Using a Macromodel to Predict RFI in Bipolar Operational Amplifiers

A macromodel containing two transistors can accurately predict how amplitude modulated RF sig­ nals with RF carrier frequencies in the .05 to 100 MHz range are demodulated in a bipolar operational amplifier (pA741 op amp) to cause undesired low frequency responses related to the modulation en­ velope 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.