I am being told that the circuit below will regulate current, I am wondering how true that is and how exactly does it do it if so. As far as I can tell: two NPN transistor with their bases biased by the two resistors. This looks to be a negative feedback loop that holds current steady, but I don't quite understand the loop and if it really does regulate current.

A Google search shows me that a current regulator of which I think is the same as a current mirror normally has the bases of both transistors on one node of which doesn't seem to be the case in this circuit.

Here is the circuit:


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ They are both NPN. \$\endgroup\$ Jun 1, 2014 at 10:27
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    \$\begingroup\$ Keep in mind that a current mirror can not work with discreet components. And bjt's "control" terminal is called base, not gate. \$\endgroup\$ Jun 1, 2014 at 10:27
  • \$\begingroup\$ @ apalopohapa my bad.. thanks for the heads up. and @ Vladimir Cravero thanks for that didnt know! \$\endgroup\$ Jun 1, 2014 at 12:22

1 Answer 1


It is a current limiter, and the current limit is set by the 12 ohm resistor. The idea is that when enough current flows through that resistor, Q2 is turned on, pulling down the base of Q1, "strangling" it.

If Q2 turns on with a Vbe of, say, 0.7V, then the current is limited to 0.7V/12ohms = 58mA.

To understand it, first imagine that Q2 is not there. Basically Q1 is turned on via its base resistor and lets through as much current as it can. If only there was 'something' that would start turning it off when current reaches a settable amount... and that's where Q2 comes in, and the 12ohm resistor is really a current sensing resistor.


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