The aim of this research programme was to design and develop a novel CMOS
current conveyor, to improve areas such as bandwidth, slew rate, gain, and Powe-
Supply Reject Ratio (PSRR). The current conveyor can be used in low frequency
applications such as LED drivers for mobile phones and televisions, and high
frequency applications such as mixers for up/down converters used in anything from
radios to mobile phones.
The initial part of the research looked into improving the Power Supply Rejection
Ration (PSRR) of the current follower (mirror) by increasing its output impedance.
Several types of current mirror were compared using analytical and simulation
methods, using a new generic low frequency transistor model which was used to
highlight the differences in impedance between BJT and CMOS current mirrors. It
was found that the best type of mirror was the regulated cascode current mirror which
offered the largest value of output impedance when built from CMOS transistors.
Work then moved onto the voltage follower. By initially using a typical CMOS source
follower, it was found that the voltage gain suffered from low values
transconductance, drain/source resistance, and a larger than expected value of source
resistance, which was extracted from simulation and was found to be around 300-
350Q. The best design was a two stage un-buffered amplifier which offered the best
Power Supply Rejection (PSRR) voltage gain and bandwidth.
Several different types of current conveyor (CCII+) were simulated and the results
were compared. It was found that the best types of current conveyor were the cascode
type conveyors which offered a voltage gain error of less than 1%. The regulated
cascode type current conveyor offered the highest figure of PSRR that of around
60dB.
Finally the new cascode type current conveyors were used to build examples of
current feedback operational amplifiers (CFOAs), and the cascode type CCIl+ offered
a voltage gain error of less than I%, largest bandwidth and best P SRR