4.12 Practical Considerations in Transistor Layout 99a common-mode signal and can be rejected. Symmetrical circuits also prevent un-equal signal delays again ensuring that signals are properly lined up. 4.12.3 MatchingTo achieve good matching it is important that components being matched are close to-gether and are oriented in the same direction. When multiple components (for exam-ple, resistors, capacitors, or transistor fingers) need to be matched, additional dummy components are sometimes added at the edges, as shown in Figure 4.24. Dummy components are not connected to the circuit, but are there to reduce edge effects.A common way to obtain better matching between several components is to interleave subparts of these components. If these components are transistor fingers, and if the source is common to both the transistors, they can be placed in the same device well as shown in Figure 4.25. Another technique is to place components such as transistors in a common centroid configuration, in which each component is a parallel combination of sub-components, which are arranged in a pattern on the die so that process variations in the x or y direction will affect subcomponents of each transistor. As an example, two transistors M1 and M2 each of size W/L can be made up of two subtransistors each of size 0.5 W/L as shown in Figure 4.26. In this way process variation in the x or y directions will affect each transistor in the same way and matching will be maintained. However, the common centroid approach results in the need for more interconnect resulting in more parasitic capacitance, and as a result frequency re-sponse may be poorer.As another example of a tradeoff, triple-well transistors can provide more drive at low voltages, but typically may have worse matching because of the typical require-ment of more spacing between such transistors compared to regular transistors. Figure 4.24 Dummy components to reduce edge effects.Figure 4.25 Interleaved transistors having a common source. Each transistor has four fingers.