[Amps] Understanding S Parameters

[Steve Thompson]

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.

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 at 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 at xs4all.nl
>> > To: amps at 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 at contesting.com] On Behalf Of Drax Felton
>> > Sent: dinsdag 16 juni 2015 21:53
>> > To: amps at 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.