another output Z question

T

tempus fugit

Guest
Hey all

I'm still trying to find the output Z of a low gain JFET preamp I'm working
with. I'm using an MPF102, in common source configuration. I have a 1K
resistor going from the 9v supply to the drain, with the source connected to
ground. There is no spec for gO on the data sheet, so I can't just use 1/gO,
and I figure the output Z is at least somewhat dependent on the drain
resistor and the load Z (which is 10K min). I know that the common source
configuration has a high output Z, but how high is high? Would it be 1000s
of ohms, or 10s of 1000s?

Thanks
 
"tempus fugit" <toccata@quitspammingme.ciaccess.com> wrote in message
news:a4be1$499e0324$d1d89e2d$1725@PRIMUS.CA...
Hey all

I'm still trying to find the output Z of a low gain JFET preamp I'm
working
with. I'm using an MPF102, in common source configuration. I have a 1K
resistor going from the 9v supply to the drain, with the source connected
to
ground. There is no spec for gO on the data sheet, so I can't just use
1/gO,
and I figure the output Z is at least somewhat dependent on the drain
resistor and the load Z (which is 10K min). I know that the common source
configuration has a high output Z, but how high is high? Would it be 1000s
of ohms, or 10s of 1000s?

Thanks

Wait - I found a data sheet that gives the yOs as 200 uS which would give an
Click to expand...
output Z of 5000 ohms. Is this a close enough approximation or will the
output Z vary much with different drain and load resistances?

Thanks
 
On Thu, 19 Feb 2009 20:13:38 -0500, "tempus fugit"
<toccata@quitspammingme.ciaccess.com> wrote:

Hey all

I'm still trying to find the output Z of a low gain JFET preamp I'm working
with. I'm using an MPF102, in common source configuration. I have a 1K
resistor going from the 9v supply to the drain, with the source connected to
ground. There is no spec for gO on the data sheet, so I can't just use 1/gO,
and I figure the output Z is at least somewhat dependent on the drain
resistor and the load Z (which is 10K min). I know that the common source
configuration has a high output Z, but how high is high? Would it be 1000s
of ohms, or 10s of 1000s?
Click to expand...
---
Since you've got a 1k pullup to the positive rail, with the JFET cut off
you're looking at 1000 ohms of resistance/impedance between the rail and
the load.

As you drive the JFET, its resistance will change from quasi-infinite
when it's cut off to quasi-zero when its fully enhanced, so the output
impedance of your circuit will vary with JFET drive.


JF
 
On Thu, 19 Feb 2009 20:20:37 -0500, "tempus fugit"
<toccata@quitspammingme.ciaccess.com> wrote:

"tempus fugit" <toccata@quitspammingme.ciaccess.com> wrote in message
news:a4be1$499e0324$d1d89e2d$1725@PRIMUS.CA...
Hey all

I'm still trying to find the output Z of a low gain JFET preamp I'm
working
with. I'm using an MPF102, in common source configuration. I have a 1K
resistor going from the 9v supply to the drain, with the source connected
to
ground. There is no spec for gO on the data sheet, so I can't just use
1/gO,
and I figure the output Z is at least somewhat dependent on the drain
resistor and the load Z (which is 10K min). I know that the common source
configuration has a high output Z, but how high is high? Would it be 1000s
of ohms, or 10s of 1000s?

Thanks

Wait - I found a data sheet that gives the yOs as 200 uS which would give an
output Z of 5000 ohms. Is this a close enough approximation or will the
output Z vary much with different drain and load resistances?

Thanks
Click to expand...
Zout will be about 1K in parallel with the fets's 5K, say 800 ohms.

John
 
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