The doping gas flow over the wafer is perturbed by the surface height changes which will make the transistor doping profiles different for each transistor. That is why I suggested a low value of 1.005 for making a small offset. The input transistors are made intentionally large so that the areas do not change much when the linear dimensions change from the oxide etching tolerances. If you are trying to find the range of offsets for an op amp design, you will have to vary the beta of all transistors as well as their areas and early voltages. The added floating voltage source is the best way to add offset to a macromodel. My suggestions were on how to put in some offset for seeing what effect it has on circuit performance.
This model is consistent with the observation that in a real op amp, the output is zero when there is a difference in the input ( V + ≠ V – ) and that a real op-amp produces a nonzero output when V + = V –. The presence of offset can be encapsulated by assuming that the real Op Amp input/output transfer characteristic is y = A ( V + – V – + e ) where e is the error in the differential input to the ideal Op Amp. The transfer function of an ideal Op Amp is described by the equation y = A ( V + – V – ), where y is the output A is the gain, with A → ∞, V + is the voltage at positive input terminal and V – the voltage at the negative input terminal of the Op Amp. In this article, a generalized method is proposed to compute offset in the output when an Op Amp with an input offset e is used in the circuit. The presence of offset voltage is a well-understood phenomenon and is described in various literature and textbooks. In addition, they can reduce the dynamic range of the output if significant in value. Offset voltage of an Op Amp results in an error at the output for DC signals. In such applications, the presence of offset voltage cannot be ignored unlike in a signal processing chain where DC offsets can be easily filtered out with a single capacitor. One such environment is DC measurement systems.
Idealized models of the Op Amp, namely, infinite values of gain, bandwidth, input impedances and output admittance and zero values of input offset voltage and bias currents, are a good first-order approximation for analyzing Op Amp-based circuits.ĭeviation from ideal behavior can be incorporated into analysis depending on the environment in which the Op Amp is operating. Although functionally simple, they exhibit complex behavior as the Op Amp itself is a carefully crafted sub-circuit consisting of more than a dozen transistors. Op Amps are among the most widely used components in systems design of electronic circuits.
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