Reconciling Spice results and hand calculations: unexpected problems

In an earlier paper, we analyzed and offered solutions to several discrepancies that often arise when students seek to understand and reconcile their hand calculations with SPICE simulation results. This paper presents several additional commonly encountered but quite unexpected problems that frustrate and confuse students. In particular, we discuss the difficulties that arise from the use of Miller compensation in the middle gain stage of a typical bipolar op amp, the discrepancies that occur when a transistor is operating in “high level injection region,” the techniques for minimizing the voltage offsets when the input transistors match perfectly, and the difficulties in calculating CMRR caused by the difference between the ac and dc gains of current mirrors.