Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~28~7~ ,
--1--
ELECTRONIC DEVICE MEASUREMENT APPARATUS
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for
measuring characteristics of electronic devices, such as
transistors, diodes, etc.
Known measurement devices will be described in
detail hereinbelow.
SUMMARY OF THE INVENTION
In accordance with an aspect of the invention
there is provided an electronic device measurement
apparatus, comprising: voltage supply means for supplying
a voltage between first and second terminals of an
electronic device under test: a current detecting resistor
inserted between said voltage supply means and the second
terminal of said electronic device; a voltage divider
connected between one terminal of said current detecting
resistor adjacent to said voltage supply means and the
first terminal of said electronic device; a first buffer
amplifier having an input terminal connected to an output
terminal of said voltage divider; a second buffer amplifier
~213~
having an input terminal connected to the second terminal
of said electronic device; and an operational circuit to
receive output signals from said first and second buffer
amplifiers and a signal at said one terminal of said
current detecting resistor; wherein a characteristic of
said electronic device is measured in accordance with a
voltage across said current detecting resistor and an
output signal from said operational circuit.
Since the voltage divider for dividing the voltage
between the first and second terminals of the DUTY is not
connected to the common junction of the second terminal of
the DUTY and the current detecting resistor and the input
impedances of the buffer amplifiers are very high, only
the current flowing between the first and second terminals
of the DUTY flows through the current detecting resistor
and this current can be detected accurately. As the
result that the first buffer amplifier receives the first
terminal voltage divided by the voltage divider, this
first buffer amplifier does not receive the high voltage
and it is not needed to be floated. When the voltage
divider divides the voltage between the first and second
terminals of the DUTY an error voltage occurs because of
the voltage across the current detecting resistor. This
error voltage is compensated by applying the output signals
from the first, second and third buffer amplifiers to the
operational circuit. The second buffer amplifier prevents
I ~22~7~
the current at the second terminal of the DUTY from flowing
into operational circuit, and the third buffer amplifier
prevents the current from flowing from the power voltage
source of the second buffer amplifier through the output
terminal thereof and the operational circuit to the power
voltage source of the DUTY Thus, the measurement can be
accomplished accurately.
It is, therefore, an object of the present
invention to provide an apparatus which measures
characteristics of electronic devices accurately.
It is another object of the present invention to
provide an electronic device measurement apparatus which
detects accurately a current flowing a DUTY regardless of a
voltage divider connected to the DUTY
It is a further object of the present invention
to provide an electronic device measurement apparatus
which does not need a complex and expensive floating
; buffer amplifier.
Other objects and advantages of the present
invention will become apparent to those having ordinary
skill in the art when taken in conjunction with the
accompanying drawings.
DRAWINGS
FIG. 1 shows a block diagram of a conventional
electronic device measurement apparatus;
FIG. 2 shows a simplified block diagram of
I ~28~7~
another conventional electronic device measurement
apparatus;
FIG. 3 shows a simplified block diagram of a
third conventional electronic device measurement
apparatus; and
FIG. 4 shows a circuit diagram of a part of a
electronic device measurement apparatus according to the
present invention.
RETAILED DESCRIPTION OF THE INVENTION
An electronic device measurement apparatus
called generally a curve trace is useful to measure
characteristics of electronic devices, such as transistors,
diodes, etc. A typical one of conventional electronic
device measurement apparatus is illustrated in FIG. 1,
I wherein an AC voltage from a line source is applied
through a switch 10 to a power supply circuit 12 and a
variable transformer 14. The power supply circuit 12
produces do voltages for each circuit of the measurement
apparatus, and the variable transformer 14 supplies an AC
voltage having a controlled amplitude to a primary winding
of a transformer 16. A plurality of taps are provided at
a secondary winding of the transformer 16, and one of
these taps excluding the lowest position tap is selected
by a switch 18. A diode 20 is a half-wave rectifier which
\
- 5 - ~22~7~
rectifies the AC voltage from the switch 18. These
elements 10-20 construct a floating collector supply
circuit 21. The cathode voltage of the diode 20 is
applied to a first terminal of a device under test (DUTY),
S e.g., the collector of a transistor 26 through a load
resistor 24 selected by a switch 22. The base of the
transistor 26 receives a step bias signal from a bias
supply circuit 28 and the emitter thereof as a second
terminal is grounded. The lowest position tap of the
secondary winding of the transformer 16 is connected
to the emitter of the transistor 26 through a collector
current detecting resistor 32 selected by a switch 30.
A voltage divider 36 selected by a switch 34 divides the
collector voltage Vc of the transistor 26, and the divided
voltage is applied to a horizontal deflection plate of a
cathode ray tube (CRT) 40 through an amplifier 38 having a
high input impedance. A differential amplifier operettas
as a high input impedance voltage detector which detects a
voltage across a resistor 32 (being substantially pro-
portion Al to the collector current Icy and applies into a vertical deflection plate of the CUT 40. Thus, the
CRT 40 may display the Vc-Ic characteristic curve of the
transistor 26. In FIG. 1, the DUTY 26 is the common emitter
transistor. However, any electrode of the transistor may
be grounded, and the DUTY may be other electronic devices,
such as a diode. In any cases, the voltage-current
6 ~22~3~7~i
characteristic of the DUTY may be displayed on the CRT 40.
