Statistics for Demodulation RFI in Inverting Operational Amplifier Circuits

Abstract

The paper describes an investigation to determine statistical variations for RFI demodulation responses in operational amplifier (op amp) circuits. AmplitudeModulated (AM) RF signals were injected into the op amp signal input terminals to produce undesired demodu­ lated responses at the 1 kHz AM-frequency. The RF frequency was varied over the range 0.1 to 400 MHz. First, 30 to 35 741 op amps were tested in an invert­ ing amplifier circuit with 10 kfl input resistor and 100 kfl feedback resistor (voltage gain A^ = -10) with­ out and with RFI suppression capacitors (27 pF). Mean values and standard deviations were determined for the demodulation RFI which was characterized by a nonlinear transfer function H2. When the RFI suppression capa­ citors were included, the experimental mean values for H2 were suppressed from 10 to 35 dB over the RF fre­ quency range 0.1 to 150 MHz except at 0.15 MHz where only 3.5 dB suppression was observed. Next, experi­ mental and NCAP computer simulation results are com­ pared for a 3-stage op amp circuit without and with RFI suppression capacitors. Macromodels were used for the 741 op amps in the NCAP simulations. Finally, the first op amp stage input resistor and feedback resistor were increased by a factor of ten. Four op amp types were inserted in the first stage. The op amps investigated were the 741 bipolar op amp (npn input transistors), the LM10 bipolar op amp (pnp input transistors), the LF355 JFET-bipolar op amp (JFET input transistors), and the CA081 MOS-bipolar op amp (MOSFET input transistors). Approximately 30 of each type were tested. Mean values and standard deviations were measured for demodulation RFI. At RF frequencies below 1 MHz, the CA081 op amp mean demodulation RFI response was 20 to 40 dB less than that of the other op amps. Beginning at 1 MHz, the LF355 op amp mean demodulation RFI response decreased rapidly with in­ creasing RF frequency and above 3 MHz was less than that of the CA081 op amp. From 3 to 200 MHz CA081 and LF355 mean demodulation RFI ‘responses were 20 to 30 dB less than that of the 741 op amp. The LM10 op amp mean demodulation RFI response decreased slowly from 3 MHz where it was the same as the 741 op amp to 200 MHz where it was between that of the CA081 and LF355 op amps.