Bipolar Transistor Characterization
Contents
1. Syntax de lt oct dec lin gt lt Sourcel gt lt Start gt lt Stop gt lt Incr gt lt source2 gt de 101010m Cancel Figure 6 DC Sweep I R1 ov 1v 2V 3V 4V 5V 6V 7V 8V 9V 10V Figure 7 I V Plot for a 100 ohm Resistor Analysis 2 Diode Curve Trace Draft the schematic shown below in fig 8 Be sure to use a 1N4148 diode Set the DC sweep to be from 0 4V to 1 8V in 10m increments The current flow through the diode as a function of the voltage across it is shown in fig 9 Note the exponential nature of the curve and the eventual straightening out due to the series resistance in the diode D1 V1 1N4148 dc V1 0 4 1 8 10m Figure 8 LTSpice Schematic for Diode I V Plot I D1 0 4V 0 5V 0 6V 0 7V 0 8V 0 9V 1 0V 1 1V 1 2V 1 3V 1 4V 1 5V 1 6V 1 7V 1 8V Figure 9 I V Plot for a 1N4148 Diode Analysis 3 NPN BJT Curve Trace Since a BJT is a three terminal device the procedure for the DC sweep will change from the previous examples Draft the schematic shown below in fig 10 The two resistors are not necessary for simulation but they will provide a result that is more similar to actual lab experiments To get an accurate characterization of the BJT the voltage source should be directly connected to the base and the emitter resistor omitted dc V1 010 10m V2 0 2 0 2 Figure 10 LTSpice Schematic for NPN BJT I V Plot Edit the simulation command to match the values shown in fig 11 Note that a s2. verify which channel represents which axis Generally channel one is X and channel two is Y Fig 20 below shows what happens if 5V is applied to the X input and the Y input independently Note the scale factors Spend some time getting comfortable with X Y mode ENM NIEN E ee Figure 20 5V applied to X and Y inputs Next set up the function generator to output a fairly low frequency 20 100Hz triangle wave This will serve as the linear voltage sweep just like in LTSpice simulations Set the amplitude to a value of about 5V If the voltage is set too high then the function generator may not be able to drive the load This will vary between different instruments A schematic of the test set up is shown below in fig 21 The voltage sweep is connected to the X input and the junction between the main resistor and the sampling resistor connects to the Y input It is up to you to pick the value of the sampling resistor Ideally the resistor should be as small as possible since this reduces error but the noise of the scope may not make the measurement meaningful The reason for using 1X probes is to reduce the noise as well You will have to adjust the scale factors in order to see the desired slope Otherwise the X Y plot may look like a straight line An example of a resistor I V plot on a scope is shown in fig 22 Note the difference in scale factors If you understand the concepts presented in this lab then it will be simple to figure out the val
3. EE320L Electronics I Laboratory Laboratory Exercise 6 Current Voltage Characteristics of Electronic Devices By Angsuman Roy Department of Electrical and Computer Engineering University of Nevada Las Vegas Objective The purpose of this lab is to understand current voltage characteristics of various passive and active electronic components and how to interpret these characteristics for the design of electronic circuits Equipment Used Dual Output Power Supply Oscilloscope with X Y Capability Breadboard Jumper Wires Resistors Capacitors Small Signal diodes 1N418 or equivalent NPN and PNP transistors 2N3904 2N3906 or similar N Channel MOSFETs ZVN3306A or similar N Type JFET J310 or whatever low cost small signal JFET is available at hand 1x Scope Probes If available Background The first two words that come to mind when one thinks of electronics are voltage and current Every electrical phenomenon can be described in some form based on these two fundamental properties Despite this much of electrical engineering devolves away from these fundamentals and understanding 1s obscured by adding layers upon the foundation of voltage and current The question that needs to be asked of any electronic component is how the voltage is related to the current This understanding brings powerful insight into the design of electronic circuits The simplest electronic component is the resistor The resistor has a simple relationsh
4. Note the changes in the DC sweep values from the previous examples The 100k resistor to ground is not needed for simulation but is needed in a practical lab in order to prevent stray voltages from appearing on the gate of the MOSFET when the voltage source is disconnected or if for some reason the gate is coupled with a capacitor to the voltage source The 10 ohm resistor is not needed either but the voltage drop through this resistor will translate to current flow in our lab experiment M1 V1 2N7002 V2 0 om Se 0 10 dc V1 0 10 10m V20 51 Figure 16 Schematic for an NMOS I V Plot Id M1 Figure 17 Drain Current for Different Gate Voltages Analysis 6 N Channel JFET Curve Trace Draft the schematic shown in fig 18 The schematic symbol for the N channel JFET is named njf in the component selector in LTSpice Note the changes from previous examples most importantly the flipping of voltage source V2 The JFET is a depletion mode device Unlike previous three terminal devices which have increasing collector or drain current with increasing base or gate voltage the JFET exhibits decreasing drain current with increasing gate current With zero volts applied to the gate a JFET conducts its maximum current The physics of JFET devices is outside the scope of this lab and the interested reader should consult the references listed The simulation results are shown in fig 19 dc V1 0 10 10m V2 0 30 5 Figure 18
