We need to bias the jfet channel for 50% on, 50% off. When a jfet is
"on", it grounds its winding with its phasing for RF to IF transfer.
Alternating jfets flips the signal to the opposite polarity. Rapidly
flipping the signal polarity through this stage at the LO rate does
the mixing.
Thus there is no DC current, but we are trying to turn on/off AC
paths. Since jfet pinch off thresholds vary so much from device to
device, separate bias is best.
As a matter of fact, if you had a bunch of these, it would be nice to
match Idss and pinchoff voltage for these two jfets.
- Dan
Sent from my Windows Phone
-----Original Message-----
From: ext Joe Street
Sent: 2/28/2014 5:10 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: KISS Mixer Musings
These FETs are not really 'biased' in the normal sense anyways because
the drains are open circuit for DC so there is no bias current flowing.
What you are doing is raising and lowering the potential of the whole
channel. Variation in the device fabrication processes result in device
to device variance in transconductance so perhaps in this strange
circuit balance is more important??
On 28/02/2014 4:58 PM, Tayloe, Dan (NSN - US/Tempe) wrote:
That might change the balance of the mixer, but does not change the
bias on each gate.
- Dan
-----Original Message-----
From: minima-bounce@xxxxxxxxxxxxx
[mailto:minima-bounce@xxxxxxxxxxxxx] On Behalf Of ext Sandeep Lohia
Sent: Friday, February 28, 2014 2:56 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: KISS Mixer Musings
http://4.bp.blogspot.com/-fIABMtTpyUE/Ut9yfPotc0I/AAAAAAAAAUQ/IG6TveVonrw/s1600/under+R&D.jpg
NOTE : not yet tested live...
IF you have two separate bias pots,
You could bias the two JFETs with separate pots and adjust for
similar drain
Just thinking about this. I remember that the transconductance of
JFETS
varies quite a bit so the fact that the signals are not symetrical
might
be due to an imbalance in the device characteristics?
Please take a look at the link and then any volunteers who can try
and explain this to me most welcome!
