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|PIN diode bias question... - RF Cafe Forums|
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Post subject: PIN diode bias question... Posted: Wed Nov 23, 2005 6:29 am
Joined: Fri Aug 27, 2004 11:25 am
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.
First 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 question.
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.