Input stage circuit for an operational amplifier with enhanced input offset voltage trimming capabilities
An input stage circuit for an operational amplifier includes first and second differential pairs connected in parallel between positive and negative input terminals. Each differential pair comprises a pair of transistors that are intentionally and systematically mismatched. The mismatching of each transistor pair creates a pre-trim input offset voltage for the circuit. However, a unique current is utilized to bias each of the first and second differential pairs. By adjusting the differential between the bias currents, a composite input offset voltage is created that combines with the pre-trim input offset voltage to yield a total input offset voltage for the circuit that approaches zero. Additionally, adjusting the differential between the bias currents simultaneously trims the temperature coefficient of the total input offset voltage to zero while using limited power and producing minimal noise.
1 . An input stage circuit for an operational amplifier having first and second input terminals, the input stage circuit comprising:
(a) a first differential pair connected to the first and second input terminals, the first differential pair comprising a pair of transistors that are mismatched; and
(b) a second differential pair connected to the first and second input terminals in parallel with the first differential pair, the second differential pair comprising a pair of transistors that are mismatched;
(c) wherein the first differential pair is biased with a first current and the second differential pair is biased with a second current, the first and second currents being unequal;
(d) wherein mismatching of the pair of transistors in each of the first and second differential pairs creates a pre-trim input offset voltage for the input stage circuit;
(e) wherein at least one of the first and second currents is adjusted relative to the other to reduce the pre-trim input offset voltage created by the input stage circuit.
2 . The input stage circuit as claimed in claim 1 wherein at least one of the first and second currents is adjusted relative to the other to create a composite input offset voltage which is combined with the pre-trim input offset voltage to create a final input offset voltage of zero.
3 . The input stage circuit as claimed in claim 2 wherein mismatching of the pair of transistors in each of the first and second differential pairs creates a pre-trim temperature coefficient of input offset voltage.
4 . The input stage circuit as claimed in claim 3 wherein at least one of the first and second currents is adjusted relative to the other to reduce the pre-trim temperature coefficient of input offset voltage to zero.
5 . The input stage circuit as claimed in claim 2 wherein the pair of transistors in each of the first and second differential pairs is mismatched in value.
6 . The input stage circuit as claimed in claim 5 wherein the pair of transistors in each of the first and second differential pairs is mismatched in value with respect to emitter area.
7 . The input stage circuit as claimed in claim 5 wherein the pair of transistors in each of the first and second differential pairs has a mismatch ratio of n:n+1.
8 . The input stage circuit as claimed in claim 7 wherein each transistor in the first and second differential pairs is a bipolar junction transistor.
9 . The input stage circuit as claimed in claim 7 wherein each transistor in the first and second differential pairs is a metal oxide semiconductor field-effect transistor.
10 . The input stage circuit as claimed in claim 7 wherein each of the first and second differential pairs has an output, the outputs from the first and second differential pairs being combined to form at least one output terminal.
11 . The input stage circuit as claimed in claim 10 wherein the outputs from the first and second differential pairs are combined by an amplifier.