Silicon Valley Computer SHUGART 706 User Manual download pdf (Page 54)

FIG. 2- MEASURE

INPUT

OFFSET

VOLTAGE

grounding

the non -inverting input.

With a bi-

polar

power

supply, the inverting

input is held

virtual ground

by the op -amp's output.

FIG.

3- OFFSET VOLTAGE AND

CURRENTS

may

be measured with this

circuit, depending

on the state of

and

S2.

As before.

IB,

refers

to the inverting input

and IB, refers to the non -inverting input.

Input

offset

voltage

Another result

of mismatched differen-

tial-

amplifier inputs is offset

voltage,

Voti.

The error is reflected as

voltage

the output

when

both of

the inputs are

grounded.

A test circuit for measuring Vos

shown in Fig. 2.

Looking at the circuit,

however. we

see that only one input is

directly tied to ground; the other is at a

virtual ground.

That concept is important

in understanding how op -amps work.

One

of the basic properties of an ideal op -amp

is that its differential input

voltage

is zero.

Hence, when

the non -inverting input is

tied low,

the sum of the

voltages

at the

inverting input must

also be zero, or at

ground. That sum

consists of the input

signal and the feedback.

In an ideal

device.

with

no input signal

there

would

be no output.

But, in a real

device mismatched base -to- emitter (or

gate -to-

source,

as the case may be)

volt-

ages between the input transistors

will

cause a DC offset

voltage

at the output.

Input offset

voltage

defined

as the

volt-

age

necessary

at the

inputs

to bring the

output to zero.

Returning to

Fig.

2, in that configura-

tion, the op -amp attempts to balance its

inputs by forcing the inverting input

volt-

age high or low to compensate for

any

voltage

difference between the two in-

puts.

The result

(Vos) can be read directly

with

voltmeter.

A circuit that allows you to measure all

three characteristics (offset

voltage

and

the input bias

currents) is shown in

Fig. 3.

Vos

is measured

by closing both

switches

and

S2. The

value

is read

directly from

the

meter.

The

input

bias currents

are measured by

alternately opening

each switch.

With

open

and S2 closed, we

read an offset

voltage

that

we'll

call

VI.

Closing SI and

opening

S2 gives

V2.

IBI is

calculated

from

this formula:

IBt

Vos

IB2

is calculated in

similar manner:

IB2 = V2 Vos

The calculated bias

currents are displayed

microamps. IB is

derived by averaging

1BI

and IB2;

Ios

is the difference between

them (in microamps).

Variations

offset parameters

The values

Ios

and

Vos

are depen-

dent on the

gains of the input transistors.

Generally,

the lower the

gain of a tran-

sistor,

the less the offset

parameter

will

affected. Lower input

gains, however,

tend

to increase input

bias currents, and

that

reduces input impedance.

In addi-

tion, open -loop gain

of an amplifier

also

has a pronounced

effect on

the overall

influence

of the offset parameters.

Temperature

effects

Offset

differences can

be affected

greatly by temperature

change. It's not

the

offset

itself that is the

culprit. Offset

volt-

age and current

can be compensated

for in

a given

circuit simply

by using trimmer

potentiometers.

The problem is

in the tracking

of the

offset

values.

Even "perfectly"

matched

transistors

have slightly

different tem-

perature

curves. The differences

arise be-

cause

of slight temperature

gradations

within

the semiconductor

substrate of the

device itself, and because

of impurities

that are introduced

during the manufac-

ture of

the device.

The result is that one

transistor's leak-

age current or offset voltage

can change at

a rate that is faster

than that of

the other

transistor, thereby

creating mis- matched

outputs. For

example, an offset voltage

that is handily

compensated for at room

temperature

may prove too much

to han-

400

350

300

250

200

150

100.

-75

-50 -25 0 +25 +50

+75 +100

+125

TEMPERATURE-

°C

FIG.

4 -INPUT BIAS CURRENT is

function

temperature.

dle at 125 °E. The

overall effect is

called

temperature drift.

All op -amps

are designed to minimize

the effects

of temperature

drift -some

just do it better than

others. The effects

temperature

drift can be measured

using

the circuit shown

in Fig. 3.

With

both SI

and S2 closed, record

the

value

of Vos

room

temperature. Now

raise the tern -

perature

and note how Vos

changes.

With

properly tracking

transistors, the

change

should

be slight, if it's

noticeable at all.

The table in

Fig.

shows how

input bias

current

of a typical

device

varies

with

changes in temperature.

Temperature

drift is normally

ex-

pressed

numerically in

terms of tem-

perature coefficient.

The temperature

co-

efficient of offset voltage

is calculated

from

this formula:

TEMPERATURE COEFFICIENT =

ovos

and

the temperature

coefficient of

offset

current

is calculated:

TEMPERATURE

COEFFICIENT

abs o

Temperature

coefficient usually is spec-

ified in

units of microvolts per degree

Centigrade (p.V

/ °C)

and in

units of

micro-

amps

, nanoamps, or picoamps per

degree

Centigrade (µA / °C, nA / °C, or pA / °C),

depending on the type of

device.

Remember, we're not

concerned

with

the actual change in input

bias -it

will

fluctuate. What

concerns us is that the

individual input currents change

together.

The smaller

the temperature coefficient,

the better the

two

inputs

track.

Aging

Another

factor that influences how an

op -amp

works

aging. Op -amp charac-

teristics have a tendency to change as op-

erating time increases.

Change

especially noticeable in the offset

values.

Devices that change much can be incon-

venient

to use, because the

equipment

which

they're used may have to calibrated

fairly

often.

Aging is

accelerated

with

continuous

exposure to elevated temperatures -such

as that frequently experienced inside an

enclosure.

Fortunately,

aging

is not a

con-

tinuous process. Most

changes

take

place

within

the first 100 hours of operation.

Once

stabilized,

you can expect an

ampli-

fier

to perform

very

reliably over the next

10,000 hours or so.

In fact. forced aging is one

way

to in-

crease

life

and enhance

the

performance

of a part, as

shall learn later on in this

series.

Common

-mode rejection ratio

Another

consequence of

imperfect

matching is common -mode

voltage.

Nor-

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Silicon Valley Computer SHUGART 706 User Manual Page 54

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