Active load

If the bias current is fixed for some performance reason, any increase in load resistance automatically leads to a lower voltage for Vout.

In contrast, using the active load of Figure 2, the AC impedance of the ideal current source is infinite regardless of the voltage drop VCC − Vout, which allows even a large value of VCB.

Active loads are frequently used in op-amp differential input stages, in order to enormously increase the gain.

In contrast, an active load presents to the source a resistance value varied by electronic control, either by an analogue adjusting device such as a multi-turn potentiometer or, in automated test setups, by a digital computer.

The heat-dissipating devices (usually transistors) in an active load therefore have to be designed to withstand the resulting temperature rise, and are usually cooled by means of heatsinks.

Figure 1: Basic NPN common base circuit with resistor load (neglecting biasing details). Signal is applied at V in , output taken from node V out may be a voltage or a current.
Figure 2: Basic NPN common base circuit (neglecting biasing details). Current source I C represents an active load.