Abstract
A compact power-efficient class-AB Miller op-amp is introduced. It uses a simple auxiliary circuit that enhances the op-amp’s gain-bandwidth product and helps to drive a wide range of capacitive and resistive loads with high static and dynamic current efficiency. Simple Miller compensation is used to obtain stability over a wide range of loading conditions. The op-amp’s simulation and experimental results in strong inversion with $15\mu \text{A}$ bias current and in sub-threshold with 250nA bias current are shown. Its performance is measured in terms of dynamic and static current efficiency figures of merit FOMCEDyn and FOMCEStat. and using the conventional small-signal figure of merit FOMSS. Experimental results of op-amps fabricated in a 130nm CMOS technology are shown that validate the proposed approach.
Highlights
It can provide relatively large positive output currents and a maximum negative output current that is limited by the quiescent current of the output transistors IMOutQ determined by MON
The value of τ depends on the RC time constant of a high pass network used in the op-amp
A limitation of the Free-Class-AB approach is that it can hold large peak negative output currents only during a period of time that is determined by the time constant τ =RLargeCBAT
Summary
A drawback of the basic scheme of Fig. 3(a) is that the peak positive variation of the voltage at node X is limited to a maximum value VSGQP (but it is smaller in practice) This relatively small positive change in X transferred to node Y can lead to just a modest boosting in the negative output current (and in the negative slew rate and CE). The control circuitry increases the static power dissipation and the op-amp supply requirements and might not significantly boost the peak negative output current For this reason, most of the reported schemes based on the direct battery implementation are characterized by a CE value that is not much higher than that of a conventional class-A op-amp. The addition of the control circuit in this and other class-AB approaches (i.e., in [4]) increases the total quiescent power dissipation of the op-amp, reducing current efficiency and leading to a relatively modest boost of the peak negative output current.
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