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Biasing Three-Terminal Devices
for Test
Application Note 376-1
A precise solution for
component evaluation and
sub-assembly testing...
Introduction Biasing Methods Figure 1 shows a generalized
three-terminal device. On this
Measuring the operating Commonly, a three-terminal
device, terminal 3 is a bias input
characteristics of a wide variety of device test requires that two
and terminal 2 is a control input.
three-terminal devices can be parameters (voltage or current) be
Terminal 1 is the common return
accomplished with a single test defined as fixed, and applied to
for both dc sources as shown in
configuration eliminating the need the device. The other parameters
Figure 2.
to rewire or change are then measured. There are
instrumentation. This reduction in many methods of accomplishing In the case of a bipolar transistor
set-up time is especially valuable this. As each method is described, connected in the common emitter
in environments where many the benefits and drawbacks will configuration, terminal 3 is the
different types of components be enumerated, then the following collector, terminal 1 is the emitter,
need to be evaluated, such as method will address the and terminal 2 is the base. The
failure analysis labs, R&D, and drawbacks of the previous one. control variable could either be
incoming inspection. the current into pin 2, or the
voltage from pin 1 to pin 2. If beta
The types of tests which can be
is of interest, it would be simplest
accomplished using the biasing
to control current, and if Gm is of
methods discussed below include,
interest, then the choice would be
but are not limited to; measuring
voltage control. AC ground could
ac characteristics and dc
be any of the three terminals, or
parameters and fault testing.
none of them.
Typical components which may
be evaluated in this manner
include; pnp and npn bipolar
transistors, enhancement and
depletion mode FETs, ICs, and
SCRs. Two-terminal devices, such
as diodes, can also be tested by
simply not using the third test
node (node #2). Figure 1. Generalized Three-terminal
Device
It is assumed that the dc sources
(or multiple output source) used
for all of the configurations
described have a minimum set of
features. This includes computer
control of the output and internal
monitoring of output voltage and
current. If the supplies cannot be
controlled by computer, then
adjustments must be made
manually. If the power supplies
cannot meter their output voltage
and current, then appropriate
metering instruments must be
Figure 2. Biased Generalized
added to the configurations. Three-terminal Device
2
The Simplest Biasing Method by the computer. In order to keep The Self-Controlling Biasing
the collector current at the Method
Continuing with the bipolar
desired value, continuous
transistor example, the device to The circuit in Figure 4 does not
measurements must be made, the
be tested is an RF power require controller intervention to
results transmitted to the
transistor which is not mounted keep the biases at the desired
controller, new values calculated
on a heat sink. Hfe (dc gain) is to levels, so iterations by the
(based on some algorithm), and
be measured at different values of computer are no longer a problem.
then finally the source 1b must be
1c at a fixed Vce. The biasing
adjusted. This ties up the Source 1e is programmed as a
method chosen is shown in
controller leaving less time to do current source. Source Vc is
Figure 3.
other measurements. The second almost equal to Vce (Vc + Vbe =
The voltage source Vce is set for drawback of this method is the Vce). No runaway is possible with
the specified level, and the current possibility that the controller will this circuit, because 1c is no
source 1b is adjusted until 1c not be able to act quickly enough longer set indirectly with 1b with
reaches its specified value. to avoid potentially destructive the associated time lags that
thermal runaway. worsen the heating problem. Vce
At first, this simple circuit seems will remain correctly regulated to
adequate. All dc values (Vce, 1c, 1b, Clearly, it is desirable to develop a
the programmed value (within
Vbe) can quickly be read by the method of biasing which does not
approximately 0.6 volts). This
metering circuits internal to the require controller intervention.
0.6 volt offset is the major
power supplies. All dc parameters drawback of this system.
(Beta, Gm, leakages) are easily
calculated from the measured
values. It does, however, have two
drawbacks. First, the control of 1c
by adjusting 1b requires iteration
Figure 4. The Self-Biasing Controlling
Method
Figure 3. The Simplest Biasing Method
3
Self-Controlling Biasing with The Flexible Biasing Solution Notes On The DC Power
Programmed Voltage Offset for Three-Terminal Device Sources
Correction Testing
To effectively test a variety of
To improve upon the accuracy of The problem which occurred in devices using the circuit in
Vce, the power supply sense leads the last configuration can be Figure 6, the power sources must
of source Vc can be connected as solved by replacing Vc with either have the following characteristics:
shown in Figure 5. a bipolar source or equivalently,
two back-to-back sources as 1. adequate programming resolution
shown in Figure 6. Notice that the and accuracy
source at the emitter terminal was 2. readback of voltage and current
also modified to provide a similar 3. adequate readback resolution and
bi-polar capability. accuracy
4. a large allowable voltage
The configuration in Figure 6 differential between output and
allows precise dc bias conditions sense terminals
to be regulated automatically, 5. the ability to source and sink
prevents runaway conditions, and current
does not consume extra controller
time. This configuration can be An example of a power supply
used to test a wide variety of two solution which will meet all the
and three-terminal devices. requirements above for a wide
range of components under test is
Figure 5. The Self-controlling Biasing the HP 6626A Multiple Output
Method with Programmed Voltage System Power Supply. It has two
Offset Correction
25 watt outputs and two 50 watt
Power supply Vc will adjust it's outputs. The 25 watt outputs can
output voltage so that the voltage produce 0.5 amps each and the
at the sense leads (Vce) equals the 50 watt outputs can produce
programmed value. The power 2.0 amps each.
source will actually be supplying a
HP Model 6626A has programming
larger potential, Vce + Vbe. Of
and readback resolution of up to
course, the power supply must be
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