Transistor base as a diode

The most common case that comes to mind is a current mirror circuit, where you need an input diode with essentially the same I/V curve as your output transistor.

The way I read that, is as the OP mentioning a transistor with its CB shorted and the BE used as a diode - as opposed to in a configuration such as a CM. One application that springs to mind is that it yields a slightly lower Vf than a regular diode, the collector will begin to draw current at a VBE of around 0.5V. This is also very dependent on temperature, I recall that the old Germanium AC128 was often used in this way as a temperature compensator in the convergence panels of old delta-gun CTVs. Also most Si transistors exhibit a zener voltage in reverse - usually around 5V or so.

Sometimes the reason is much more mundane: Logistics. If the board doesn't need many diodes but already has transistors then using a transistor instead can reduce the number of BOM line items by one. Saves inventory costs, rigging costs etc.

That is the same configuration, for the input side of the current mirror.

But if you ever make them conduct in reverse like this (as a Zener) then those devices should be treated as suspect parts from then on. It has been alleged to do funny things to the noise performance etc. Certainly on some IC processes it is frowned upon to let that junction break down, even at low current levels.

Chris

The logarithm characteristics of a perfect diode are shared by this kind of connected transistor; so-called 'ideal' diode behavior is used in logarithm converters, and few 'switch' diodes (which are optimized for low capacitance) or rectifier diodes (which are optimized for high currents) can compete with a good low-noise transistor.

This is sometimes called a "super diode"; it has a maximum current rating equal to the maximum collector current of the transistor, and tends to have a nice low forward voltage drop at moderate currents compared with average small signal diodes, but the main advantage is that the diode characteristics match the characteristics of other transistors in the circuit so you get (for example) good current mirrors , especially if both are part of the same IC, so good thermal tracking and in some situations low distortion or little error over a wide operating current range.

Mark A

It avoids much of the IR voltage drop of using the b-e diode alone since most of the current flows c-e.

Graham

They make pretty good Zeners also if you need one around that voltage region that is heat sinkable! :)