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This is a very crude method. But it does take into account the working voltage, the Z of the circuit (here about as simple a Class C amplifier that you can get). It also takes account of the working frequency. All of these factors affect the output Z
Adjust the resistor until you see half the initial peak voltage reading.
The output impedance of your transistor is equal to the ohmic resistance of the load resistor once adjusted to half the no load peak voltage.
REMEMBER THIS IS SIMULATION
The EC simulator is very good for many circuits. However, it doesn't adjust the reactance of simulation transistors, such that, supply voltage and frequency don't affect the results much. A real transistor will change its reactance with both operating frequency and importantly, with supply voltage. The latter is due to the variable capacitor effect of PN junctions. In simulation of many circuits I have deducted that there is an inbuilt fixed output capacitance of approximately 5pF when using the EC transistor model.
IN OTHER WORDS
1) measure the peak output with the switch open.
2) close the switch. Then make the load resistor a value such that the peak voltage falls to 50% (half) of the peak voltage as measured when the switch was open.
3) The output impedance value of the transistor, is the value of the resistor, which when placed, caused the output RF voltage to fall by 50%.
Note: Measuring is only safe with this method for circuits where the expected power is less than approximately 1W, for the no-load voltage will rise proportionally
EXAMPLE
A 1Watt example of a given transistor, at 50 Ohms might result in 10 Volts peak voltage.
At 100 Ohms the peak voltage is at 40 Volts
At 200 Ohms the peak voltage rises to 80 Volts
Note that a BD139 does not support much more than that - see the data sheet. (Mentioned because this transistor is very popular with low power PA stages in QRP HAM radio circles - mainly because its very low cost)
So this is important stuff!
The longer way round this is to do loads of pointless calculation. This is very good for your brain and if you like that type of thing, great!
There's a problem with this calculated method. It may not reflect the device under test at all for many reasons which I'm not going to delve into here.
Suffice to say, its better to test the real device!
The output impedance will determine the type of coupling to the filter or next stage. Be it a transformer or capacitor or more complex coupling topology.
Please note that the choke (inductor) at the collector of the transistor will have to be adjusted for operation at different frequencies.
EXAMPLE
If you lower the frequency to 1 MHz, the choke must be something in the region of 8uH
You can make a self supporting air core coil
20 mm diameter - plastic water pipe
7 mm length
6 turns 1 mm copper wire
Or you could just buy one of those chokes that look like resistors.
1001001
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