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Laboratory Session 5 - The Transistor

There is no template document for this laboratory as you have seen what is required in the previous laboratories; however, any of the tables can be cut-and-pasted into your final report. 

It would be useful if you could have a copy (electronic or otherwise) of the notes on transistors to help you with the responses to the specific questions. The notes are available here as "Lecture Slides on BJTs"
Please submit your laboratory write up to: http://moodle.dcu.ie/mod/assign/view.php?id=27754

As there are three digital multimeters required for this laboratory (one with high quality microamp readings) and a variable voltage supply, attendance is compulsory.

Lab Objectives:

  • To obtain the output characteristics (IC vs. VCE) of a NPN transistor 
  • To calculate some of the transistor’s parameters from the measurements taken
  • To wire a common-emitter amplifier and check that it behaves as we have discussed in lectures in its quiescent state.

Equipment:

EE223 Electronics Kit, with these components in particular:

  • 2N2219A Transistor - Datasheet attached at the bottom of the page (illustrated in Figure 1)
  • 1MΩ Potentiometer (Pot) and 10KΩ Potentiometer (Pot)
  • 22K Resistor (or two 47K resistors in parallel) 
  • Minilab (lab instrument station)

Procedure:

1. 2N2219 (NPN) Transistor Output Characteristics (IC vs. VCE)

The collector current (IC ) of a transistor is mainly dependent upon two variables: the collector-emitter voltage (VCE) and the base current (IB ). This dependency is normally shown as a “family” of graphs: for each of a number of different values of IB , the graph of IC  vs. VCE is plotted.     

                                   

2. Component Assembly and Work-Station Set-up

  • Ensure that the power is off. Assemble the circuit shown in Figure 2 on your breadboard using the components supplied. The transistor Q1 in Figure 2 is the 2N2219A as summarised in Figure 1.

Figure 1. The 2N2219A Transistor Pin Diagram (note: figure on left illustrates the bottom side of the transistor)

  • On the Minilab, all points marked with the GND or COMMON symbol are internally connected. Use the fixed 5V supply (lower right-hand corner, Brown & Green terminals) for VBB and the 0 to +15 V variable Regulated Power Supply (usual Red & White) for VCC.
  • Note: Make sure to connect the White terminal directly to the Green terminal below it so that the 0V Terminal is grounded.
Figure 2. The Common-Emitter Configuration (no meters)

  • Wire the circuit as illustrated in Figure 2. Note that the 1MΩ potentiometer has the numbers 105 written on the side (i.e. 10x105Ω = 1,000,000Ω). Do not use the potentiometer with 103 written on the side. Figure 3 illustrates the construction of the potentiometers - the single pin on its own is connected to the wiper (the middle leg on the multi-turn potentiometers) and the other two pins are connected to either end of the resistive material. As you rotate the wiper the further it gets from A the greater the resistance between A and W (i.e. if W is turned full clockwise in Figure 3(a)). You can use your digital multimeter set to "Ω(20M)" to measure the resistance across the 1MΩ potentiometer to ensure that you understand its operation. Please do not short the pins of the potentiometer when you are inserting it into the breadboard (just like you do with chips). If there is no 22kΩ resistor in your kit use two 47kΩ resistors in parallel, which gives a value of 23.5kΩ. Alternatively, you could use two 10kΩ resistors in series.

(a)                                                                (b)
Figure 3. (a) An example of the internals of a potentiometer. You can see that the W pin is connected to the wiper that turns with the dial over the resistive material. The A and B pins are connected at either end of the material, meaning that when the wiper is turned fully to the left that the resistance between W and A is at the minimum, but the resistance between W and B is at the maximum. (b) shows some different styles of multi-turn potentiometer. The multi-turn potentiometer in your kit allows 15 full 360 degree turns to go from 0Ω to 1MΩ.


