0195136047.pdf

238 ANALOG BUILDING BLOCKS AND OPERATIONAL AMPLIFIERS

−

+

2

3

1

5

6

10

kΩ

Pin 1 labeled “− offset null”

Pin 5 labeled “+ offset null”

V−

Figure 5.3.4Offset null terminals of 741 connected to

10-kpotentiometer and negative supply rail to zero the

offset voltage.

INPUTBIASCURRENTIB

When operated at extremely low quiescent current values, the base bias currents will be low; but

they do have to be taken into consideration. The input bias currentIBfor an op amp is defined as

the average of the two input currents with the inputs grounded (see Figure 5.3.5), i.e.,

IB=

IBp+IBn

2

(5.3.2)

A typical value for aμA 741 is 80 nA; for aμA 771 it is 50 pA. When the resistance of the source

feeding the op amp is large enough, the input bias current may have an adverse effect. As shown

in Figure 5.3.6, where the noninverting input is grounded and the inverting input is connected

to a source of 1-Mresistance and of voltagevSmomentarily at 0 V, the bias current of 80 nA

generates a voltage of−80 mV on the noninverting input.

INPUTOFFSETCURRENTIOS

The adverse effect of the input bias current mentioned would be nullified if both inputs were

connected to equal resistances (one of which could be a passive resistor) and ifIBp=IBn. The

differential input would then be zero. However, if the input bias currents are unequal, as shown in

Figure 5.3.7, there will be a finite differential input voltage. It then becomes necessary to specify

the difference between the bias currents, and this is the input offset currentIOS,

IOS=IBp−IBn (5.3.3)

A typical value forIOSis 20 nA. BothIBandIOSare usually measured with the output at 0 V; but

in practice the output voltage has little effect.

−

+

IBn

IBp

Figure 5.3.5Bias currents of op amp.

−

vS = 0 +

RS = 1 MΩ

VB = −80 mV

80 nA Figure 5.3.6Illustration of input voltage generated by bias

current.