Op-Amps Introduction

Ohm’s law is stated as V=IR, and it is fundamental to all electronics. Ohm’s law can be applied to a single component, to any group of components, or to a complete circuit. When the current flowing through any portion of a circuit is known, the voltage dropped across that portion of the circuit is obtained by multiplying the current times the resistance (Equation 2–1).

V = IR

In Figure 2–1, Ohm’s law is applied to the total circuit. The current, (I) flows through the total resistance (R), and the voltage (V) is dropped across R.

Figure 2–1. Ohm’s Law Applied to the Total Circuit

In Figure 2–2, Ohm’s law is applied to a single component. The current (IR) flows through the resistor (R) and the voltage (VR) is dropped across R. Notice, the same formula is used to calculate the voltage drop across R even though it is only a part of the circuit.

Figure 2–2. Ohm’s Law Applied to a Component

Kirchoff’s voltage law states that the sum of the voltage drops in a series circuit equals the sum of the voltage sources. Otherwise, the source (or sources) voltage must be dropped across the passive components. When taking sums keep in mind that the sum is an algebraic quantity. Kirchoff’s voltage law is illustrated in Figure 2–3 and Equations 2–2 and 2–3.

Figure 2–3. Kirchoff’s Voltage Law

Kirchoff’s current law states: the sum of the currents entering a junction equals the sum of the currents leaving a junction. It makes no difference if a current flows from a current source, through a component, or through a wire, because all currents are treated identically. Kirchoff’s current law is illustrated in Figure 2–4 and Equations 2–4 and 2–5.

Figure 2–4. Kirchoff’s Current Law