As it's a 1W transistor, the input capacitance is likely to be
less than 15nF.
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.
Steve
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" <ww1c@outlook.com> 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?
Markku
> From: pc5m@xs4all.nl
> To: amps@contesting.com
> 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))/(1S22*Gamma(outp))
>
> 73's Carel.
>
> Original Message
> From: Amps [mailto:ampsbounces@contesting.com] On Behalf Of Drax Felton
> Sent: dinsdag 16 juni 2015 21:53
> To: amps@contesting.com
> 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
uncertain
> 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 = (ZL50)/(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
impedances
> in linear form like 50.581589j My RF books do not seem to say anything
> about this.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
