mW (17 times higher), BUT note the rapid rise of harmonics from the osc (green circles) and we are no longer on the linear part of the power curve. The amplifier is now heavily into gain compression. Output of F1 is now 55mW and amp almost in full compression (no increase in input drive will provide higher output) as we have reached the flat part of the curve. Also note how the osc frequency shifted down as expected because of the higher capacitance.

Here is also the plot for power in transistor with 47k base resistor connected with input drive stepped 1-10mW.

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Finally, I biased the amplifier stage into class-C with removing the 47k resistor and adding a RF choke to gnd on the base. I ran a power sweep again. Now we can see that if we can get the drive level up to about 8mW (which it seems we can't) from the simulation data, it is possible to get up to 150mW into the 75 Ohm antenna or load before gain compression for this class starts to come in.

The 2nd last plot shows the Vp-p on the collector as the drive level is stepped up. Distortion is clearly visible as the drive level is increased to maximum (10mW)

Last plot shows power dissipation in the transistor (Watts) for various drive levels 1 to 10mW.

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So, how do we get more power? Well running the osc on 9V gives us almost 8mW drive and higher harmonics, which gives us about 130mW on the power curve. The problem is that now the frequency will drift as the 9v battery gets flat. So no easy answer apart from adding another amplifier stage.

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My transmitter takes the oscillator's output from its emitter. Many other FM transmitters take a higher level oscillator signal from the collector or from a tap on the coil for a higher output power. I didn't try it.

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Here's plots from swcad using vendor coils from their library with Q of about 100, which is about normal from winding a 100nH coil. Feedback cap is set at 10pF. Now the collector current does not go under zero. Vc is the voltage on the collector, Ic the collector current and VL the voltage over the 75 Ohm load. Ic was meaured by placing a 0.001 Ohm resistor in series with the collector terminal and using the current probe. Model for 2N3904 is still linear SPICE model and not the more accurate non-linear model used before.

Note:For some reason (better resolution?) swcad change the scale when the collector voltage and load voltage swing are plotted on the same graph. Load voltage should swing negative and positive because of the coupling cap. When it is plotted alone on a graph, it shows correctly. The p-p swing indicated is still correct.

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