# Equations.

Discussion in 'Wireless & RF Design' started by PRS, Mar 6, 2010.

1. ### PRS Thread Starter Well-Known Member

Aug 24, 2008
989
36
Equations for input z are nonsense when dealing with rf. I just went through the experience of expecting 1 M ohm resistance from a Fet source follower, only to find out that with increasing f that 1 M ohm dropped way down to about 5000 ohms. Any comments?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Stray capacitance could be one source of the discrepancy. You've not indicated the circuit configuration and component types in any detail - just that it's a source follower. What frequency is of interest to you?

3. ### Papabravo Expert

Feb 24, 2006
11,758
2,484
If you look at s-parameter plots as a function of frequency you see that nothing is constant. Input impedance is in fact a function of frequency. In particular, as you go up in frequency, things you can't see like inductance and stray capacitance are actually modifying the equivalent circuit.

4. ### PRS Thread Starter Well-Known Member

Aug 24, 2008
989
36
That's right, Papa, thanks. And I'll be the first to point out that this effect is actually included in the literature. I had forgotten how the equations were modified by rf. Those 'tiny' device capacitances become significant as we increase the frequency, but the equations don't actually change; they're just modified by including device parameters and stray capacitance, even probe capacitance. I'm going through a time of transition from thinking audio frequencies to radio frequencies. And I'm beginning to see why rf engineers use power gain rather than voltage gain as a figure of merit for an amplifier.

5. ### PRS Thread Starter Well-Known Member

Aug 24, 2008
989
36
Here's the circuit and the data. The graph of the curve looks like a low pass RC filter. It shows there is a total device input capacitance, stray capacitance, and probe capacitance of about 82 pF.

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