6 Common-Mode Limitations Inside CMOS Amplifiers

From a high level, the internal biasing of CMOS input stages looks similar to the bipolar device, but these devices are voltage controlled rather than current controlled. Depending on whether a P-channel, or N-channel device is used, the gate-to-source voltage required to bias the device generally allows for linear operation to either the negative or positive power-supply rail. Figure 6-1, Figure 6-2, and Figure 6-3 illustrate the P-channel case where the input operates linearly with input signals to the negative supply rail, but has common-mode limitations as the positive rail is approached.

Figure 6-1 illustrates the characteristic curve for the input P-channel devices. For linear operation the input transistors need to operate on the flat part of the curve where gate-to-source voltage is –0.9 V. Figure 6-2 illustrates the swing to the negative rail. The drops shown in this schematic illustrate the minimum voltages required for linear operation. Doing a Kirchhoff's walk to the negative supply, shows that the minimum voltage for linear operation is –0.2 V below the negative rail. For example, if the negative supply is –2.5 V, the input signal can swing below the negative rail to –2.7 V. Figure 6-3 illustrates the swing to the positive rail. Doing a Kirchhoff's walk to the positive supply, shows that the that the input signal can swing to 1 V below the positive rail or 1.5 V in this example. Since this op amp uses P-channel devices the op amp can swing all the way to the negative rail, but has limitations with the positive rail. An N-channel device behaves in the opposite manner.

Figure 6-1 Characteristic Curve for P-channel MOSFET

Figure 6-2 Swing to Negative Supply Rail Limitation for P-channel MOSFET

Figure 6-3 Swing to Positive Supply Rail Limitation for P-channel MOSFET