Computer-Aided Analysis of RFI Effects in Operational Amplifiers

The modified Ebers-Moll model is used to predict RFI effects in the 741 operational amplifier (op amp)-a bipolar linear integrated circuit (IC). RFI susceptibility predictions for RF incident upon the op-amp input terminals are made using a complete model, a macromodel, and a voltage-offset model. Both the batch-mode computer program SPICE 2 and the commercial interactive computer program ISPICE are used. The three sets of calculated results are essentially identical and agree within 4 dB with experimental results measured at 220 MHz. A threshold cannot be given for the RF power level at which a 741 op amp is susceptible to RFI. The level depends upon the op-amp circuit application. For op-amp circuits designed to amplify input signals in the 0.1-to 1.0-V range, RF power levels as large as -15 to + 5 dBm may be required to cause RHI susceptibility problems. For op-amp circuits designed to amplify input signals in the 1-to 10-mV range, RF power levels as low as -55 to -35 dBm may cause RHI susceptibility problems.