[Amps] measuring bandpass of a power amp with an NA

[jeff millar]

Dan...

The input impedance of a transistor at RF frequencies runs a few ohms, 
depending on a variety of factors, with high power meaning lower 
impedance. 30 dB in power means 1000:1 in power or about 30:1 in 
voltage. The network analyzer measures power in 50 Ohms, but if 
connected to a source of only a few ohms, it basically becomes a voltage 
measuring device. So, if the input matching network transforms 50 Ohms 
down to 1.5 Ohms, that would result in the measuring a 30:1 voltage 
reduction, about what you saw.

jeff, wa1hco

Dan Sawyer wrote:
> I have a couple solid state RF power amps to test. One question is: What 
> is their bandpass? I connected a network analyser drive to the output 
> and then took the NA input of the collectors. Accounting for the 
> impedance mismatch, this worked quite well, the amps showed good 
> bandpass characteristics across the 2 to 30 MHz HF range.
>
> I then connected the NA drive to the amps input and took the NA input at 
> the transistor bases. This did not work at all, the NA showed close to a 
> 30 db loss across the range. What am I missing here? It would seem there 
> should be at lease a discernible signal characteristic by this method.
>
> Thanks - Dan
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