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Can anyone tell me what a 'double diode' is used for? For example, a BAV99. I'm not sure of the correct name for it and so can't find it in any of my reference books. I've come across one in a circuit, between an output pin on a micro and an input clock pin.

Diagram

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In the case you show the diodes act as clamping diodes to protect the input against too high or too low voltages. The top diode will clamp the input to +Vs + 0.7 V, the other one will clamp the input to -0.7 V. The small advantage is that they have a the anode of one diode in common with the cathode of the other.
Other double diodes, like the BAT54 are available in different configurations, so that there's always one for when you need two connected diodes:

enter image description here

Note that even the single BAT54 comes in a SOT23 package, so placement of two single diodes requires six pins to solder, versus three pins for the double diode. Soldering cost is calculated per pin, and for simple components may exceed the cost for the part itself. And it also saves board space. Apart from the series, common anode and common cathode arrangements there are also independent double diodes, like the BAS40-07, which again may be used to save board space and cost.

The BAV99 diode is also half of a bridge rectifier, and the BAV99s has two of them for making a bridge.

Apart from less signal routing through the common connection you can also use double diodes when they need to be well matched, since they're on the same die.

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    \$\begingroup\$ Maybe throw in 'input protection' somewhere where you talk about clamping? \$\endgroup\$ – jippie Jul 4 '12 at 13:36
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    \$\begingroup\$ @jippie - No way! I mean, done :-) \$\endgroup\$ – stevenvh Jul 4 '12 at 13:38
  • \$\begingroup\$ You are too good ;o) \$\endgroup\$ – jippie Jul 4 '12 at 13:40
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    \$\begingroup\$ @jippie - Oh, I know! :-) But missed the Nobel Prize for Goodness again last year. Bah. :-) \$\endgroup\$ – stevenvh Jul 4 '12 at 15:14
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    \$\begingroup\$ Why are you getting kudos for commenting on my brilliant additions to your pretty good article? \$\endgroup\$ – jippie Jul 4 '12 at 15:59
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Another common name for it is a signal diode. This tutorial gives a good long explanation of what it is and what is can be used for.

The semiconductor Signal Diode is a small non-linear semiconductor devices generally used in electronic circuits, where small currents or high frequencies are involved such as in radio, television and digital logic circuits. The signal diode which is also sometimes known by its older name of the Point Contact Diode or the Glass Passivated Diode, are physically very small in size compared to their larger Power Diode cousins.

Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA. [...] Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off.

Also note that there is something called double diodes, but that is simply two diodes in one package (such as for example BAV74

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    \$\begingroup\$ Where's the double diode? \$\endgroup\$ – stevenvh Jul 4 '12 at 13:24
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    \$\begingroup\$ electronics-tutorials.ws is an excellent site, but sometimes... "sometimes known by its older name of the Point Contact Diode". What? The term "Point Contact Diode" hasn't been heard here since the last Napoleonic Wars. \$\endgroup\$ – stevenvh Jul 4 '12 at 13:57
  • \$\begingroup\$ Thanks, both. I will be sure to look at that tutorial in detail! \$\endgroup\$ – Peter Hull Jul 4 '12 at 14:12

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