Sources of Amplifier Noise

The main factors affecting the total harmonic distortion plus noise (THD+N) characteristics of dual-supply amplifiers are input noise and output stage crossover distortion. The THD+N performance of single-supply amplifiers is derived from the input and output stages of the amplifier. However, the input stage's impact on THD+N complicates the specification of single-supply amplifiers.

There are two single-supply amplifier topologies that can accept input signals between supplies. The topology shown in Figure 1a has a complementary differential input stage. In this topology, when the amplifier input is near the negative rail, the PMOS transistor is "on" and the NMOS transistor is "off." When the amplifier input is closer to the positive voltage rail, the NMOS transistor is "on" and the PMOS transistor is "off."

This design topology will have a large difference in amplifier offset voltage over the common-mode input range. In the input range near ground voltage, the offset error of the PMOS transistor is dominant. In the region near the positive power supply, the NMOS transistor pair dominates the offset error. Since the input of the amplifier passes between these two regions, both pairs are "on". The end result is that the input offset voltage will vary between two levels. When both PMOS and NMOS are "on", the common-mode voltage region is about 400 mV. This crossover distortion phenomenon affects the total harmonic distortion (THD) of the amplifier. If you configure the complementary input amplifier in a non-inverting structure, the input crossover distortion will affect the THD+N performance of the amplifier. For example, in Figure 2, if the input transition region does not occur, the THD+N is 0.0006%. If the THD+N test includes the input crossover distortion of the amplifier, the THD+N is equal to 0.004%. You can avoid this type of amplifier crossover distortion by using an inverting structure.

Figure 1. Complementary input stage, single-supply amplifier: a).

Single differential pair input stage with a positive charge pump: b)

Figure 2. THD+N performance of a complementary input stage single-supply amplifier.

Another major contributor to THD+N is the op amp’s output stage. Typically, the output stage of a single-supply amplifier has an AB topology (see Figure 1a). When the output signal is swept rail-to-rail, the output stage exhibits a crossover distortion similar to that of the input stage as it switches between transistors. In general, a higher level of output stage quiescent current can reduce the amplifier’s THD.

The input noise of an amplifier is another factor that affects the THD+N specification. High levels of input noise and/or high closed-loop gain will increase the overall THD+N level of the amplifier.

To optimize the THD+N performance of a complementary input single-supply amplifier, place the amplifier in an inverting gain configuration and keep the closed-loop gain low. If the system requires the amplifier to be configured as a non-inverting buffer, an amplifier with a single differential input stage and a charge pump is more appropriate.