[Amps] estimating the impedance of amp input and outputs?

[Peter Chadwick]

Dan asks
>1. How can the impedance of an amplifier stage be estimated? For 
example, if the collector of a 2N5109 is biased at 1000 Ohms and the 
emitter is DC biased at 500 Ohms and RF biased with a .005 uf is it 
reasonable to assume the output impedance is somewhere near 1000 Ohms?<
I pres you mean that the collector load is 1 kohm. To a first order approximation, at HF, the source impedance will be about 1k as far as the filter is concerned.

>2. If the base of the same stage is DC biased by 8200 Ohms to +12 and 
3300 Ohms to ground how can the input impedance be evaluated? Is it very 
low due to the RF bipass?<
The input impedance will be 8200 ohms in parallel with 3300 ohms in parallel with the transistor input impedance. That is Hfe times the emitter resistance, which is given by 26/I ohms, where I is the emitter current in mA. So you have to decide what the Hfe is, based on the emitter current v. Hfe graphs in the data sheet. Again, this is a good approximation at HF, but don't push it much above 50MHz.

>3. How can the effective input impedance of G1 and G2 on a 40673 be 
estimated? The inputs are DC biased by values of 10k to 100k.<

At HF, again figure the gate input impedance is high enough that the impedances are dominated by the external resistors.
If you are running it with much gain at 30MHs, you may need to use the Linville Stability criteria equations to decide if it's going to be stable.
73
Peter, ON/G3RZP this week.