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I come across with a transistor circuit as shown below. What is the name for this circuit? Besides, what is the working principle of it?

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Peter Mortensen
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user42423
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    It's not a circuit until all the nodes are revealed to a sufficient degree that there is no ambiguity. You show three components and I see three nodes going to unknown places; that doesn't constitute a circuit. – Andy aka Aug 22 '22 at 09:44
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    That two terminal circuit **is** a circuit on it's own, called the Vbe multiplier. – Justme Aug 22 '22 at 10:02

3 Answers3

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This goes by the name of 'amplified diode'.

Consider what would happen if there was enough voltage across the collector and emitter terminals, VCE, to put 0.7 V across the base-emitter junction by voltage divider action between R59 and R37. The transistor would be conducting a small amount of current. With 10 k and 33 k, that voltage is about 3 V.

If VCE now dropped a little, the collector current would drop a lot.

If VCE increased a little, the collector current would increase a lot.

Both of these behaviours are much the same as if you replaced these components with a diode with a 3 V forward voltage drop, with its VI plot voltage scaled accordingly.

This circuit is frequently used to bias the output stage of a class B amplifier, where it has to track the voltage drop of the several stacked VBEs that you get with complementary darlington transistors.

Neil_UK
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It's a rubber diode. The voltage drop over R37 is VBE, and the current through both resistors is approximately the same, so you have a voltage drop of aboput 4.3 × VBE over the circuit.

CL.
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  • So approximately 3.0 V? – Peter Mortensen Aug 23 '22 at 11:58
  • Very approximately, depending on current, temperature, and manufacturing variations. – CL. Aug 23 '22 at 12:25
  • You can get a circuit based on the same idea, but with a lot of precision when you replace the 0.6V VBE "voltage reference" by a precision reference, e.g. the TL431 (2.45V) or TLV431 (1.25V). On the other hand, This imprecise circuit is actually useful if the transistor is thermally coupled to some other component and can be used to compensate temperatur drift of that other component. Class AB amplifiers that use this circuit for setting idle current often have the reference transistor mounted on the power transistor heatsink. – Michael Karcher Aug 23 '22 at 19:33
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Looks like a VBE multiplier, otherwise known as an amplified diode or rubber diode. Have you pulled that section of the circuit from the output stage of a class AB power amp?

There is about 0.7 V across R37, forcing a current through R37 and R59 of about 70 µA. This forces a voltage between the collector and emitter of about 3 V and this voltage would be applied between the bases of the amplifier's output transistors to bias them into conduction with the intention of reducing crossover distortion.

Usually there would be a pot in series with R37 to adjust the VCE voltage and therefore set the output transistors biasing and quiescent current as desired.

Peter Mortensen
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