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))/(1-S22*Gamma(outp))
> >
> > 73's Carel.
> >
> > -----Original Message-----
> > From: Amps [mailto:amps-bounces@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 = (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
> impedances
> > in linear form like -50.58-1589j My RF books do not seem to say anything
> > about this.
> >
> >
> > Kb3x
> >
> >
> >
> >
> >
> >
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> >
> >
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>
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