On the other hand, the switches 30 and 34 are
controlled in accordance with a desired measurement range.
A horizontal-axis size of the display is determined by a
dividing ratio of the voltage divider 36 selected by the
switch 34, while a vertical-axis size thereof is determined
by a value of the resistor 32 selected by the switch 30.
The maximum voltage to be applied to the DUTY or the
measurement range is determined by the adjustment of the
variable transformer 14 and the selection of the switch
18. Thus, it is necessary to select the resistor 32
having the small value when measuring a large current
flowing through the DUTY and it is necessary to select
the voltage divider 36 having the large dividing ratio
when measuring a large voltage across the DUTY
As described herein before, the voltage divider 36
is necessary to adjust the measurement range. As a result
of the voltage divider 36, the current flowing through the
selected resistor 32 is the sum of the collector current
of the transistor 26 and the current flowing through the
voltage divider 36. It should be noted that the base
current of the transistor 26 flows through ground to the
bias supply circuit 28 and does not flow through the
resistor 32, because the output current value of the
collector supply circuit 21 (at the cathode of the diode
20) is equal to the input current value thereof (at the
I ~228~
lowest position terminal of the transformer 16). Thus, the
voltage across the resistor 32 is not directly proportional
to the collector current of the transistor 26 and leads to
error in the measurement result.
An another conventional measurement apparatus
overcoming this measurement error is Tektronix curve tracer
model 576, and a simplified block diagram is shown in FIG.
2. FIG. 2 shows only improved parts of the block diagram
of Fog. 1. In FIG. 2, a first voltage divider 36 con-
sitting of resistors 44 and 46 is connected between the
collector of the transistor 26 and the collector supply
side of the current detecting resistor 32, and a second
voltage divider 52 consisting of resistors 48 and 50 is-
connected in parallel with the resistor 32. Divided out-
puts from the first and second voltage dividers 36 and 52
are applied to a differential amplifier 54. It should be
noted that the value of the resistor 32 and the dividing
ratio of the first voltage divider 36 are variable. Since
a current flowing through the voltage divider 36 flows
through the voltage divider 52 instead of the resistor 32
because values of the resistors 48 and 50 are very large,
this current does not lead to the measurement error. A
voltage across the first voltage divider 36 does not
correspond to the voltage between the collector and the
emitter of the transistor 26 (the first and second
terminals of the DUTY because of the voltage across the
Jo
-8- ~28~7~
resistor 32, but an output voltage from the differential
amplifier 54 is not affected by the voltage across the
resistor 32 because a dividing ratio of the second voltage
divider 52 is set to be equal to that of the first voltage
divider 36. In this prior art, the values of the resistors
48 and 50 are set to be very large with respect to the
value of the resistor 32. However, when the resistor 32
of a high resistance is selected by the switch 30 for
measuring a small current, a small part of the collector
current of the transistor 26 is shunted into the second
voltage divider 52. Thus, the error occurs in the
measurement result.
This measurement error is overcome by Tektronix
curve tracer model 577 of which a main part is shown in
FIG. 3. In FIG. 3, the collector of the transistor 26
(the first terminal of the DUTY is connected to a buffer
amplifier 56, and an output voltage therefrom is applied
to the voltage divider 36. Since an input impedance of
the buffer amplifier 56 is very high, only the collector
current of the transistor 26 flows through the current
detecting resistor 32. Moreover, the divided output
voltage from the voltage divider 36 is accurately
proportional to the collector voltage of the transistor
26. There are various kinds of Duty including a high
voltage transistor, so that an input voltage of the buffer
-9- 1.;22~3~L7~
amplifier 56 changes in a wide range. Consequently, the
buffer amplifier 56 should be floated. The floating
amplifier needs a special power supply, a chopping
amplifier, etc. and the measurement apparatus becomes
complex and expensive in construction.
Referring to FIG. I, there is shown a circuit
diagram of a part of a preferred embodiment according to
the present invention wherein the voltage detector for
detecting the voltage across the DUTY This embodiment
uses Kelvin sensing technique for avoiding that the
measurement result is affected by contact resistances
when the DUTY is connected to the electronic device
measurement apparatus. The collector of the
transistor 26 as the DUTY is touched to contacts 58 and
60 and the emitter thereof is touched to contacts 62
and 64. This is accomplished by putting the electrode
of the transistor between the two contacts. The contacts
58 and 60 correspond to the first terminal of the
8~L7~
DUTY and the contacts 62 and 64 correspond to the
second terminal thereof. The contact 58 is
connected through the selected load resistor 24 to
one electrode of a floating collector supply 21,
and the contact 62 is connected through the
selected current detecting resistor 32 to the other
electrode of the floating collector supply 21.
The collector supply 21 may be the circuit
consisting of the transformer 16, the switch 18 and
the diode 20 shown in FIG. 1. The contact 62 is -
grounded for the base current path. The voltage
divider 36 consisting of resistors 44 and 46 is
connected between the contact 60 and the collector
supply side of the resistor 32. The resistors 44
and 46 may be changed by the switch 34 of FIG. 1.
