Electrical – What determines input /output impedance of a transistor configuration

You really need one of a number of options, none of them are hobby friendly if you are above a few MHz. I would guess you are at 2.4GHz, or at 900MHz.

Professional Options

If you might know someone whom has access to equipment that would help you have a few options.

1. VNA- Vector Network Analyzer. This can give you a smith chart over a frequency range and make sure it shows you at 50 ohms. You can also measure S11 which should be less than -20dB
2. Spectrum Analyzer- This should allow you to determine your power received at your frequency. This is not ideal, but it should get more output power as you get closer to a proper match.
3. SWR Meter-As Is noted in another answer and I always forget about, you can use an Standing Wave Ratio meter, but I would still suggest a VNA. They have significantly more features, and can do anything a spectrum analyzer or SWR meter can do.

Hacker style ways to approach it

There is another funny way to approach this, the problem being you will need to build one of a few microwave components.

You could use an isolator and measure its temperature, hotter isolator is equivalent to a worse mismatch. this is not even close to good method as at 10dBm it will be very hard to detect a temperature failure.

You could build your own mixer and feedback your output signal and mix it with your signal returning from the connection. To take away your signal from the feedback you need to use what is called a circulator. This is not fun, but it is doable.

If you would like to try either method, which is similar to creating your own VNA, I can get you more information on how to build these devices.

There are two "tricks" to answering those questions with transistor amplifiers.

The first trick is to understand that the current-transfer ratio (h_FE) of a transistor effectivly multiplies the resistance at its emitter. So, to find the impedance looking into the base, you calculate the effective resistance at its emitter, multiply this by h_FE, and then add the internal base resistance.

The second trick is to realize that the result of the calculation above is typically orders of magnitude larger than the other resistances connected to the base of the transistor and can therefore be ignored. In other words, the input impedance of a transistor amplifier is usually very close to the impedance of its bias network alone. Indeed, bias networks are very often designed so that this is the case.

The output impedance is a question of how much the output voltage changes with output current: ΔV/ΔI. The transistor itself is essentially a current source, and whatever current it is passing is shared between the various resistances connected to the emitter. Therefore, the output impedance is equal to the net emitter resistance, not including the resistance of the external load.