  • Add in two ammeters and one voltmeter as described in Figure 4. The first ammeter is on the base and measures the base current. You should use the DMM on the station to measure current in the uA range - make sure that you plug the lead into the mA socket on the DMM. The second ammeter is on the collector and measures the collector current in the mA range. The voltmeter should be connected across the variable voltage supply and measures Vcc in the 0-15V range. It is recommended that you place a piece of paper beside each meter to make it clear which digital multimeter is which, i.e. base current, collector current and collector-emitter voltage (same as Vcc).

Figure 4. The Common-Emitter Configuration (with meters)



  • Before switching on, ensure that VCC = 0V (i.e. The 0 to +15V Control knob is fully Counter-Clockwise). Please double check your circuit wiring before you power it on.

3. 2N2219 Transistor Base Current, IAdjustment

Using potentiometer RB, set the base current IB to the value in row 1 of the leftmost column of Table 1 below.

NB: This is a sensitive adjustment so it’s important to check and reset it constantly as you take the following measurements, to ensure that it is held constant at the indicated value. It may deviate when you alter other variables. 

 

4. Family of Graphs using IC, with values of VCE  and  IB

Vary the collector-emitter voltage VCE ¾ the top row of the table below gives a suggested range of values; you may need to include more at some points within this range to produce a reasonable graph ¾ and measure the corresponding values of collector current IC , recording these in row 1 of the table. Some of the readings are very sensitive and take time to settle down.

Repeat this procedure for the remaining rows of the table - set a new value for IB, vary VCE through its range, measure and record IC while checking that IB remains constant at the level indicated in the leftmost column (remember that the base current is in micro-amps, while the collector current is in milli-amps)

   VCE  V 0.0V   0.2V 0.5V 1.0V 2.0V 4.0V 6.0V 9.0V12.0V 15.0V 
 IB  uA IC mA IC mA IC mA IC mA IC mA IC mA IC mA IC mA IC mA IC mA

30uA

Row 1

0.0mA

 

 

 

 

 

 

 

 

 

60uA

Row 2

0.0mA

 

 

 

 

 

 

 

 

 

90uA

Row 3

0.0mA

 

 

 

 

 

 

 

 

 

120uA

Row 4

0.0mA

 

 

 

 

 

 

 

 

 

150uA

Row 5


0.0mA








Table 1  Measurements of Output Characteristics (Place the measured IC value in the boxes)


You will need to plot this very carefully for your write-up, but plot each of the curves now and check with one of the demonstrators that you have valid results. So, on the same page and using the same axes, plot a family of graphs: for each value of IB, plot IC (vertical or Y-axis) vs. VCE (horizontal or X-axis), labelling each with the appropriate value of IB.

Do this before performing the calculations in the next section of the laboratory.

5. Calculation of Transistor Parameters

In calculating transistor parameters you should take values from the “flat” or active region of the characteristic. Your answers should include the appropriate units. Carefully number each of these questions as 5(a), 5(b) etc. in your final write-up.

      (a)    Estimate the DC current gain hFE = I/ IB where VCE has a constant value of say 9V. Compute for each value of IB from Table 1 and take the mean value.

     (b) Using the attached datasheet, determine if your calculation in (a) is valid and within range. What is the valid range of hFE at the current temperature (assume 25 Celsius) and with a collector current and collector-emitter voltage in the range that you used.

     (c)   Estimate the small-signal current gain hfe ΔIΔIwhere VCE has a constant value of say 9V again by using the differences between the data in adjacent rows. Compare hFE and hfe.

Note: It is possible that the transistor has been damaged by thermal runaway when the current was set to be 150uA and the voltage was set high at the end of the last section. If your transistor is not behaving correctly in the next section then please test the transistor.

6. Basic Common-Emitter Amplifier Circuit


In this part of the experiment we are going to set up a very basic common-emitter amplifier circuit and we are going to design it so that we achieve an optimized operating point when the amplifier circuit is in its quiescent state - i.e. there is no input applied. We are then going to verify that the circuit behaves in this experiment as it should behave in theory. 

