PIN diode circutis no good at low frequencies? - RF Cafe Forums
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Post subject: PIN diode circutis no good at low frequencies? Posted: Thu Oct 12, 2006 9:04 pm
Joined: Thu Oct 12, 2006 8:56 pm
Can someone please help a novice understand why are PIN diode limiters, switches, attenuators etc... never characterized or available for low frequency applications (e.g. HF). What happens to make them no good down in that frequency range?
Post subject: Posted: Mon Oct 16, 2006 3:04 am
Joined: Mon Jun 27, 2005 2:02 pm
PIN Diodes are characterized only for RF/Microwave frequencies because their resistance (In forward bias) is not constant at low frequencies. Usually this resistance increases at low frequencies (Below 10MHz) and is not stable with changing the forward current.
Hope this helps.
Post subject: PIN DiodesPosted: Thu Oct 19, 2006 3:54 pm
Joined: Wed Feb 22, 2006 3:51 pm
A bit more explanation for "low frequency" use of PIN diodes:
PIN diode operation depends on the carrier (hole or electron) lifetime in the intrinsic region. The longer the lifetime, the lower the frequency you can use the PIN diode at.
I've seen successful use of PIN diodes (with the appropriately long carrier lifetime) in an attenuator operating at 455 kHz. The designer had to search fairly hard for an adequate diode, though.
The carrier lifetime depends on the purity and defect density of the intrinsic region, so it's only somewhat under manufacturer control. I haven't seen many PIN diode data sheets that omit the lifetime as a spec - but if you're implementing a circuit on an IC, the foundry may not know what a PIN diode will do in their process, unless they've gone out of their way to characterize it.
When the signal cycle time T = 1/f is near the carrier lifetime, the PIN diode starts acting like a regular diode.
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