The source follower is a well known basic building block for CMOS design. A capacitively loaded source follower acts as a 1st-order lowpass filter and exhibits excellent linearity, especially with reduced overdrive voltages (Vov = VGS – VTH). This behavior is the opposite of other continuous-time filter implementations, where large linearity requires large Vov and, therefore, large power for a given gm since gm = 2·I/Vov). In addition, the source follower exhibits other advantages: • No parasitic poles, which avoids having to increase the power to pushing non-dominant poles to high frequen cies. • No common-mode feedback circuit (the output com mon-mode voltage is fixed by the transistor’s VGS). • Low-output impedance to drive following stages. In [1] these source-follower features have been used advantageously in the design of a 2nd-order (biquadratic) low-pass filter and a 4thorder filter, which was made by cascading two 2nd-order cells. We
S. D’Amico, M. De Matteis, A. Baschirotto
University of Salento, Lecce, Italy