Active load preference and Widlar in IC biasing

Why active load is preferred over passive load

• Much higher gain: gain = gm × load resistance. A transistor current source gives a small-signal resistance ro (50 kΩ to MΩ) while dropping only a few tenths of a volt dc. A resistor of the same value would need a very large dc drop and supply voltage.
• Gain is doubled in a differential pair: the current-mirror load converts the differential output to single-ended without losing half the signal (Ad = gm(ro2‖ro4) instead of gm·RC/2).
• Less chip area: a transistor occupies much less area than a large resistor.
• Large output swing and better use of a low supply, since the dc drop is small.
• Better CMRR and good temperature tracking (matched devices).

Advantage of Widlar current source in IC biasing

• Gives very small bias currents (µA) with small resistors: IO·RE = VT ln(IREF/IO), so a 10 µA source needs only about 12 kΩ instead of about 1.4 MΩ, saving chip area.
• Higher output resistance, Ro ≈ ro(1 + gm RE), so the current is more constant.
• The output current is less sensitive to supply voltage and β changes because of the emitter degeneration.
• Used for the low input-stage current of the 741, giving low input bias current and high input resistance.