implementation of low voltage high slew rate opamps and linear transconductors
A new class AB differential stage that operates with a single supply voltage of less than two transistor threshold voltages is introduced. This circuit has utilization in high slew rate one stage op-amps, two stage op-amps with class AB input and output stages and linear transconductors. The circuit was verified with simulations and experimentally. It it is shown to have lower voltage supply requirements than other commonly used structures reported in literature.
Opamps with low supply voltage requirements, very low static power dissipation, rail-to-rail output signal swing and high slew rate are required for many applications like for delta-sigma converters used in voice and audio CODECS. One stage opamps with class AB input stages and two stage opamps with class AB input and output stages have several advantages: a) Class A folded cascode op-amps have a slew rate which relates directly to the static power dissipation. High gain is achieved at the expense of cascoded output stages that cause a greatly reduced output swing, bandwidth degradation and increased noise. Two stage class AB op-amp architectures can achieve significant performance improvement with reduced static power dissipation and supply voltages. b) Two stage op-amps with class AB output stages allow low voltage operation, rail-rail output swing (extended dynamic range) and high gain. Several efficient low-voltage class AB output stages with quiescent current control have been reported in literature recently . A drawback of two stage op-amps that have a Class AB output stage and a class A input stage is that they can have high static driving capability but the class A input stage is still the limiting factor for the op-amp slew rate. c) Two stage op-amps with Class AB input and output stages can have a very low supply voltage requirements, high slew rate and very low static power dissipation. High slew rate leads to short nonlinear settling times and allows operation at higher clock frequencies with relatively small quiescent currents (determined by the noise floor) d) In two stage op-amps the output stage transistors have negligible contribution to op-amp noise whereas in one stage op-amps all transistors contribute to equivalent input noise. e) If the differential pair transistors remain in saturated mode then class AB differential input stages have a linear DC transconductance characteristic. This results in reduced open loop distortion which is very important for high precision delta-sigma applications and for implementation of linear OTAs for continuoustime OTA-C applications
Several efficient class AB output stages for utilization in two stage op-amps with low supply voltage requirements have been reported recently . Some class AB input stages have been also reported in literature . In what follows a comparison of commonly used class AB input differential stages is presented and it is shown that the new structure has lower supply requirements. Commonly used implementations of class AB MOS differential amplifiers correspond to implementations of the same basic scheme shown in Fig. 1a. and are characterized by a class AB DC transconductance characteristic (I1,I2 vs. Vd) similar to the one shown in Fig. 2c. If both transistors remain in saturation the differential transconductance characteristic I1-I2 vs Vd is linear and the circuit can be used as a linear transconductor in OTA-C applications.