Using NCAP to Predict RFI Effects in Operational Amplifiers

Abstract

Applications of the Nonlinear Circuit Analysis Program NCAP to calculate RFI effects in electronic circuits containing discrete semiconductor devices and small scale linear bipolar integrated circuits (differential pairs) have been reported upon previously. The objective of this paper is to demonstrate that the computer program NCAP can also be used to calculate RFI effects in linear bipolar integrated circuits (ICs) of greater complexity. The IC reported upon is the μA741 operational amplifier (op amp) which is one of the most widely used ICs. The μA741 op amp was used as the active component in a unity gain buffer amplifier. The computer program NCAP was used to predict how amplitude modulated RF signals are demodulated in the op amp to cause undesired low frequency responses. The predicted and measured results for RF frequencies in the .050 to 100 MHz range are in good agreement. Contained in the paper is a complete set of NCAP parameter values for all the transistors in the μA741 op amp. These parameter values may be used as typical data for similar transistors in other bipolar integrated circuits.