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This version (23 Mar 2017 15:52) is a draft.Approvals: 0/1The Previously approved version (14 Dec 2012 21:35) is available.
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Table of Contents
Activity 4. A BJT Curve Tracer
Objective:
The purpose of this activity is to investigate the collector current, IC vs. collector voltage, VCE characteristics of the BJT. The kit of parts the student has will contain a number of transistors (ideally both NPN and PNP devices of various types). Many of the activities in this series make use of the matching or relative size of the devices in their operation. The student, after successfully configuring the curve tracer, should serialize and measure their devices and sort this inventory of transistors by various parameters such as gain (beta) , VBE etc.
Background:
The variable analog outputs supplied by the Discovery hardware are voltages. The BJT collector current is controlled by the base current. The AWG output voltage must be converted into a suitable current to drive the base terminal of the device under investigation. A simple resistor can be used to convert a voltage into a current, as shown in figure 1. However, only if the voltage across the resistor is known or controlled in some way. In this simple circuit, the base current IB = (VAWG1 - VBE)/100KΩ. We can set VAWG1 to known values but we don’t know the exact value of VBE. We can of course remove an estimate of the VBE mathematically. This is still only an estimate.
Figure 1 Simple IC vs VCE circuit
Materials:
ADALM2000 Active Learning Module
Solder-less Breadboard
1 - 100KΩResistors
1 - 100ΩResistor
1 - small signal NPN transistor (2N3904 or SSM2212)
Directions and Setup:
Build the simple curve tracer circuit shown in figure 1. The green boxes indicate where to connect the ADALM2000. Using the custom waveform editor in the Scopy AWG tool, construct a stair-step waveform with 5 levels. Be sure to reset so that you are starting with flat line at 0%. First set the type to constant. Then with start set to 0%, length set to 20% and offset set to -100% click on generate function. There should now be a line at -100% from 0 to 20%. Next change the start to 20% and the offset to -50% and then click on generate function again. There should now be a line at -50% from 20% to 40%. Next set the offset to +50% and the start to 60% and then click on generate function again. There should now be a line at 0% from 40% to 60% and a line at 50% from 60% to 80%. Finally set the offset to 100% and start to 80% and click on generate function one last time. There should now be a final line at 100% from 80% to 100%. Click on save and your new waveform should be in channel 2. Now at this point set the frequency to 40Hz, the amplitude to 2 V and the offset to 2.6 V. The waveform in the display should start at 0.6V and increase in 1 V increments to 4.6 V (0.6, 1.6, 2.6, 3.6, 4.6) Each step should be 5 mSec long for a total of 25 mSec. In AWG channel 1 configure a triangle wave with an amplitude of 2.5 V and an offset of 2.5V (wave should swing from 0 to 5V). Set the frequency to 200 Hz ( 5 times the 40 Hz of channel 2). Comparing the waveforms in channel 1 and channel 2, the triangle wave in channel 1 should go through one cycle from 0 to 5 V and back to zero during the time of one step in the waveform in channel 2. It will probably be necessary to set the phase of channel 1 to 270 degrees to make them line up in this way.
You should export your newly created stair-step waveform to a .csv file for future use.
Procedure:
The 1 V steps in the voltage driving the 100KΩ base resistor will produce approximately 1 V/100KΩ or 10 uA steps in the base current. Using the scope in XY mode plot channel 1 on the horizontal axis (VCE) and channel 2 (IC) on the vertical axis. You should see a set of 5 curves of IC vs. VCE, one for each of the 5 different base current levels. These base current levels should be approximately 0, 10uA, 20uA, 30uA and 40uA. It may be necessary to slightly adjust the 0.6V level of the first step of AWG2 up or down slightly to insure it is right at the initial turn on value ( IB=0 and IC=0) of the transistor you are testing.
Questions:
- From the measured data calculate the current gain Beta ( IC/IB ) for each device.
- Using the curve for the highest base current step, calculate the early voltage for each device.
- Calculate the Beta Early voltage product ( b*VA) for each device.
- Compare your results with manufacturer specifications for each device measured.
