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PIN diode bias question... - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: PIN diode bias question... Posted: Wed Nov
23, 2005 6:29 am
Joined: Fri Aug 27, 2004
Location: Cape Town, R.S.A.
A fundamental question regarding PIN diodes and their biasing
– How does one calculate the minimum DC voltage required to ensure that
a PIN diode remains OFF in the presence of RF?
In a specific
system implementation, where a PIN diode RF switch is used to cold switch
500 W RF pulsed signal, a shunt diode configuration is used. In this
existing system a reverse bias of 50V is used to switch the diodes OFF,
but it was shown experimentally that the reverse voltage could be dropped
as low as 25V before the RF began to bias the diodes ON.
of all I understand that 500W RF power corresponds to a RF voltage swing
of 450Vp-p, if 50 ohm is assumed. This on top of a -50V bias (for switching
the diodes OFF) means that the signal is swinging between -275V and
+175V, which one would think would bias the diodes ON during the positive
cycle. The minority carrier lifetime of the PIN diode however is related
to the fact that this does not happen and the diode remains OFF for
the complete duration.
The question then remains, what is the
relationship between minority carrier lifetime, RF power, frequency
and other PIN diode specifications and this minimum reverse bias voltage
to ensure that the diodes remain OFF.
Am I missing something
obvious? I've searched through various application notes and other literature,
which somehow always seem to ignore this requirement.
Post subject: Posted:
Wed Nov 23, 2005 11:44 pm
Joined: Tue Sep
07, 2004 3:09 pm
This is an interesting
As I know when used as attenuators, pin diode long
minority carrier lifetime drives the hability of the diode to provide
good intermodulation caracteristic.
In a two tone intermodulation
test the enveloppe being non-constant, I see this also as the hability
to resist bias change by the enveloppe modulation.
I wonder if
the equations for linearity of a pin diode attenuator in high attenuation
condition would be useful... like the one from MACOM or Alpha. You would
be using a criterion for extremely low distortion meaning the diode
absorbs no power.