In continuous-time filter design, high output impedance
current-mode active devices are needed to enable filtering
at low frequencies and reduce filtering errors [7, 8]. As
many COA-based filters are presented in the literature [9 -
11], it would be very useful to design a COA with very
high output impedance.
In this paper, high-drive high-output impedance COA is
presented. Both the input and output stages present classAB
operation which enables high drive capability with
low quiescent current. At the output stage of the proposed
COA instead of simple cascode one regulated cascode
(RGC) [12] stage is used to achieve very high output
resistance.
Figure 1. Circuit symbol of fully differential COA
II. PROPOSED COA
Figure 2 shows the CMOS realization of the proposed
two-stage COA with the transistor dimensions and DC
values reported in Table 1 and Table 2 respectively. Both
input and output stages operate in class-AB which enables
high input-output current swing and drive capability.
Input resistance equations of the proposed COA are:
capacitance (Cc) is inserted at the high
impedance node series with M13 which operates like a
resistance. M13 is modelled with the resistance - Rc.