The Gummel Plot
The Gummel plot is the combined plot of the collector and base currents (IC and IB) of a transistor vs. the base-emitter voltage, VBE, on a semi-logarithmic scale. This plot is very useful in device characterization because it reflects on the quality of the emitter-base junction while the base-collector bias, VBC, is kept at a constant. A number of other device parameters can be garnered either quantitatively or qualitatively directly from the Gummel plot: the DC gain, Beta, base and collector ideality factors, nIb and nIc; series resistances and leakage currents. (http://en.wikipedia.org/wiki/Gummel_plot)
Figure 2 Circuit to generate a Gummel Plot
Figure 3 Example Gummel Plot
(From: http://www.synopsys.com/Tools/TCAD/Pages/hbtprocessing.aspx)
A more accurate approach to create IB
Objective:
We need to somehow remove VBE from the equation in figure 1 which sets IB. The circuits shown in figures 4 and 5 perform the function to force VAWG1 across the 10KΩ resistor independent of the value of VBE.
Materials:
ADALM2000 Active Learning Module
Solder-less Breadboard
1 - Dual Op AMP (such as ADTL082)
4 - 10KΩ Resistors
1 - 1KΩ Resistor
1 - 100Ω Resistor
1 - small signal NPN transistor / PNP transistor (2N3904, 2N3906, SSM2212, SSM2220)
2 - 4.7 uF decoupling capacitors
1 - 1 uF filter capacitor
Directions and Setup:
Below are schematics for both a common emitter and a common base BJT curve tracer test circuit for use with the Discovery Lab hardware. It uses one dual opamp (ADTL082) powered from the +/- 5 Volt board supplies. In the common emitter configuration, one opamp serves as a virtual ground at the base terminal to convert the voltage steps from waveform generator W2 into base current steps through a 10KΩ resistor. The collector voltage is swept using a ramp from generator W1. VCE is measured differentially by scope inputs 1+, 1-. The collector current, IC is measured by scope inputs 2+, 2- differentially across a 100Ω resistor. A ratio of 100 for the base and collector resistors is used because beta, the collector current to base current gain, is often approximately 100. The voltage on the base terminal can be offset to either +2.5V or -2.5V (or 0V) to increase the possible VCE swing (by -2.5 for NPN or +2.5 for PNP). The +2.5V is generated by a voltage divider from the +5V supply and the -2.5V is generated by inverting the +2.5V with the second opamp in the dual op-amp (ADTL082). The base and emitter connection can be interchanged to configure the device under test (DUT) in the common base mode. The resistor values are changed to 1KΩ for both in this configuration. This ratio of one is appropriate given that alpha, the ratio of emitter current to collector current, is very close to one. The voltage from W2 now sets the emitter current and the ramp on W1 sweeps the VCB and is measured differentially with 1+, 1-. The collector current IC is measured differentially across the 1KΩ resistor with 2+, 2-.
Figure 4, Common Emitter configuration
Figure 5, Common Base configuration
The following characteristic curves where taken using various NPN and PNP transistors in the common emitter configuration. A 10KΩ base resistor and 100Ω collector resistor was used in both cases.
Figure 6 NPN IC vs. VCE
NPN, beta is approximately 166.
Figure 7 PNP IC vs. VCE
PNP, beta is approximately 200.
The following characteristic curves where taken using the same NPN and PNP transistors in the common base configuration. A 1KΩ emitter resistor and 1KΩ collector resistor was used in both cases
Figure 8 NPN IC vs. VCB
Figure 9 PNP IC vs. VCB
Further reading: http://en.wikipedia.org/wiki/Transistor_curve_tracer
Measuring Transistor VBE
Objective:
The purpose of this activity is to characterize the base emitter voltage of a BJT. The VBE vs. Collector current characteristic of the transistor is a key factor in circuits. Often transistors are used in pairs where the matching of the VBE is important to proper circuit operation. In other cases the difference between the VBE of two or more transistors is exploited in the operation of the circuit. Size matters, VBE is a strong function of the size of the transistor, the emitter region at least.
