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Opamp Offset Parameters and Saturation
In AC analysis, a DC operating point is calculated at the beginning of the analysis. The circuit is then linearized around this calculated point. In simulating a circuit with an operational amplifier that uses a Micro-Cap .model statement, you have to be conscious of the effect that the three offset parameters may have on that DC operating point calculation. In high gain configurations, the offset parameters may wind up saturating the opamp at the operating point. The saturation would skew any expected results.
The three offset parameters are VOFF, IOFF, and IBIAS. VOFF is the input offset voltage. IOFF is the input offset current. IBIAS is the input bias current. In many data books, the offset parameters are only specified with their maximum values, so those would be the values specified in the library. The figure below displays a schematic that contains two identical circuits. Each of the circuits contains a pair of cascading opamps designed to produce a high gain. The top circuit uses the LF347 opamp model from the Micro-Cap library. The bottom circuit uses the LF347 model also, but the three offset parameters have been edited so they would not affect the simulation. VOFF has been set to 0, IOFF has been set to 0, and IBIAS has been set to 50f.
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The first figure below displays the DC operating point voltages for both circuits. The second opamp in the top circuit is saturated at -9.5 volts, while the second opamp in the bottom circuit is still in its linear area of operation at -6.8 volts. These are the voltages to which the circuit is to be linearized to. Since the top circuit is saturated, the full gain of the circuit will not be available for small signal analysis. This is apparent in the bottom figure. The blue plots are for the top circuit, and the red plots are for the bottom circuit. Due to the offset saturation, the gain has dropped from 107dB to 14dB.
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