Current Mirror:

The circuit in Fig. 6.1 shows a current mirror, which is often used as constant-current-source. It utilizes a pair of matched transistors Q1 and Q2 The fundamental idea behind the current mirror is that base-emitter voltages V_BE for both transistors are equal.

Since V_be1=V_be2, then according to equation (), I_c1=I_c2, as long as both devices are in forward active mode. We use this idea to establish a current source. We first set I_c1 to a specific value. This then sets I_c2 to the same value, regardless of the value of R₂. I_c2 can thus be considered to be a current source, since its value does not change no matter the value of R₂. (Provided of course that the BJT's do not saturate.)

To set I_c1, we take advantage of the short between the collector and base of Q1. We then obtain the following loop equation: