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Re: [Amps] Understanding S Parameters

Subject: Re: [Amps] Understanding S Parameters
From: Steve Thompson <>
Date: Wed, 17 Jun 2015 22:17:51 +0100
List-post: <">>
As it's a 1W transistor, the input capacitance is likely to be less than 1-5nF.

Don't get too hung up about three decimal places of accuracy, reflection coefficients anywhere near 1 are prone to huge errors in measurement. As Alex says, the input looks like a capacitance in parallel with a very high resistance (or in series with a very small resistance if you prefer).

Up to 100MHz the angle of reflection is pretty much linearly related to frequency which allows you to calculate the equivalent capacitance - somewhere around 10pF I think.


Quite so.
However, at low frequencies the input impedance of a fet is about 1 to 5 nF
in parallel with a quite high resistance. You might say a pure reactance,
more or less.
On the other hand, terminations affect the stability of a device wuite
strongly and they can cause a state of negative resistance at the input-
particularly some feedback capacitance from source to gate dou tp imperfect
source grounding, bue to package parasitic inductance.
Alex 4Z5KS
On Jun 17, 2015 10:19 AM, "Markku Oksanen" <> wrote:

If my memory serves me correctly:
Is it not so that S11 refers to the input port "input impedance" of a two
port network?So, in this case the input impedance is just simply about 1
ohm with small reactance (phase angle -3.6).SWR at the input should be
around 50.
Without S12 or S21 you should not be able to know stability?

> From:
> To:
> Date: Tue, 16 Jun 2015 22:45:08 +0200
> Subject: Re: [Amps] Understanding S Parameters
> Hi Drax,
> If S11 > 1 it means it is not unconditional stable, i.e. it might
> oscillate...
> Your formula for input impedance is only valid if s12 or s21 = 0, and I
> think that is not the case.
> Otherwise: Gamma(Inp)=S11+(S12*S21*Gamma(outp))/(1-S22*Gamma(outp))
> 73's Carel.
> -----Original Message-----
> From: Amps [] On Behalf Of Drax Felton
> Sent: dinsdag 16 juni 2015 21:53
> To:
> Subject: [Amps] Understanding S Parameters
> I am trying to understand S Parameters for a RD00HHS1 transistor.
> The S11 parameters listed in the datasheet are as follows:
> 10 MHz                1.002 (mag)     -3.6(ang)
> 30 MHz                1.003           -9.9
> 50 MHz                1.005           -16.8
> 100           1.007           -33.5
> 150           0.989           -49.8
> I'm trying to estimate the input impedance at 0.475MHz.  But I am
> what it means when the mag is greater than 0.999.
> Using the 10 MHz S11 value because it's available I get:
> Gamma = 1.002 = (ZL-50)/(ZL+50) --> ZL= -50050 ohm impedance or a VSWR of
> negative 1001:1.
> What am I doing wrong?  What does this mean?
> My other attempts at calculating input Z also result in negative
> in linear form like -50.58-1589j My RF books do not seem to say anything
> about this.

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