Materials:
Various -- small signal transistors (NPN and PNP)
1 - Dual Op AMP (such as ADTL082)
1 - 1KΩResistor
2 - 4.7 uF decoupling capacitors
Directions:
The circuit below, figure 8, can be used in conjunction with the Analog Discovery Lab hardware board to accurately measure the VBE vs. emitter current of an NPN transistor. Emitter current is used in this measurement rather than collector current but IE and IC are essentially the same given a high beta transistor. The op-amp supplies the base current and any bias current that might flow in the 2+ scope input while forcing the emitter of the transistor to the (virtual) ground potential. Negative voltages applied by waveform generator W1 set the emitter current through the 1KΩ resistor. The same circuit can be used to measure PNP transistors by connecting the collector to Vn rather than Vp. Positive voltages applied by generator W1 set the emitter current through the 1KΩresistor.
Figure 10 Circuit to measure VBE
Hardware Setup:
The generator output W1 should be configured for a 100 Hz triangle wave with 2 volt amplitude and 2 volt offset (for an NPN device). The single ended input of scope channel 2+ is used to measure the voltage at the base of the transistor (optionally connect 2- to the emitter to remove any input offset of the op-amp). The setup should be configured with channel 1 connected to display the output of W1 and channel 2 connected to display the base voltage. The emitter current is calculated as the voltage of W1 / 1KΩ.
Procedure:
where:
VT is the Thermal Voltage
IC is the collector current
VBE is the base-emitter voltage
∆VBE is the base-emitter offset voltage
k is Boltzmann's constant
q is the electron charge
T is the absolute temperature
ln is the natural log
IS is the collector-emitter saturation current
When the collector-emitter saturation currents (emitter area) are equal, they cancel each other out, and Equation 4 reduces to Equation 5
The IE vs. VBE data for two different size transistors is plotted in the next two graphs.
Figure 11 IE vs. VBE
Figure 12 IE vs. VBE
The difference between these two curves, or delta VBE, is plotted here. It can be seen that this difference is relatively constant over a wide range of current and is about 66 mV. From our VBE equations this calculates to an effective emitter area difference of 12.7 between the two devices.
Figure 13 DVBE 2S1815 / 2D438
Questions:
- From the measured data calculate the current gain Beta ( IC/IB ) for each device.
- Using the curve for the highest base current step, calculate the early voltage for each device.
- Calculate the Beta Early voltage product ( b*VA) for each device.
- Compare your results with manufacturer specifications for each device measured.
Alternative Method
The circuits used earlier to make these measurements use the differential nature of the scope input channels of the Analog Discovery hardware. This was done, in part, to facilitate the conversion of the AWG’s voltage output into a current suitable to drive the transistor’s base. You may not always have access to instruments with differential input capability, such as standard bench top Oscilloscopes or the Analog Explorer student lab system. The following configuration, shown in figure 10, allows both the collector emitter voltage VCE and the collector current IC to be measured with ground referenced, singled ended, scope inputs while still converting the AWG voltage output into a suitable base current.
Figure 14 Alternate curve tracer circuit
The circuit is built from one dual op-amp (ADTL082 for example or two single amplifiers) and a handful of resistors. Amplifier A1 is configured as a current to voltage converter such that the collector of the device under test (DUT) is forced to (virtual) ground and the voltage seen at its output represents IC = VScope2/100. The second amplifier, A2, is configured as what is known as the “improved” Howland current source. The staircase voltage from AWG2 is converted into a current and applied to the base of the DUT. The emitter of the DUT, and thus the VCE is swept by the voltage of AWG1. The Howland current source will supply a fixed current independent of the emitter (and base) voltage. Since the collector of the DUT is at ground, the voltage ramp supplied by AWG1 must be negative, i.e. from -5V (or whatever the maximum negative voltage from the source is) to 0V. This is the minor concession that must be made to accommodate the single ended signal measurements. The VCE as measured by Scope1 will need to be inverted to display a positive VCE on the horizontal axis.
This same circuit can be used to measure PNP devices by simply configuring the ramp signal to be positive, i.e. from 0V to the maximum positive swing of the source.
For further reading on the Howland current source:
http://www.cirrus.com/en/pubs/whitePaper/199210-Apex-Versatile_current_source_circuits.pdf
http://www.analog.com/static/imported-files/application_notes/236037846AN_843.pdf
http://michaelgellis.tripod.com/howland.html
http://falstad.com/circuit/e-howland.html
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