Macromodel Predictions for EMI in Bipolar Operational Amplifiers

A macromodel containing two transistors can accurately predict how amplitude-modulated RF signals with RF carrier frequencies in the 0.05-to 100-MHz range are demodulated in a bipolar operational amplifier (¿ A741 op amp) to cause undesired low-frequency responses related to the modulation envelope of the RF signals. The original op-amp macromodel, which was developed by Boyle et al., was modified by adding four capacitors (each 4 pF) to account for parasitic-capacitance effects in the integrated-circuit op amp. The macromodel can be used when the RFI signals are incident upon the op-amp input terminals. The use of the macromodel leads to a substantial reduction in computer time and expense.