Principle of biasing circuits in IC design

In discrete circuits transistors are biased with resistor divider networks and bypass/coupling capacitors. In ICs this is not practical, so a different principle is used:

• Resistors take large chip area (a 1 MΩ resistor needs far more area than a transistor) and have poor absolute tolerance (±20%), but ratios and matching of devices are very good.
• Capacitors larger than a few tens of pF cannot be made, so coupling and bypass capacitors are not possible; stages are direct coupled.
• Transistors are cheap, small and very well matched because they are made side by side in the same process and are at the same temperature.

So IC bias circuits use constant current sources made from transistors (current mirrors). One reference current is set by a single resistor and a diode-connected transistor, and is copied to all stages by current mirrors (current steering), often with Widlar sources for small currents. The current sources also act as high-resistance active loads and tail sources, giving high gain and high CMRR, and their currents track temperature and supply changes together.