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A new switched-capacitor (SC) common-mode feedback (CMFB) circuit for fully-differential operational amplifiers (op-amps) is presented. By reducing the amplifier capacitive load with respect to conventional SC-CMFB schemes, the proposed solution guarantees a significant improvement of the op-amp speed performance. A typical SC integrator employing the new CMFB has also been designed in 0.35 μm CMOS technology. Simulation results show that, for a given power consumption, the op-amp settling time can be about halved by using the proposed CMFB instead of the conventional one.
Introduction: SC-CMFB circuits are widely employed to stabilise the common-mode output voltage of hilly-differential op-amps used in typical SC applications [1, 2], owing to some important advantages that they offer with respect to their continuous-time counterparts. In fact, SC-CMFBs are characterised by high linearity, do not introduce parasitic poles on the common-mode loop and are suitable for low-voltage operation, because they do not limit the op-amp output swing [3], The schematic of the most well-established and extensively used SC-CMFB circuit [1, 3] is shown in Fig. 1. C i and C2 are the capacitors of the CMFB, φι and φ2 are the two non-overlapping clock signals of the SC circuit, Vqi and Vq2 are the op-amp output voltages the common mode of which has to be fixed, Vcm is the desired common-mode level, Vi, is the reference bias voltage and Vbus is the bias voltage of amplifier transistors controlled by the CMFB. The function of the circuit is based on the charge exchange between Ci and C2 during the two clock phases. If Vi, is properly chosen close to Vbias. the common mode of Vqi and Vq2 settles to a steady-state value of about Vcm after an initial transient period.
On the basis of the operating principle of the circuit in Fig. 1, alternative more complex SC-CMFB schemes were also proposed in the past. Unfortunately, all the SC-CMFB configurations presented in the literature led to an increase of the total op-amp capacitive load, caused by the connection of the circuit capacitors to the amplifier output nodes [3], This increase is generally significant because sufficiently large values of C i and C2 are needed to limit the deleterious effects owing to circuit non-idealities (such as parasitic capacitances, and leakage currents and...