Comparative RFI Performance of Bipolar Operational Amplifiers

TLDR: The RFI s u s c e p t i b i l i t i e s o f b i p o la r BJT op amps in a u n i t y gain b u f f e r a m p l i f i e r c o n f i g u r a t i o n are determined from measurements o f demodulation r e s p o n s e s.

Abstract: The RFI s u s c e p t i b i l i t i e s o f b i p o la r BJT op amps in a u n i t y gain b u f f e r a m p l i f i e r c o n f i g u r a t i o n are a s s e s s ­ ed us ing a second order t r a n s f e r f u n c t i o n . Values f o r th e second order t r a n s f e r f u n c t i o n are determined from measurements o f demodulation r e s p o n s e s . The RF c a r r i e r frequency was v a r i e d from 50 kHz t o 100 MHz. The AM modulat ion freque ncy was 400 Hz. Experimental r e s u l t s show t h a t a t RF f r e q u e n c ie s above 1 MHz th e b u f f e r amp­ l i f i e r employing an LM10 op amp i s c o n s i d e r a b l y l e s s s u s c e p t i b l e t o RFI than i s a pA741 op amp. In the RF frequency range 10 MHz t o 50 MHz where the demodula­ t i o n i n t e r f e r e n c e e f f e c t s a re the most i n t e n s e , the d i f f e r e n c e in s u s c e p t i b i l i t y between t h e two b u f f e r a m p l i f i e r s i s about 15 dB. Computer s i m u l a t i o n s can ac co unt f o r th e d i f f e r e n c e in RFI s u s c e p t i b i l i t y , i f th e s i z e s o f p a r a s i t i c s u b s t r a t e c a p a c i t a n c e s in op amp macromodels a re a d j u s t e d . In cr e a s in g the v a l u e s f o r the p a r a s i t i c s u b s t r a t e c a p a c i t a n c e s cau ses the pr e d i c t e d RFI t o i n c r e a s e . Thus, the p a r a s i t i c sub­ s t r a t e c a p a c i t a n c e v a l u e s re qui re d f o r th e pA741 macromodel are l a r g e r than th o se req u ire d f o r the LM10 macromodel .