The main specifications of amplifiers required for
High Frequency Switched-Capacitor (HF SC) filter are high gain, fast response and good accuracy.
Different cascode Operational Transconductance Amplifier (OTA) topologies have been presented and compared in this
paper. The amplifier structures have been analyzed
in terms of their Alternating current (AC), Direct
Current (DC) and transient performance parameters using
T-Spice simulations. The multipath amplifier with enhanced gain, approximately one and half times unity
Gain Band Width (GBW) as that of Current-Mirror Cascode (CMC) amplifier and more than 30% improvement in
slew-rate values with respect to Folded Cascode (FC) amplifier has been presented in this paper.
High Frequency Switched-Capacitor (HF SC) filters with a sampling frequency higher
than 100 MHz require an amplifier with settling time below 3-4 ns for good performance
(Wang and Embabi, 2003 and Thandri and Martinez, 2006). The settling time consists of slewing
and linear settling portions. During linear settling, the amplifier can be modeled with its
small-signal equivalent and this time depends on Gain-BandWidth (GBW) and phase margin.
The Direct Current (DC) gain must also be high enough to ensure accurate settling.
It is difficult to design an amplifier with both high gain and high bandwidth.
The Folded Cascode Operational Transconductance Amplifiers (FC OTAs) have always
been traditionally preferred for HF SC applications (Olivera and Martynez, 1999) because of
their high DC gain and GBW. The telescopic OTA provides better current efficiency and
hence consumes less power, but suffers from limited output swing (Quinn et al., 2000). |
