This is a current mirror, a device that uses the current in one half of the circuit to control the current in the other half. The current is the same in both halves. The switch on the left changes the current in the left half, which is mirrored in the right half. The switch on the right causes the resistor to be shunted, but the current mirror ensures that the current does not change. The emitter-base junction of Q1 acts as a diode. The current through it is set by the resistor circuit below it. Connecting the base to the collector allows the base current to flow, so the transistor can stay in the active mode. Since the base of Q1 is connected to Q2, they are at the same voltage, so the emitter-base junction of Q2 must have the same amount of current flowing through it. (It acts as a diode, so the current is determined by the voltage across it.) Nearly the same current flows through both halves of the circuit. The only difference is that the base currents from Q1 and Q2 flow through the left half rather than the right half. In this circuit we use high-beta transistors so that these base currents are as small as possible.
This page is a utility for simulating токовое зеркало online with specified initial values.
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