I build a common emitter amplifier based on BFP420 BJT, and would like to measure its gain and input and output impedance. As the gain should be within 25dB, so I connected a 40dB 50 ohm input/output attenuator between CH0 and the input of the amplifier to avoid driving the amplifier into saturation, then connected the output of the amplifier to CH1.
The negative dBs readings of S21 were making sense to me after adding up logically 40dB to come up with the actual amplifier gain. However as the inserted attenuator impedance at CH0 is 50 ohm, the readings of S11 was showing as almost flat 50 ohm across the frequency range which is expected.
To measure amplifier input impedance (S11), I removed the attenuator and connected amplifier input directly to CH0 and connected amplifier output to a 50 ohm load. As the amplifier is driven into saturation, I am not sure if the S11 readings are correct, and I can rely on for small signal.
I repeated the same but swapping the amplifier input and output to measure the output impedance (S22).
Is this the right way to measure input and output impedance for such amplifier.in the absence of power control on the nanoVNA? Is there a better way?
