Using positive feedback to overcome gmro limitations in scaled CMOS amplifier design

The use of positive feedback as a solution to intrinsic gain degradation in scaled technologies is discussed. Criteria for increasing gain while keeping the system stable are derived in terms of traditional feedback theory as well as a modified amplifier model. The amplifier model, in an attempt to standardize positive feedback analysis on generic amplifiers, includes non idealities that traditional feedback theory does not, including finite input impedance and non-zero output impedance. Both treatments show that as amplifier open loop gain decreases, positive feedback can more easily be applied to increase that gain at a cost of a slightly more than one-to-one tradeoff with the amplifier bandwidth. This analysis shows that the concept of positive feedback is most useful in high bandwidth single stage amplifiers where gain is at a minimum. It is applied to a differential stage in 65 nm technology and is shown to increase the gain from 12.61 dB to 27.25 db.

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