If you actually want to build a differential amplifier using the diagram in the previous section, you have a problem. You can buy the resistors, transistors and the battery in any shop, but where to buy a DC current source? The answer is simple. Don't go looking for it, because no one sells one. We have to create one ourselves.

There are several ways to do this. We'll use a so called current mirror.

Again, both transistors have the same characteristics. V_R3=9V-0.6=8.4V, so I_R3=8.4V/8.2k=1.02mA. Since the transistors have the same characteristics and V_BE1=V_BE2, both collector currents must be the same: I_C3=I_C4=I_R3=1.02mA.

You can obtain any current flow you like by adjusting R3.

To use the current source in the differential amplifier, connect the collector of T3 to the emitters of T1 and T2.