SYSTEM AND METHOD FOR OPTIMAL BIASING OF A TELESCOPIC CASCODE OPERATIONAL TRANSCONDUCTANCE AMPLIFIER (OTA)

摘要

A system and method of biasing a telescopic cascode operational transconductance amplifier is provided to prevent or reduce the likelihood that the inputs to the amplifier do exceed the input common mode voltage range for the amplifier. The system and method provides a bias control circuit for the differential input transistors and tail current transistor of the operational amplifier such that their respective Vds-Vdsat is maintained substantially constant. To accomplish this, the biasing system and and method uses a bandgap voltage source that typically produces a highly stable voltage that is substantially temperature and process invariant. The bandgap voltage source is used to generate bias voltages applied to the gates and drains of the differential input transistors that maintains their and the tail current transistor's Vds-Vdsat substantially constant. There are several advantages of the system and method for biasing a telescopic cascode OTA. First, by keeping Vds-Vdsat substantially constant for the tail current transistor, this transistor is prevented from operating in its linear region, which would otherwise cause a decrease in the bandwidth of the amplifier. Second, by keeping (i.e. Vds-Vdsat) is substantially constant for the input transistors, these transistors are prevented from operating in their linear region, which would otherwise cause a reduction in the output impedance and the DC gain of the amplifier. Third, Vds-Vdsat for the input and tail current transistors can be maintained relatively low in order to minimize the reduction of the output swing of the amplifier.