The present invention relates to a CMOS buffer circuit for liquid crystal display (LCD) drivers, which includes a single stage operational transconductance amplifier (OTA) with a differential of transistors for receiving a differential input voltage, a bias current source coupled to the differential pair and a single-ended output, the first bias current generating stage with a differential pair of transistors coupled to receive the differential input voltage to produce an output current in an output current path in response to a positive differential input voltage, a second bias current generating stage with a differential pair of transistors coupled to receive the inverted differential input voltage to produce an output current in an output current path in response to a negative input voltage, wherein the output current paths of both bias current generating stages are combined in a common current path and the current in the common current path is mirrored to the bias current source of the single stage OTA, so as to increase the bias current through the bias current source in response to an increasing magnitude of the differential input voltage.
1. A CMOS buffer circuit for liquid crystal display (LCD) drivers, comprising:
an single stage operational transconductance amplifier (OTA) with a differential pair of transistors for receiving a differential input voltage, a bias current source coupled to the differential pair and a single-ended output,
a first bias current generating stage with a differential pair of transistors coupled to receive the differential input voltage to produce an output current in an output current path in response to a positive differential input voltage,
a second bias current generating stage with a differential pair of transistors coupled to receive the inverted differential input voltage to produce an output current in an output current path in response to a negative input voltage,
wherein the output current paths of both bias current generating stages are combined in a common current path and the current in the common current path is mirrored to the bias current source of the single stage OTA, so as to increase the bias current through the bias current source in response to an increasing magnitude of the differential input voltage.
2. The CMOS buffer circuit according to claim 1 , wherein the first and the second bias current generating stages are substantially similar to each other except that the first bias current generating stage is coupled to receive the first differential input voltage and the second bias current generating stage is coupled to receive the inverted input voltage.
3. The CMOS buffer circuit according to claim 1 , wherein the current through the bias current source of the OTA increases exponentially in response to an increasing magnitude of the differential voltage input to the current mirror from the two bias circuits.
4. The CMOS buffer circuit according to claim 3 , wherein in each bias current generating stage has two diode coupled load transistors coupled to the transistors of the differential pair and a constant current source coupled to the common source connection of the differential pair, the CMOS buffer comprising further a first and second transistor having their gates coupled to the gates of the respective load transistor and a common source connection coupled to the drain of a third transistor, the drain of the first transistor being coupled to ground and the drain of the second transistor being coupled to a constant current source, wherein the source of the third transistor is connected to a supply voltage and the gate of the third transistor is coupled to the drain of the second transistor, wherein a fourth transistor is coupled by its gate to the third transistor to mirror the current through the third transistors to the fourth transistor.
5. The CMOS buffer circuit according to claim 1 , wherein the output of the OTA is connected to a negative input of the differential pair to operate as a voltage follower.