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參數資料
| 型號： | AN-24 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | A Simplified Test Set for Op Amp Characterization |
| 中文描述： | 一種簡化的測試設置運算放大器表征 |
| 文件頁數： | 2/10頁 |
| 文件大?。?/td> | 412K |
| 代理商： | AN-24 |
The triangular wave generator is a dc stabilized integrator
driven by the output of the square wave generator and pro-
vides a
g
5V output at the square wave frequency, inverted
with respect to the square wave.
The purpose of these various outputs from the power supply
section will be discussed in the functional description.
BIAS CURRENT TEST
A functional diagram of the bias current test circuit is shown
inFigure 1. The output of the triangular wave generator and
the output of the test circuit, respectively, drive the horizon-
tal and vertical deflection of an oscilloscope.
The device under test, (cascaded with the integrator, A
7
), is
connected in a differential amplifier configuration by R
1
, R
2
,
R
3
, and R
4
. The inputs of this differential amplifier are driven
in common from the output of the triangular wave generator
through attenuator R
8
and amplifier A
8
. The inputs of the
device under test are connected to the feedback network
through resistors R
5
and R
6
, shunted by the switch S
5a
and
S
5b
.
The feedback network provides a closed loop gain of 1,000
and the integrator time constant serves to reduce noise at
the output of the test circuit as well as allowing the output of
the device under test to remain near zero volts.
The bias current test is accomplished by allowing the device
under test to draw input current to one of its inputs through
the corresponding input resistor on positive going or nega-
tive going halves of the triangular wave generator output.
This is accomplished by closing S
5a
or S
5b
on alternate
halves of the triangular wave input. The voltage appearing
across the input resistor is equal to input current times the
input resistor. This voltage is multiplied by 1,000 by the
feedback loop and appears at the integrator output and the
vertical input of the oscilloscope. The vertical separaton of
the traces representing the two input currents of the amplifi-
er under test is equivalent to the total bias current of the
amplifier under test.
The bias current over the entire common mode range may
be examined by setting the output of A
8
equal to the amplifi-
er common mode range. A photograph of the bias current
oscilloscope display is given as Figure 2. In this figure, the
total input current of an amplifier is displayed over a
g
10V
common mode range with a sensitivity of 100 nA per vertical
division.
TL/H/7190–2
FIGURE 2. Bias Current and Common
Mode Rejection Display
The bias current display of Figure 2 has the added advan-
tage that incipient breakdown of the input stage of the de-
vice under test at the extremes of the common mode range
is easily detected.
If either or both the upper or lower trace in the bias current
display exhibits curvature near the horizontal ends of the
oscilloscope face, then the bias current of that input of the
amplifier is shown to be dependent on common mode volt-
age. The usual causes of this dependency are low break-
down voltage of the differential input stage or current sink.
OFFSET VOLTAGE, OFFSET CURRENT TEST
The offset voltage and offset current tests are performed in
the same general way as the bias current test. The only
difference is that the switches S
5a
and S
5b
are closed on
the same half-cycle of the triangular wave input.
The synchronous operation of S
5a
and S
5b
forces the ampli-
fier under test to draw its input currents through matched
high and low input resistors on alternate halves of the input
triangular wave. The difference between the voltage drop
across the two values of input resistors is proportional to the
difference in input current to the two inputs of the amplifier
under test and may be measured as the vertical spacing
between the two traces appearing on the face of the oscillo-
scope.
Offset voltage is measured as the vertical spacing between
the trace corresponding to one of the two values of source
resistance and the zero volt baseline. Switch S
6
and Resis-
tor R
9
are a base line chopper whose purpose is to provide
a baseline reference which is independent of test set and
oscilloscope drift. S
6
is driven from the pulse output of the
function generator and has a duty cycle of approximately
1% of the triangular wave.
Figure 3 is a photograph of the various waveforms present-
ed during this test. Offset voltage and offset current are
displayed at a sensitivity of 1 mV and 100 nA per division,
respectively, and both parameters are displayed over a
common mode range of
g
10V.
TL/H/7190–3
FIGURE 3. Offset Voltage, Offset Current
and Common Mode Rejection Display
2
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