I need the detail lower-cut-off frequency of BAP55LX
The PIN diode must be forward biased (Low Loss or ON State) so that the stored charge, Qs, is much larger than the RF induced charge that is added or removed from the I-region cyclically by the RF current.
This relationship is shown by the inequality: Qs >> Irf / (2*Pi*F)
Storage charge can be evaluated as Qs = If*tau
So, a diode behaves in PIN mode operation if F>>Irf/If *1/(2*Pi*tau) [1]
Below the transit time frequency of the I-region, the PIN diode behaves as a PN junction diode, ie, it rectifies the RF voltage.
Qs stored charge
If = Forward Bias Current
Irf = rf signal current
tau = Minority Carrier Lifetime
BAP55LX has tau = 0.27us thus using equation [1]: F >> 0.6MHz *Irf/If
Low frequency range depends on the RF and bias currents but rough estimation is F>6 MHz.
So, this device can be used as RF switch/attenuator at frequencies above few MHz.
Reference document "THE PIN DIODE CIRCUIT DESIGNER'S HANDBOOK":
https://ieee.li/pdf/essay/pin_diode_handbook.pdf
Have a great day,
Pavel
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