Both bipolar and CMOS op amps have an input differential pair used as the first stage of the amplifier (see Figure 1-5, and Figure 1-6). For bipolar devices, the offset voltage is mainly due to the mismatch of the base to emitter junction voltages (Vbe1 and Vbe2). This mismatch is due to photo-lithography effects, doping gradients, and stress introduced during packaging of the device. Similarly, the offset in CMOS devices is from the gate-to-source voltage mismatch, and the mismatch is due to the same types of fabrication issues as with bipolar.

The V_OS in bipolar and CMOS op amps can be reduced by adjusting the value of Ros1 or Ros2. This process is called trimming and can be accomplished through laser-trim or package-level-trim (e-Trim®). In both cases the V_OS is measured and resistors are adjusted to minimize the offset. In laser-trimming, sections of a thin film resistor are physically cut away to increase the resistance (see Section 1.3). Package-level-trim uses digital communications to open switches or fuses in a binary weighted resistor network (see Section 1.4). A third way that op amp V_OS can be minimized is by using an internal self-calibration circuit. These types of amplifiers are called zero-drift amplifiers and are covered in Section 4.

The temperature drift of a bipolar op amp is linear and is directly proportionate to offset (see Figure 1-7). In fact, each millivolt of offset yields about 3.3µV/C of offset drift. Thus, trimming the offset to zero on a bipolar op amp also trims the drift close to zero (see Figure 1-7). Conversely, the drift of CMOS devices is nonlinear and is not proportional to offset as with the bipolar (see Figure 1-8). Trimming the temperature drift on a CMOS device requires adjustment of resistors R₁, R₂, and offset is trimed with R_OS1, and R_OS2 (see Figure 1-6). Furthermore, the CMOS op amp offset must be measured at multiple temperatures in order to trim the temperature drift, whereas the bipolar device temperature drift can be trimmed at a single temperature. The additional complexity of the CMOS V_OS and the V_OS-drift trim makes it challenging to achieve the same performance as the bipolar device.