The dividing ratio of the voltage divider 36 is
variable and defined as n. Thus, R44=(n-l)R46,
wherein R44 and R46 represent the values of the
resistors 44 and 46, respectively.
An output terminal of the voltage
divider 36 is connected to an input terminal of a
first buffer amplifier 66, the contact 64 is
connected to an input terminal of a second buffer
28~
amplifier 68, and the collector supply side of the
resistor 32 is connected to an input terminal of a
third buffer amplifier 70. These buffer amplifiers
are, for example, operational amplifiers connected
as voltage followers. Since currents flowing
through the contacts 60 and 64 are much less than
currents flowing through the contacts 58 and 62
because of the high resistance of the voltage
divider 36 and the high input impedances of the
buffer amplifiers, the voltages across the DUTY can
be detected without being affected by the contact
resistances. An output voltage from the buffer
amplifier 66 is divided by resistors 72 and 74 and
applied to a non-inverting input terminal of a
buffer amplifier 76. The output voltages from the
buffer amplifiers 68 and 70 are applied to an
inverting input terminallof the buffer amplifier 76
through resistors 78 and 80, respectively. A
feedback resistor 82 is inserted between an output
terminal and the inverting input terminal of the
buffer amplifier 76. The resistances of the
resistors 78 and 80 are changed in accordance with
the dividing ratio of the voltage divider 36 as
,, ~228~7~;
described hereinafter. These resistors 72, 74, 78
through 82 and the operational amplifier 76
construct an operational circuit. Resistors 84 and
86 are inserted between the contacts pa and 60 and
between the contacts 62 and 64, respectively, the
values of the resistors 84 and 86 being much larger
than the contact resistance. Thus, voltage levels
of the contacts 58 and 60 are equal to each other
and voltage levels of the contacts 62 and 64 are
equal to each other, even if the DUTY is not
connected. The output terminal of the amplifier 76
may be connected to the amplifier 38 of FIG. 1 and
both the terminal of the resistor 32 may be
connected to the differential amplifier 42 of FIG.
1.
A current of the voltage divider 36 flows
from the collector supply 21 through the contacts
58 and 60 (a small part of the current flows
through the resistor 84) and the voltage divider 36
to the collector supply 21. A current for the DUTY
26 flows from the collector supply 21 through the
contact 58, the DUTY 26 (the collector-emitter path
of the transistor 26), the contact 62 and the
8~7~i
resistor 32 to the collector supply 21. Thus, the
current flowing through the resistor 32 is not
affected by the voltage divider 36. Since the
current from the bias supply circuit 28 flows
through the base-emitter junction of the transistor
26, the contact 62 and ground to the bias supply
circuit 28, this current does not flow through the
resistor 32. Consequently, the current flowing
through the resistor 32 is only the current flowing
between the first and second terminals of the DUTY
26, so that the current of the DUTY can be detected
accurately.
The voltage at the upper terminal of the
voltage divider 36 is the voltage at the first
terminal of the Depth collector of the transistor
26), however, the voltage at the lower terminal of
the voltage divider 36 is the voltage at the second
terminal of the Depth emitter of the transistor
. 26) offset by the voltage across the contact
resistance of the contact 62 and the voltage
across the resistor 32. Assuming that the
voltages at the upper and lower terminals of the
voltage divider 36 are respectively Vc and VG and
the voltage at the second terminal of the DUTY is
VEX the divided output voltage ED of the voltage
divider 36 is as follows:
VD=(VC-vG)/n+vG
Since the gains of all the buffer amplifiers 66,
68 and 70 are +1, the output voltage V0 is as
follows:
Vo={R74/(R72+R74)}{R82/(R78//R80)+1}VD
-(R82/R78)vE
-(R82/R8o)vG
wherein R72 through R82 represent the values of
the resistors 72 through 82, respectively, and
R78//R80 represents the parallel resistance of
the resistors 78 and 80. Assuming that
R72=R74=R82, R78=nR72 and R80={n/(n-l)}R72,
V0 is as follows:
Vo=(Vc-VE)/n
Thus, the voltage V0 is the voltage between the
first and second terminals divided by n. The
operational circuit) compensates the
voltages across the contact and the resistor 32.
The buffer amplifier 70 is provided instead of
directly connecting the lower terminal of the
~L2~8~7~
, ,~, Jo
--aye--
resistor 80 to the right side of the resistor 32,
so that the current from the power terminal of the
buffer amplifier 68 is prevented from flowing
through the resistor 32. Therefore, the voltage
between the first and second terminals of the DUTY
and the current flowing through the DUTY can be
measured accurately with a simple construction.
It should be noted that the buffer amplifiers may
be emitter follow amplifiers or voltage follow
amplifiers.
While I have shown and described herein
the preferred embodiment of my invention, it will
be apparent to those skilled in the art that many
changes and modifications may be made without
departing from our invention in its broader
aspects. Therefore, the scope of the present
invention should be determined only by the
following claims.
