Folded cascode operational amplifier with gain enhancing base current compensation

In a folded cascode operational amplifier having:

• a) a differential input stage comprising first and second differentially connected input transistors with respective current circuits, a first input current source supplying current differentially from a first reference voltage line to the current circuits of said first and second input transistors, and first and second input resistors connected in series between the current circuits of said first and second input transistors, respectively, and a second reference voltage line,
• b) a gain stage comprising first and second bipolar active load transistors having their collector-emitter circuits connected on one side through respective first and second active load current sources to said first reference voltage line and on the other side to said first and second input resistors, respectively, a bias circuit for said active load transistors comprising (i) a bias current source connected on one side to said second reference voltage line and (ii) a bias transistor with its base connected between the collector-emitter circuit of said first active load current source and said first active load transistor, and its collector-emitter circuit connected in common with the other side of said bias current source to the bases of said first and second active load transistors, a first bipolar gain transistor having its base connected between said second active load current source and the collector-emitter circuit of said second active load transistor, and a final gain current source connected between one side of the collector-emitter circuit of said gain transistor and said second reference voltage line, and
• c) an output stage comprising an output current source, an output bipolar transistor having its base connected between said gain current source and the collector-emitter circuit of said gain transistor, and its collector-emitter circuit connected in series with said output current source between said first and second reference voltage lines, and an output terminal connected between said output current source and the collector-emitter circuit of said output transistor to supply an output current, with changes in the output current producing corresponding changes in the gain transistors collector-emitter current,
• wherein the improvement comprises:
• a first current compensation bipolar transistor having its collector-emitter circuit connected to supply current from said first reference voltage line to the collector-emitter circuit of said gain transistor, and its base connected between said first active load current source and said first reference voltage line, a change in the output current from said output terminal being reflected by said output transistor to change the collector-emitter current of said gain transistor and thus its base current, said first current compensation transistor together with said first active load current source, first active load transistor, first and second input resistors and second active load transistor comprising a current compensation loop that responds to a change in the gain transistor's collector-emitter current by supplying an associated change in the gain transistor's base current, thereby increasing the amplifier's transconductance.

In a folded cascode operational amplifier having an output stage for supplying a variable output current load, a gain stage having a first gain transistor, said gain transistor including a control circuit and a current circuit whose current varies as a function of the output current, and a differential input stage whose differential input voltage varies as a function of the current through the first gain transistor's control circuit, the improvement comprising:

• a first current compensation circuit connected between the current circuit and the control circuit of said first gain transistor to supply changes in the first gain transistor's control current associated with changes in the current through its current circuit, and thereby increase the amplifier's transconductance, said first current compensation circuit comprising a first compensation transistor having a current circuit connected in series with the current circuit of said first gain transistor such that the first compensation transistor's control current approximately tracks the first gain transistor's control current, with the remainder of the first current compensation circuit actuated by changes in the first compensation transistor's control current to supply a compensating current to the control circuit of said first gain transistor,
• a second gain transistor that has a current circuit and a control circuit and is nominally matched with the first gain transistor and connected to balance the first gain transistor in the gain stage, and
• a second current compensation circuit connected between the current and control circuits of said second gain transistor to supply changes in the second gain transistor's control current associated with changes in the current through its current circuit, said second current compensation circuit including a second compensation transistor having a current circuit connected in series with the current circuit of said second gain transistor such that the second compensation transistor's control current approximately tracks the second gain transistor's control current, with the remainder of the second current compensation circuit actuated by changes in the second compensation transistor's control current to supply a compensating current to the control circuit of said second gain transistor.