The bias is set by the ratio of resistances R2 and R3. It can be anything from zero,or ground potential, to + 12 V, the supply voltage. Normally it will be a couple of volts.Capacitors C2 and C3 block dc to or from the input and output circuitry (whateverthat might be) while letting the ac signal pass. Resistor R4 keeps the output signal frombeing shorted out through the power supply.A signal voltage enters the common-emitter circuit through C2, where it causes thebase current, IB to vary. The small fluctuations in IB cause large changes in the collectorcurrent, IC. This current passes through R4, causing a fluctuating dc voltage to appearacross this resistor. The ac part of this passes unhindered through C3 to the output.The circuit of Fig. 22-9 is the basis for many amplifiers, from audio frequenciesthrough ultra-high radio frequencies. The common-emitter configuration produces thelargest gain of any arrangement. The output is 180 degrees out of phase with the input.Common-base circuitAs its name implies, the common-base circuit, shown in general form by Fig. 22-10,has the base at signal ground.The dc bias on the transistor is the same for this circuit as for the common-emittercircuit. The difference is that the input signal is applied at the emitter, instead of at thebase. This causes fluctuations in the voltage across R1, causing variations in IB. The re-sult of these small current fluctuations is a large change in the dc current through R4.Therefore amplification occurs.Common base circuit40922-9Common-emitter circuit configuration.