5. Schematic for an N Channel JFET I V Plot Id J1 OV 1V 2V 3V 4V 5V 6V TV 8V 9V 10V Figure 19 Drain Current for Different Values of Gate Voltage Prelab Deliverables 1 Submit screenshots of each prelab s schematic and simulation output 2 You must also include an Altium schematic Altium netlist and PCB layout for the circuits in prelabs 1 through 5 Each PCB layout must include footprints for all components used and can be auto routed or manually routed You may use either thru hole footprints or surface mount footprints for each component For the voltage and current sources just put a two pin header and label them VCC and GND Also include a grounding plane and make sure your traces are wide enough for the increase in current To determine the trace width use a PCB trace calculator If you are unsure how to use Altium please click on the Lab Equipment Learning Tutorials Manuals Downloads link on the UNLV EE Labs homepage and read the Altium tutorial or watch the videos https faculty unlv edu eelabs index html navi main_labequipment Laboratory Experiments Experiment 1 Resistor I V Characteristic Curves Measure the I V characteristics of the 100 ohm resistor in fig 5 First set the scope to X Y mode If you are unsure how to do this consult the user manual for the scope since it will be different for every scope In X Y mode there is no time axis Both axes are in terms of voltage Apply a known voltage to both channels and
6. e Voltages Analysis 4 PNP BJT Curve Trace For a PNP device the schematic needs to be modified in order to get a result that is similar to the NPN BJT Draft the schematic shown below in fig 13 The PNP is flipped so that the emitter is the most positive terminal The base bias voltage is referenced to the positive supply instead of ground The values for the DC sweep do not change from the previous example That is the main reason for using this configuration There are a few alternate ways to create I V plots for a PNP BJT with the end result being the same after proper manipulation of the curves Fig 14 shows the plot of the collector current The plot can be flipped by right clicking on the trace label for Ic Q2 and adding a minus sign to the expression as shown in fig 15 This procedure or some other variation needs to be done for P type MOSFETs or JFETs as well in order to match the polarities of N type devices dc V1 0 10 10m V2 0 2 0 2 Figure 13 Schematic for PNP BJT I V Plot OV 1v 2V 3V 4V 5V 6V TV 8V 9V 10V Figure 14 Collector Current for Different Base Voltages iT Expression Editor Filc Q Default Color L Attached Cursor none Enter an algebraic expression to plot Ac Q2 F Delete this Trace md Ic Q2 OV 1v 2V 3V 4v 5V 6v TV 8v 9V 10V Figure 15 Flipping the Collector Current Analysis 5 N Channel MOSFET Curve Trace Draft the schematic shown in fig 16
7. econd source needs to be swept as well The second source is the voltage applied to the base Note that the number of steps for the second source is the number of curves that will appear in the final plot One can use as many steps as desired but if the steps are too close together the plot will appear too busy It is better to pick a few key points The second source is stepped from 0 to 2V in increments of 0 2V resulting in 10 curves Plotting the current through the collector gives the result shown in fig 12 AC Analysis DC sweep DC Transfer AC Analysis DC sweep DC Transfer Compute the DC operating point of a circuit while stepping independent sources and Compute the DC operating point of a circuit while stepping independent sources and treating capacitances as open circuits and inductances as short circuits eating capacitances as open circuits and inductances as short circuits 1st Source 2nd Source Name of 1st Source to Sweep W1 Name of 2nd Source to Sweep Wz Type of Sweep Type of Sweep Start Value Start Value O Stop Value 10 Stop Value 2 Increment 10m Increment 0 2 syntax de lt ockdec line lt Sourcel gt lt Start gt lt Stop gt klaner lt sources gt Syntax de coct dec line lt Sourcel gt lt Starty lt Stope elmer esource2 gt adc Y1 01010m W2 0 20 2 adc 01010m W2 0 202 Figure 11 DC Sweep for BJT I V Plots Boma Ic Q1 Figure 12 Collector Current for Different Bas
8. ip between voltage and current The current flowing through a resistor is the voltage across the resistor divided by the resistance of the resistor The equations below show all the relationships between voltage current and resistance R 4 V I xR gt I A gt x aed I R The value of a resistor can be determined by applying a known voltage across it and measuring the current flowing through it If this was to be plotted on the X Y plane it follows that voltage should be on the X axis since it is the independent variable and current should be plotted on the Y axis since it is the dependent variable This could be reversed but generally due to the ease of creating variable voltage sources this was the convention adopted The plot in fig 1 created using LTSpice shows three resistor values and their current voltage or I V characteristics It is tempting to think of resistance as slope but in this plot the slope is really the reciprocal of resistance or conductance Thus for the 0 5 ohm resistor at 10V the current is equal to V 10 20 A It is important to always be aware of the orientation of the axes Sometimes I V plots are flipped in order to make certain data more clear For this lab the plots will always be shown with voltage on the X axis and current on the Y axis I R1 20A Rt 18A 16A 14A 12A 10A 8A 6A 4A 2A OA OV 1V 2V 3V 4V 5V 6V 7V 8V 9V 1 ov Figure 1 I V Characteristics of 0 5Q 1Q a
