Greetings:
I don't use zeners to protect high-speed inputs because zeners have a lot of junction capacitance. Thus I use reverse biased small signal diodes to the rails.
I have recently discovered these gadgets and similar ones at
Now from the Cj specs, they leave pin 3 NC. So I would figure that pin
1 should go to the signal line, and pin 2 to ground. That way, if pin 1 goes to more than Vbr, then current will conduct through both diodes, clamping pin 1 to Vbr.Notice the specs are unclear whether the zener characteristics are measured at just the zener terminals at pins 1 and 3. I would suspect this is true, so the actual clamp voltage would be Vbr+0.7V or so.
Now the question is, why does the reverse biased zener in series with the forward biased diode have a low total Cj ???
I understand how it is that reverse biased diodes have decreasing capacitance with increasing reverse bias, but what happens with a forward biased diode escapes me. Or is it simply that the small signal diode has a low capacitance even with zero bias? A look at a 1N4148 datasheet confirms this, but doesn't tell much about what happens with fwd bias.
So what happens?
P.S. Idiotically, the Vishay datasheet also neglects to indicate the continuous current spec for this gadget.
Good day!