A ±2.45V-Swing CMOS Telescopic Operational Amplifier

A telescopic cascode opamp typically has a higher frequency capability and consumes less power than other topologies. The disadvantage of a telescopic opamp is severely limited output swing. In a conventional telescopic opamp shown in Figure 1, all transistors are biased in the saturation region. Transistors M1-M2, M7-M8 and the tail current source M9 must have at least Vds,sat to offer a good common-mode rejection (CMRR). To allow for process and temperature variations, a small safety margin Vmargin is often added to their Vds to ensure saturation. The maximum differential output swing of a telescopic opamp is shown to be 2Vsup-10Vds,sat-6Vmargin ,where Vsup is the supply voltage. With typical Vds,sat of 200mV and Vmargin of 100mV , the output swing is limited to 2Vsup-2.6V . In a 3V supply system, this represents a 45% reduction of available output swing.