The common-base amplifier has a good voltage gain, low input impedance, and high current gain.

A common-emitter amplifier has the advantages of good voltage, current, and power gain, but the disadvantage of a relatively low input impedance.

A coupling capacitor couples dc to an amplifier.

The output resistance of the common-emitter amplifier is approximately equal to the collector resistance.

A Darlington pair provides a very low input impedance.