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The Transistor Curve Tracer

I. Introduction

It is frequently necessary to physically measure transistor parameters such as current gain, breakdown voltages, and impedance. A transistor curve tracer is an instrument designed to provide the circuit conditions required to make these measurements.

One parameter of particular importance is the beta (b), or dc current gain, of the

transistor. This discussion addresses this particular measurement.

II. Principle of Operation

The curve tracer can generate and display a family of curves of collector current, Ic, versus collector-to-emitter voltage, VCE, for various values of base current, IB. From this display, the current gain, b, can be directly determined.

Three basic functional circuits are used to generate this display:

1. a sweep voltage generator for control of the collector voltage,

2. a base current source which can be controlled to provide a number of equal increments of base currents with each sweep of the voltage generator,

3. a timing source to change the base current at the start of each voltage sweep.

Figure 1 shows the waveform of the sweep voltage generator, Vs, each sweep occurring with a time period ,T . This is the collector supply voltage which is repetitively applied to the transistor. Refer to Figure 3 and note that the collector voltage, Vce, will provide the horizontal (x-axis) sweep of an oscilloscope.

Figure 2 shows the output of the base current source. Note that for each consecutive voltage sweep the base current, IB, is increased in equal incremental steps, with the steps synchronized to the beginning of each collector voltage sweep. Usually, 10 or 12 increments of base current are used for the display. As the last increment period ends, the base current generator repeats the step sequence, providing a continuous stable display.

Curve tracers in the RCA Lab use the 60 Hz power line frequency as the synchronizing signal for the collector sweep voltage and base current steps.

Figure 3 is a simplified diagram showing how the curve tracer display is generated. The bold lettering indicates the selector switches on the front panel of the curve tracer which are used to to set the proper test conditions. The conditions shown in Figures 1, 2, and 3,

apply to a npn transistor; for a pnp transistor, opposite polarity (negative) voltage and currents would be selected.

The collector- to- emitter voltage, Vce, provides the horizontal sweep, while the voltage across the current sensing resistor, Rc, which is proportional to collector current, provides the vertical sweep, resulting in a family of curves of Ic versus Vce for a series of equal increment changes in base current.

Figure 1 Collector Sweep voltage waveform                 Figure 2 Base current waveform

Note: Front panel selector switches are printed in bold lettering.

Figure 3.  A Simplified Block Diagram of a Basic Transistor Curve Tracer
 

III. Determining Current Gain (b)

The display shown in Figure 4 is the characteristic for an npn transistor.

Figure 4. Characteristics of an npn transistor
 

The current gain of the transistor is determined from:

b= current gain = DIc/DIB

where DIB is the setting of the Step Selector switch.

The slope of the load line is determined by the dissipation limiting resistor, RL , selected in the Collector Sweep control section. This resistor must be selected so that the maximum allowable collector current, Ic, specified for the transistor is not exceeded for VCE = 0 Volts.

If the data sheet for the transistor is not readily available, make a conservative estimate based on the size of the transistor. Small transistors don't have much heat dissipation capability so they should be limited to about 50 mA and 40 V; higher power transistors usually have a body which allows the unit to be attached to a heat sink. You can assume that they can handle 1 or 2 amps at 40 V.

b will vary depending on the collector current drawn, decreasing as the current increases. The gain should be measured in the voltage/current region in which the transistor will be operating.

Transistors under test can become hot. Use caution in handling the transistor.

IV. Display Examples

Example 1

NPN Silicon Transistor Type 2N3904

Maximum Ratings:

Collector-Emitter Voltage ...................40 V

Continuous Collector Current ............ 200 mA

Curve Tracer Settings

Collector Sweep

Peak Volts Range .............................................. 0 - 20

Peak Volts ............................................ 0 (full CCW)

Polarity ................................................................ + (NPN)

Dissipation Resistor .......................................... 200 Ohms

Adjust the spot intensity to a reasonable level to avoid burning the screen. Focus the spot using focus and astigmatism controls.

Vertical Scale..............................................................Collector Current 10 mA/div

Horizontal Scale .................................................. Collector Volts 2 V/div

The vertical and horizontal control blocks have spot position controls. Since this transistor is a n NPN type, the VCE axis will extend from the left to the right of the display, and IC will extend upward. Position the spot at the lower left corner of the grid as the zero position.

Base Step Generator

Polarity .................................. +

Step selector ......................... .05 mA /step

Series Resistor ..................... Not applicable.

Display settings .................... Repetitive, 240 steps/sec

Transistor Test Panel ............................................... Emitter grounded

With test panel Selector Switch in the center position, insert the transistor leads into either A or B socket, observing the designations for C, B, and E. The Configuration switch should be in the Emitter Grounded position. Select the appropriate transistor and increase the collector voltage to maximum using the variable voltage control in the Collector Sweep control block. The resulting display should be similar to that shown in Figure 1. Since the direction of current flow in a pnp transistor is the reverse of that for an npn transistor, the collector-to-emitter voltage and the direction of the base current must be reversed, resulting in the characteristic shown in Figure 4.

Example 2

PNP Silicon Transistor Type 2N3906

Maximum Ratings:

Collector-to-emitter voltage ................. - 40 V Continuous collector current ............ - 200 mA

Figure 5 - PNP Transistor Characteristic

The 2N3906 is the complementary transistor to the 2N3904.The current gains are the same order of magnitude, as are the absolute values of the maximum ratings. The difference is in the polarities associated with applied voltages and in current flow direction.

Being complementary, the settings used for the 2N3904 may be used for the 2N3906, except that the polarities of the collector voltage and the base current flow direction must be reversed. With the transistor selector switch of the Test Panel in the center position, the 2N3904 may be removed and replaced with the 2N3906. Switching the polarities of the base current steps in the Base Step Generator block and the collector voltage in the Collector Sweep are the only setting changes required. Since the voltages and currents are reversed, select the upper right corner of the grid as the starting position Select the transistor and increase the peak collector voltage to maximum. The display should be similar to that shown in Figure 5.

Example 3 2N3055 NPN Power Transistor

One of the more frequent measurements of bare made on the 2N3055 NPN power transistor whose absolute maximum ratings are:

Collector-to-emitter voltage ................................. 60 V

Continuous collector current .............................. 15 A

Total device dissipation ...................................... 115 W

An outline of the 2N3055 is shown in Figure 6.

Figure 6 - Bottom View of 2N3055 Transistor

Curve Tracer Settings

Collector Sweep

Peak Volts Range ................ 0 - 20

Polarity ....................................... + (NPN)

Dissipation Resistor ................ 2 Ohms

Vertical Scale ............................................... 500 mA/div
 
 
 

Horizontal Scale ........................................ 2 V/div

Base Step Generator

Polarity ......................................... +

Step Selector ............................... 10 mA/step

Test Panel ....................................................... Emitter grounded

Caution : Under these test conditions the 2N3055 can become

extremely hot, and a severe burn can result if touched !!!!



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