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USEFUL INFORMATION FOR USERS OF TEKTRONIX INSTRUMENTS
NUMBER 18 FEBRUARY 1963
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PRINTED I N U.5.A
INTRODUCTION I faedback If current is applied to the -input, it
TO ,vault1 teiitl to develop voltage across the
OPERATIONAL AMPLIFIERS mpcdruice of ieetlbacli element, and move
(he -input ;~\v;iyfrom ground potential. T h e
sutput, hoivever, slvings in the opposite di-
rection, pro\.itling current to balance the
II n E out input current and hold the -input at
ground. T f the impedance of the feedbacl.:
point elenleiit is high, the output voltage must
(b) I)ccome quite high to pro\~ideenough current
to 11al;mce even n small input current.
Iiip~rt lilcii~eirt %, Coilzvrts Itzpzrf S i p c r l
to C-~trreilt
Since w e more often have to deal with
J \dt;ijie r;itl~erthan current signals, an adtli-
0 tion:il element is used in most operational
E out ;implifier applications, designated "Zi" (input
inipctl;unce). This is an impeclance placed in
~ u i l
point ser-ics wit11 the -input, converting into
(c) cr/rrril/ that parameter of the input signal
\vliich we want to appear a s voltage :it the
Figure 1. Conventional Operational Ampli output (Figure l - c ) .
plifier Symbols.
l i Zi and Zr ;ire both resistors (Figure 2 ) ,
tlie operational ;