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A new class AB output stage for CMOS op-amps, with accurate quiescent and minimum current control, is proposed. The proposed stage can be operated with a supply voltage close to the threshold voltage of the transistor. A dynamic biasing scheme allows it to operate over a wide range of supply voltages. Simulation results are provided that are in good agreement with expected values.
Introduction: The demand for portable electronic equipment has driven industry to produce circuit designs with very low supply voltage. Although two oxides are presently available in many analogue technolo- gies, digital compatibility forces analogue circuits to operate with sup- ply voltages close to the threshold voltage of the transistor.
Several low-voltage (VDD .. 1.8 V) class AB output stages have recently been reported [1 - 5]. Monticelli's scheme [1] achieves an accu- rate quiescent current control by means of a translinear loop at the expense of a high voltage supply (larger than two VGS). In [2], a folded version of Monticelli's output stage was proposed to achieve operation with a low supply voltage. The output stages proposed in [1, 3 - 5] do not keep control of the minimum current through the output transistors. In addition, the output stage proposed in [3] has a bad class AB behav- iour, that proposed in [4] requires a complex circuitry for quiescent cur- rent control, and that proposed in [5] requires a voltage doubler to operate with a low supply voltage. Other output stages using a feedback loop for quiescent current control can also be found in the literature, but they have a slower speed and may have stability problems owing to the feedback loop.
Proposed class AB output stage with quiescent and minimum current control: In this Letter a new CMOS class AB output stage is presented with simple and accurate quiescent and minimum current control (Fig. 1). In Fig. 1, the bias current Ib in the low voltage differential pairs M1p - M3p and M1n - M3n [6] accurately determines the quiescent output...