Seek help。 What is the role of D1 on the graph?
Thanks in advance.
Thanks in advance.
An externally hosted image should be here but it was not working when we last tested it.
With MPSA92 and the two 680R resistors it makes up a circuit called a current mirror. The diode would more correctly usually be the same type transistor as the MPSA92, thermally in contact with it, connected to be a diode (C and B would be connected together, so CB to E would act as a diode). In this case the author of the schematic has 'saved' one transistor by replacing it with a standard diode, but the performance of the circuit is slightly degraded (still good enough for that amplifier, though).
It's function is not only to provide a voltage drop, but also one that acts like a B-E junction including thermal dependency.
It's function is not only to provide a voltage drop, but also one that acts like a B-E junction including thermal dependency.
Ideally, the diode drop tracks the vbe. Even if you use a diode connected transistor it still doesn't, because of the huge difference in power dissipation (read: temperature). Using the 4148 gets you close enough and it won't get a lot better anyway unless you go to a more sophisticated current mirror.
At one time diodes were cheaper than transistors, but maybe not anymore.
At one time diodes were cheaper than transistors, but maybe not anymore.
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