Chapter 1Direct Current CircuitsThese lectures follow the traditional review of direct current circuits, with emphasis on two-terminal networks and equivalent circuits. The idea is to bring you up to speed for what isto come. The course will get less quantitative as we go along. In fact, you will probably ﬁndthe course gets easier as we go.1.1Basic ConceptsDirect current (DC) circuit analysis deals with constant currents and voltages, while al-ternating current (AC) circuit analysis deals with time-varying voltage and current signalswhose time average values are zero. Circuits with time-average values of non-zero are alsoimportant and will be mentioned brieﬂy in the section on ﬁlters. The DC circuit compo-nents considered in this course are the constant voltage source, constant current source, andresistor. Electronics also deals with charge Q, electric E and magnetic B ﬁelds, as well as,potential V . We will not be concerned with a detailed description of these quantities but willuse approximation methods when dealing with them. Hence electronics can be consideredas a more practical approach to these subjects. For the details look at your classical physicsand quantum mechanics courses.1.1.1CurrentThe fundamental quantity in electronics is charge and at its basic level is due to the chargeproperties of the fundamental particles of matter. For all intensive purposes it is the electron(or lack of electrons) that matter. The role of the proton charge is negligible.The aggregate motion of charges is called current II =dqdt,(1.1)where dq is the amount of positive charge crossing a speciﬁed surface in a time dt.Be awarethat the charges in motion are actually negative electrons. Thus the electrons move in theopposite direction to the current ﬂow.The SI unit for current is the ampere (A). For most electronic circuits the ampere is arather large unit so the mA unit is more common.6