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Why cant we replace diode connected BJTs by simple diodes in a current mirror circuit?

user30271
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    Do you mean replacements of BJT's with the diodes?? – Atom Oct 10 '13 at 08:23
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    Please add a small schematic to show exactly what you are asking. Draw the current mirror with BJTs and then show a modified version that has diodes instead. – Joe Hass Oct 10 '13 at 11:10
  • There's nothing wrong with the question... The reason is that in order to "mirror" you want the diode connected one to be identical (except perhaps in size) to the device (BJT) doing the driving of the current. The diode connected one will set the "right" base-emitter voltage for the both of them. If you use a diode, then it's a different device and the BE voltage on the driver BJT won't be what you want for the current you want. – jpcgt Mar 30 '16 at 18:50
  • No, there is a problem with the question, its vague. A schematic would be appropriate, props to @johnfound for getting one. – Voltage Spike Mar 30 '16 at 20:49

2 Answers2

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"Diode connected" BJT actually is not so simple. It works in active mode actually and the collector current is a function of the base voltage.

Side note: It is not very widely known, but the collector current of the BJT actually depends not on base current, but on the voltage across BE junction.

For the proper work of the current mirror the two transistors must be as equal as possible, so that is why it is not a good idea to replace one of them with diode.

Nevertheless, some simple discrete components schematic can use diodes, but only if there is some current negative feedback, as in the below schematic. But even there, using BJT is much better.

schematic

simulate this circuit – Schematic created using CircuitLab

johnfound
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If you are referring to biasing diodes that are found in audio amps, then these diodes are called mirror diodes. They are called that because they are a mirror of the output transistors.

By "mirror" it means that the characteristics of the output transistors are the same as these diodes. This is because they come from the same batch of wafer as each other.
This is important because they are being used to provide a voltage verses temperature point in which the amplifier as it gets hotter will need to have the bias dialled back in order to prevent overheating and destruction.

Typically there are more than 1 diodes combined into 1 package and secured to the output transistors or heatsink. Single diodes make this hard to do effectively thus the special packaging and purpose, specific to the amps transistors.

Russell McMahon
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user105250
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