Karel Kulhavy wrote:
thermal noise due to base resistance were your limiting factor.
Because only RF transistor has transition frequency of 25GHz.
Fight, but you said you were going to use this where a 2N3904 would have
enough speed so why do you need a 25 GHz Ft? Also you said you were
going to run at a current level low enough to drop the Ft down to 300 MHz.
Because 2N3904 will have enough speed at 5mA collector current which
generates huge shot noise. 25GHz Ft will have 300MHz speed at I don't
know 1uA of current which will generate substantially less noise?
CL<
lets see, if you have a low impedance source then your input refered
noise at 5 mA will be on the order of 0.4 nV/sqrt(Hz) ignoring base
resistance which you really can't. At 1uA of collector current, your
input refered noise will be around 290 nV/sqrt(Hz). Now you probably
can ignore the base resistance! So you're 37 dB worse in terms of SNR.
Now, if you have a high impedance source, than this is a different
story. In that case, as John has mentioned, you're better off with a
FET input.
For a given input voltage (signal), your output current in a common
emitter amplifier drops linearly with bias current while your noise
drops with the sqrt() of bias current so you lose.
-Dan
