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CCD/Constant Current Diode!

cjdelphi

Oct 26, 2011
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So i went looking for a CCD and found little so i had look to see what one was!

Funny looking diode lol
 

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Harald Kapp

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Nov 17, 2011
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This "diode" is based on a depletion mode JFET (Junction FET as opposed to MOSFET).
The "typical" MOSFET, as you probably know it, is an enhancement mode type. Without a gate source voltage it doesn't conduct between source and drain.

A JFET operates differently. The gate is not insulated from teh source-drain channel by an insulating material (the "O" in MOS), rather the insulating barrier is made of a pn-junction (the "J" in JFET). Without reverse bias, this junction is small and the JFET conducts current between source and drain without a gate-source voltage.
By applying a gate-source voltage in reverse direction, the pn-junction is reverse biased and extends into the channel, thus pinching off the channel -> current is decreased. The channel is depleted of charge carriers, that's why this type of transistor is called "depletion mode" transistor (opposed to MOSFETS where typically, but not exclusively, the number of charge carrries in teh channel is enhanced by the gate-source voltage, hence "enhancement mode" transistor).

The "constant current diode" operates as follows: without Vgs there is a currrent through the transistor. This current creates a voltage drop across the resistor in the source path. This voltage drop creates a negative gate-source voltage for the JFET. The negative gate-source voltage pinches off the channel, thus reducing the drain-source current, which in turn decreases the gate-source voltage. The circuit is in equilibrium when the voltage drop across the resistor caused by drain-source current matches the gate-source voltage for that drain-source current through the transistor.

All of this trimmed and packed into a single package makes for a CCD.
 
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