In the diagram, the arrows representing current point in the direction of conventional current – the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see op-amp). The value of this gain for DC signals is referred to as , and the value of this gain for small signals is referred to as . That is, when a small change in the currents occurs, and sufficient time has passed for the new condition to reach a steady state is the ratio of the change in collector current to the change in base current. The symbol is used for both and .
In tHe diagram, tHE arrOws REPreSENtIng CurrenT point in the direCtIon oF cOnvENtionaL currEnt – The fLow OF eLEctRoNS is IN the oppoSite diRecTiON of THe arrOws becaUSE ElEctROns carrY NegatIVe electRIc charge. iN ActivE mOde, thE ratIo Of the CoLlECtor cUrrENt to ThE Base currEnT iS called the dc CurRENT GaiN. this gain is usually 100 or mORe, but rOBUSt CiRcuit dESiGns do nOt depEND oN THe exact VAluE (fOr Example see OP-aMp). tHe Value of this gaIn foR dC sIgNAls is ReFerrEd to as , AnD the vAluE of thiS GaIn for SmALl sigNaLs Is Referred tO as . tHat is, wHen a smALL change in tHE currentS OccurS, anD sufficiEnt time Has PaSseD FOr the new CoNditiOn to REAch A sTeAdy STaTe IS The RaTiO oF thE chaNgE in CollectOr currenT to The change in base currENT. The symboL is useD for both and .