IC741 Ver la hoja de datos (PDF) - Unspecified

Número de pieza

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Fabricante

IC741

Operational Amplifier

Unspecified 

2.4 Measurement of input bias current

As mentioned eralier, input bias current is very small in magnitude - so, measuring it directly is not

a good idea. However, it can be measured cleverly using the following circuit.

Ra

(a)

R1

+Vcc

−

Ra

(b)

+Vcc

−

+

Rb

−Vcc

Vout

R2

+

Rb

−Vcc

Vout

Figure 3: Circuits to measure input bias currents Ib1 and Ib2, respectively.

Fig. 3(a) is just the circuit for an inverting amplifier, with the input grounded. So, the voltage at

the inverting input terminal should be ideally zero. But from the circuit above, one can see that

the voltage at the inverting input has two contributions - one, Vout reduced by the potential divider

made

out

of

Ra

and

Rb,

i.e.,

V Rb

Ra+Rb out

-

two,

the

voltage

drop

over

the

R1

if

there

is

a

non-zero

input bias current flowing. Thus, we can write

Vi

=

Ra

Rb

+

Rb

Vout

−

Ib1R1

=

0

(3)

or

Ib1

=

1

R1

Ra

Rb

+

Rb

Vout.

(4)

If Ra = 10 kΩ, Rb = 780Ω and R1 = 1 MΩ, we get

Ib1

=

1

1

× 106

780

10000 +

780

Vout

=

10−6Vout

13.82

A

=

Vout

13.82

µA.

(5)

(In the original manual, the value of Rb is wrongly assumed to be 330 Ω, which leads to the wrong

conclusion,

Ib1

=

10−6

330

10000+330

Vout

≈

Vout

31.3

µA)

Input bias current Ib2 can be similarly measured using the circuit in Fig. 3(b), which represents

a non-inverting amplifier, with the input grounded through the resistor R2. The voltage at the

non-inverting terminal would be Ib2R2, which gets amplified to give Vout. Using the relation for

non-inverting gain, one can write

Vout

=

Vi

Rb

+

Rb

Ra

=

Ib2

R2

Rb

+

Rb

Ra

.

(6)

So, the inverting input bias current is given by

Ib2

=

1

R2

Ra

Rb

+

Rb

Vout.

(7)

For R2 = 1 MΩ, one gets

Ib2

=

1

1

× 106

780

10000 +

780

Vout

=

10−6Vout

13.82

A

=

Vout

13.82

µA.

(8)

4

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