Bipolar op amps consist solely of bipolar junction transistors (BJTs). A wide range of performance specifications are available, ranging from low performance, widely used relics such as the LM324, to the more contemporary op amps such as the LM2904B and the precision op amp OPA828.

Substituting bipolar NPN transistors for Q₁ and Q₂ in the circuit of Figure 3-1 and setting $\mathrm{R}={\mathrm{R}}_{\mathrm{C}}$ provides the basic NPN bipolar differential input circuit shown on the left in Figure 4-1. Small resistors can also be placed at the emitter of the devices to improve linearity and speed at the cost of increased noise and decreased open-loop gain. This is normally done because it increases stability, but the effect is not discussed here.

In the bipolar process, V_OS is created primarily by differences in the base width, emitter area, and doping levels of the base and collector of transistors Q₁ and Q₂ (see Gray and Meyer [2]). These errors create differences in the bias currents flowing in the base of the differential pair. The overall result is a difference in the V_BE values of Q₁and Q_2, which causes the differential voltage V_OS to appear across the op amp inputs.

When the inputs are grounded, a loop is formed as shown on the right in Figure 4-1. Kirchoff's Voltage Law (KVL) is then used to obtain Equation 13, rewritten in the form of Equation 14. V_BE is defined in Equation 8, where the term kT/q is known as the thermal voltage (V_T), I_C is the collector current, and I_S is the reverse saturation current. Then is substituted into Equation 14 and manipulated into the form in Equation 4:

The errors introduced in Equation 4 by the I_C terms are due to the mismatch in the RC resistors. The I_S-term errors are due primarily to mismatches in the area of the emitter and the width and doping of the base (see Gray and Meyer [2]). The value of V_T (kT/q) is material-dependent (for example, 26 mV for silicon) and is inherent in all transistors. This term has the largest impact on V_OS and its drift with temperature. As T changes, V_OS predictably changes, as shown in Equation 5.