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A Technique for Correcting Systematic Offset of Static/Quasi-Static Current Mirrors

IP.com Disclosure Number: IPCOM000032608D
Publication Date: 2004-Nov-09
Document File: 3 page(s) / 63K

Publishing Venue

The IP.com Prior Art Database

Abstract

A technique and apparatus for correcting systematic offset between a mirror voltage and an output voltage in a current mirror is disclosed. A correction loop incorporates an operational amplifier coupled to the output and mirror voltage of the current mirror. The operational amplifier feeds back to regulate a supply voltage of the current mirror, which forces the mirror voltage of the operational amplifier toward that of the output voltage.

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Title

A technique for correcting systematic offset of static/quasi-static current mirrors

Problem

Current mirrors are used in many applications, but one of the wide use of current mirrors is to accurately copy a current from one branch to another. Examples for such applications are integrator charge pumps where two currents signals are subtracted, or another application is in folded cascade amplifiers, where current mirrors are used to copy the current of the first branch with limited headroom to another branch with smaller transistor stack and thus more headroom, or in simply where a differential signal needs to be converted into a single-ended signal, current mirrors are used to copy the negative signal current into the positive signal branch resulting in a single-ended signal. However, one main problem with current mirrors is that due to channel length modulation effect in mirror transistors and the fact that voltage on two sides of the mirror is not always equal, the copied version of the current will not exactly be equal to the original current. This difference in current results is some systematic input referred offset, that affects the overall performance of the system the mirror is used in. The conventional way of addressing this problem is to increase the effective channel length modulation of the mirror devices by using cascode structures. The problem with cascade structures is that they typically need higher headroom and thus not attractive for the existing trend where the processes are going toward lower supply voltages.

Solution

Figure 1 shows the basic structure of a PMOS current mirror. The input devices in this example, which in general can have different forms, can receive their input from different sources such as output of another amplifier or up/down pulses of a phase detector output. The output voltage of the stage, Vo, is set based on the state of the other preceding and following circuits, and independent of the mirror state. For example, if used as an integrator, the output voltage may be used to tune the frequency of a VCO in a PLL loop. That means that the output voltage, Vo, moves independent of mirror node voltage, Vm. As a result of this voltage difference between Vo and Vm, the mirror PMOS transistor, as well as input NMOS transistor experience different drai...