9. lied to the collector or drain terminal Build the test set up shown below in fig 24 In this set up a DC voltage supply is connected to the base through a 10k or similar value resistor When the voltage applied is zero then the I V characteristics should just be a horizontal line As the voltage is raised the curve should take the shape shown in fig 25 Repeat this for a few values of base voltage 2N3904 SS RSampling DC VOLTAGE SUPPLY FUNCTION GENERATOR Figure 24 Test Set up for NPN BJT I V Curves Pun Tigi i 120 0ms 0 0000 Figure 25 Example I V Curve for an NPN BJT at a Particular Base Voltage Save each image and then combine onto one graph by hand If you have a scope with high memory depth and infinite persistence then you may be able to plot all the curves on the same screen Label all axes appropriately Furthermore label the value of the base voltage applied to each curve in your graph Experiment 4 PNP BJT I V Characteristic Curves The test set up needs to be changed for a PNP transistor to reflect differences in polarities required Build the test set up shown in fig 26 Follow the same procedure as the previous experiment DC Power Supply Function Generator Figure 26 Test Set up for PNP BJT I V Curves Experiment 5 N Channel MOSFET I V Characteristic Curves Adapt the schematic shown in figure 16 to create your own test set up Do not omit the resistor from the gate to ground otherwise y
10. nd 2Q resistors One may wonder what the value of plotting the I V characteristics of resistors is since it is simply a straight line In reality most resistors have some form of voltage coefficient that 1s their value changes with the voltage applied across the resistor This change is generally nonlinear and can be quite complex to describe mathematically Figure 2 shows the I V characteristics of the resistors with deviation from the ideal highlighted in blue This graph is by no means typical of resistors its sole purpose is to show that resistors aren t always linear devices Generally this consideration is reserved for niche applications such as power electronics In this lab the focus will be on more widespread nonlinear devices such as diodes and transistors I R1 OV 1V 2V 3V 4V 5V 6V 7V 8v 9V 10V Figure 2 I V Characteristics of non ideal 0 5Q 1Q and 2Q resistors The semiconductor diode is a two terminal nonlinear device The current through the diode as a function of voltage across it is given by Vp I I e VT _ 1 The derivation of this equation and the meaning of its variables are outside the scope of this lab and the interested reader is referred to the additional resources section It is obvious that the current follows an exponential relationship to the applied voltage The I V characteristics of a 1N4148 switching diode are shown in figure 3 The exponential shape is clearly visible however it straighte
11. ns out around 0 9V This is because of a series resistance that is part of the diode s package A larger diode would continue to display an exponential characteristic at higher voltages I D1 300mA 27 0mA 240mA 210mA 180mA 150mA 120mA 30mA OmA 0 40V 0 46V 0 52V 0 58V 0 64V 0 70V 0 76V 0 82V 0 88V 0 94V Figure 3 I V Characteristic of a 1N4148 Diode Creating l V plots of two terminal devices is straightforward vary the voltage across the terminals and measure the current flowing through the device For a three terminal device the question becomes what to do with the third terminal A stepped voltage or current can be applied to the third terminal which will result in a family of characteristic curves on the same graph An example of an I V plot for a 2N3904 bipolar junction transistor BJT is shown below in fig 4 The traces shown are the currents flowing into the collector terminal of the BJT for different values of current applied at the base terminal Basically many different I V curves are laid out onto the same graph to see how changing one variable the current flowing through the base changes the current flowing through the collector Each trace represents a base current from 0 to 100uA in 25uA increments I V characteristics of three terminal devices are indispensable in understanding the behavior of these types of devices The basic idea is to change the voltage or current at one terminal hold it consta
