0195136047.pdf

PROBLEMS 263

5.3.4In order to minimize output voltage offsets in
practical op-amp circuits, one provides a dc path
from each input terminal to ground, makes each
input terminal see the same external resistance
to ground, and uses external balancing circuits,
if necessary, to null any remaining output offset
voltage.
(a) Consider the input-offset voltage-nulling cir-
cuit for an inverting amplifier shown in Figure
P5.3.4 (a). LetR 1 = 1 .5k, R 2 =22 k,
R 6 =100 k, R 5 =500 k, R 4 = 200 ,
andV=12 V. Find the range of input offset
voltages that can be generated at terminal 2 of
the op amp. Also findR 3 such that the input
terminals see the same external resistance to
ground.
(b) To examine the effects of input bias currents
on the inverting amplifier, consider the circuit

shown in Figure P5.3.4(b). Show that it is

desirable to chooseR 3 , which is equal to a

parallel combination ofR 1 andR 2. Compare

the residual output voltage to what occurs if

R 3 were zero.

(c) Reconsider the circuit of part (b). LetR 1 =

5k, andR 2 =70 k. Let the op-amp bias

currents beIb 1 =50 nA andIb 2 =60 nA, but

otherwise let the op amp be ideal. Determine

the value ofR 3 that should be used. Also, when

the input signal is zero, find the residual output

offset voltage.

5.3.5A noninverting op-amp circuit and its closed-loop

representation are given in Figure P5.3.5. Obtain

an expression for the closed-loop transfer function

H(ω)=Y (ω)/X(ω)and comment on how it

behaves for large loop gain [i.e., when the product

H 1 (ω)H 2 (ω) is large].

−

+

vo

vi

R 1 R 2

R 5

R 6

R 3

R 4

1

3

(a)

2

+V

−V

vo

Ib 1

1

2

(b)

+

−

v 1

v 2

I 1 I 2

R 1 R 2

Ib 2

R (^3) I
b 2
Figure P5.3.4