Folded cascode field-effect transistor amplifier with increased gain

Abstract

A fully differential CMOS operational amplifier (10) of the folded cascode type has a differential input network (12) and a transimpedance stage (14). The transimpedance output stage (14) is made up of two similar negative and positive signal branches (32,34) connected in parallel with each other between supply voltage nodes (V+,V-). Each signal branch includes a P-channel current source transistor (48,60), a cascode transistor (50,62), an N-channel a pull-down transistor (40,52), and an N-channel current source transistor (42,54), all connected in tandem, respectively, between a positive supply voltage node and the drain of a feedback transistor (44,56), which has its source connected to a negative supply voltage node. The drains of the feedback transistors of the two branches are joined. A second cascode transistor (64,66) has its source connected to the source of the first cascode transistor (50,62) and its drain connected to the source of the pull-down transistor (40,52) to increase the output impedance at the drain of the first cascode transistor without affecting the signal current-carrying capability of the signal branches (32,34) by permitting the cascode transistors and the pull-down transistors connected to the output nodes to have a reduced width-to-length ratio.