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.
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.
On Jun 17, 2015 10:19 AM, "Markku Oksanen" <firstname.lastname@example.org> 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
Without S12 or S21 you should not be able to know stability?
> From: email@example.com
> To: firstname.lastname@example.org
> 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
> 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:email@example.com] On Behalf Of Drax Felton
> Sent: dinsdag 16 juni 2015 21:53
> To: firstname.lastname@example.org
> 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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