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

[Will Matney]

Dan,

On semiconductors, it about easier to reference the spec sheets on the individual part. That is if your building the amp around their recommended specs. Most all transistors used for RF amps show both the input and output impedances on the sheets. Transistors will have a low impedance of something like 1.5 ohms each for a total somewhere around 3 ohms for two in push-pull, and FETs' will have a high impedance. The difference is because transistors are current controlled, and FETs' are voltage controlled. I'd go on the net to a place like datasheet.com and see if I could look them up. That's what I use all the time.

Best,

Will

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On 3/28/06 at 7:19 AM Dan Sawyer wrote:

>All,
>
>I am on a quest to 'analyze' the original 104 G board. This was replaced 
>by the 104a that was completely redesigned. This board has a bad rep as 
>being a poor performer, I would like to figure out why. At the end is 
>the question: was it more the result of 'system' level selections such 
>as parts or filter design, or was it more the result of detail level 
>design.
>
>This leads to the questions:
>
>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?
>
>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?
>
>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.
>
>Sorry for the basic questions, thanks in advance.
>
>Dan - kb0qil
>
>
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