12. nt and then see how the current flows through the other two terminals while changing the voltage applied across those two terminals This concept can be extended to any three or more terminal device such as vacuum tubes transistors JEETs MOSFETs silicon controlled rectifiers SCRs thyristors unijunction transistors UJTs IGBTs HEMTs etc The beauty of testing the I V characteristics of a device is that the user can design a circuit for that device without knowing or understanding any of the physics or fundamental operation of the device If a new electronic device were to be invented the first thing the inventor would do is make an I V plot Ic Q1 OV 2V AV 6V 8V 10V 12V 14V Figure 4 I V Curves for a 2N3904 BJT Most instruments in an electronics engineering laboratory are designed to measure voltages The oscilloscope is such an instrument There are current probes available both Hall Effect based for DC currents as well as transformer based for AC currents These probes have many limitations and tend to be expensive As such it is common to use a sampling resistor to measure currents in many practical circuits A sampling resistor 1s placed in series with the device under test and the voltage drop across it is proportional to the current flowing in the device Of course adding a resistor in series with a device will modify the operation of the device under test For example if the resistor causes a
13. of specialized probes one of which can only measure AC and the other can measure AC and DC This is a picture of diode I V characteristics done on a scope with low noise The sampling resistor is 0 1 ohm Knowing the ability of your test equipment is important
14. ou may get unusual results based on stored charges on a floating gate Create a family of curves as in the BJT examples Experiment 6 N Type JFET I V Characteristic Curves Adapt the schematic shown in figure 18 to create your own test set up Do not omit the resistor from the gate to ground otherwise you may get unusual results based on stored charges on a floating gate Create a family of curves as in the BJT examples Postlab Deliverables 1 All images graphs and percent error calculations indicated in the experiment descriptions 2 Questions a What is the name of a dedicated instrument that creates I V characteristics How does this instrument work Hint there are two different types of instruments that do this function One of them is not in use anymore b Why did we always place our sampling resistor between our device under test and ground What would happen if instead the sampling resistor was placed between the voltage source function generator in this lab and the device under test How would the I V characteristics change hint think about offset voltages and BJT saturation c If there was a situation where one had to insert a current sampling resistor in a circuit and one side could not be grounded what can be to get an accurate current reading hint look up differential measurements d What are two other methods of measuring current in a circuit using an oscilloscope These methods both involve using two types
15. significant voltage drop it may cause the device under test to stop working properly For this reason a sampling resistor should always be as small as possible The limitation for how small the resistor can be usually depends on the noise of the test equipment being used a very small voltage drop may be below the voltage resolution of the measuring instrument Prelab Analysis 1 Resistor Curve Trace The most basic I V plot is that of a resistor Draft the schematic shown below in fig 5 R1 V1 100 dc V1 010 10m Figure 5 LTSpice Schematic for Resistor I V Plot Next edit the simulation command and select the DC sweep tab and enter the values as shown below in fig 6 All other simulations in this lab will be of the DC sweep type A DC sweep tells LTSpice to sweep a voltage or current source from one value to another in the given increment The entered values tell LTSpice to sweep the source V1 from 0 to 10V in increments of 10mV 10mV was chosen as an increment to give a smooth looking curve Larger or smaller increments can be used as needed based on the situation IRE Edit Senulation Come Transient AC Analysis Compute the DC operating point of a circuit while stepping independent sources and treating capacitances as open circuits and inductances as short circuits 1st Source Name of 1st Source to Sweep V1 Type of Sweep Linear Start Value 0 Stop Value 10 Increment 10m
16. ue of the sampling resistor used based on the scope picture X Channel R1 10 r Y Channel 0 S Rsampling ale Figure 21 Test Set up for Resistor I V Curves SD 200m BGR gt 6 750 Figure 22 Example I V Curve for 100 Ohm Resistor Print out the scope image and label the X and Y axes with the appropriate units by hand Calculate the percentage error introduced by using the chosen sampling resistor Experiment 2 Diode I V Characteristic Curves Measure the I V characteristics for the 1N4148 diode An example test set up is shown below in fig 23 The procedure is the same as for the resistor However keep in mind that the diode s voltage range of interest is fairly low around 0 1 5V At high voltages there will be significant current flow and it may be excessive for the function generator X Channel 1N4148 Y Channel Rsampling R Figure 23 Test Set up for Diode I V Curves Print out the scope image and label the X and Y axes with the appropriate units by hand Since the diode is a non linear device it is difficult to really define a single value of error Instead comment on if you can see the straigthening out of the diode s I V curve and to what degree the sampling resistor is affecting it Experiment 3 NPN BJT I V Characteristic Curves For a three terminal device such as an NPN transistor we need to hold the voltage applied at the control terminal base or gate constant while sweeping the voltage app
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