COMMON MODE FEEDBACK STRATEGIES Common-mode feedback circuits(CMFB) are used for fully differential systems to stabilize their DC output voltage level by arranging the common-mode output current or voltage. A CMFB circuit senses the DC output voltage (common-mode voltage) while it is insensitive to the differential mode voltage. A CMFB compares the common-mode voltage with the reference voltage and feeds back the differential signal to inputs of fully differential circuit.
figure1: General common mode feedback configuration
There are several methods to implement a CMFB circuit. Three of the strategies have been reported for implementing CMFBs are given below.
Switched capacitor
Differential difference amplifier
Resistor averaged circuit Switched capacitor CMFBs can support large voltage swings however they can only be
used for discrete time systems because they need 2 clocks to work and they inject clock induced noise into the system.
Differential difference amplifier CMFBs use four identical transistors which are configured into two differential pair structures to average and compare the DC output voltages of main amplifier. Differential difference amplifier CMFBs provides large transconductance and high input resistance however they have limited voltage swing range. Input range can be improved by sizing of transistors but there is a trade of between the input range of differential difference amplifier CMFB input and the bias current which means power. Resistor averaged CMFBs use resistors to average the DC output voltages and second stage compares this common mode output voltage with the common mode reference voltage. Also, there are capacitances parallel with the resistors which are used the stabilize the system. The value of the resistors in this case is a trade-off between the speed and gain. Using resistor averaged CMFB circuit decreases the error in the common mode output voltage which is caused by the differential pairs in the differential difference amplifier CMFBs. Moreover, the voltage swing range in this case wider than differential difference amplifier CMFBs. However, using of large resistors decreases the CMFB gain.
References: Design of Fully Differential Folded Cascode Operational Amplifier by the gm/ID Methodology, N. Bako A Continuous-Time Common-Mode Feedback Circuit (CMFB) for High-Impedance Current Mode
Application, L Luh A General Description of Common-Mode Feedback In Fully-Differential Amplifiers, P.M. VanPeteghem Advanced Common-Mode Control Techniques for Low Voltage Analog Signal Processors, E. S-Sinencio Indian Institute of Technology - EE539 Lecture Notes: Analog Integrated Circuit Design; Common mode