Set up the circuit as described in Figure 5, where we use a potentiometer on the base and a potentiometer on the collector. When wiring use wires to link all of the components, so that it is easy to replace a wire with the digital multimeter in order to measure current. (Note: you can ignore the dashed lines as you will not be connecting them)
Figure 5. The Common-Emitter Amplifier Circuit

Procedure steps:
  • Turn off the power supply.
  • Disconnect the 1M Ohm potentiometer from all other components and measure the resistance across the Wiper and one of the other legs. Adjust the potentiometer until it has a value as close to 300k Ohms as is possible - record your value.
  • Wire up the circuit as described in Figure 5. Set the supply voltage Vcc at 9V (record the exact value) - measure the voltage supply with your voltmeter in order to get an accurate value. The dotted part of the circuit is not required in this experiment; however, just be aware that this would be where the signal would be inputted to be amplified.

Figure 6. (From the notes) We wish to choose the operating point of the transistor to maximise our possible clean signal output. 

  • In the notes we discussed the best choice of operating voltage is 1/2 the supply voltage Vcc. So, in this case we need to get the VCE voltage to be about 4.5V as our supply is 9V. Record the exact value you achieve. This is a suitable operating point. To do this, measure the voltage across the Collector-Emitter pins of the transistor and vary the 10k Ohm potentiometer (on the collector) until you get a voltage reading of 4.5V.
  • Turn off the power, remove the 10k Ohm potentiometer and measure the resistance value across its legs. Record this value as our Rc (Collector Resistance Value).

Figure 7. (From the notes) The Load Line analysis

  • In your final write-up plot the load line on the graph that you developed in the first part of the experiment. It should look like Figure 7. The point Vcc is 9V and the slope of the line is -1/Rc, where Rc is the resistance value that you just recorded. (Just in case, see: Using Slope and y-Intercept to Graph Lines)
  • Place the 10k Ohm potentiometer back in the circuit and measure the voltage drop across the Base Emitter. As discussed in lectures, it should be approximately 0.7V (note: don't mix up the emitter and collector when measuring this value). If this value is very far from 0.7V there is something wrong with your circuit - please check your wiring before you go on.
  • Fill in all of the values you measured in the table below - Table 2.
 
 SymbolDescription      Accurate Recorded Value
(Remember Units)
 Vcc   Supply Voltage (should be ~9V)    
 RBBase Resistor (should be ~300k Ohm)  
 VCECollector-Emitter Voltage (should be ~4.5V)  
 RcThe collector resistor value (measure)  
 VBEBase-Emitter Voltage (should be ~0.7V)  
Table 2. The Values to be completed from the above procedure

  • Draw a figure like Figure 8 in your write-up, filling in your values from Table 2. 

Figure 8. Draw this figure in your write-up replacing the values in green with your actual readings from Table 2.

  • Now, use Ohm's Law (V = IR) to calculate the values in Table 3. For example, the base current can be determined by calculating the voltage across the base resistor RB, which is VRB = Vcc - VBE. So, IB = VRB / RB.
  • Do similar calculations to determine the "Calculation Estimate" in the following table of values:
 SymbolDescriptionCalculation Estimate  Measured Value
 IBThe Base Current - use the calculation just described (should be small! in micro-Amps)  
 VOUTThe Output Voltage (Should be = VCE  
 VRCThe Voltage across the collector resistor   
 ICThe Collector Current - Use Ohm's law with the Collector Resistor  
 IEThe Emitter Current - IE = IC + IB   
 VBC

The Collector-Base voltage drop - you know VCE and VBE. VCE = VCB + VBE; however, remember that VBC has the opposite sign from VCB. 

So, VBC = VBE - VCE

  
 hFEThe transistor gain: hFE = IC / IB   
Table 3. The Values to be Calculated and then Measured

  • Finally, Measure each of the values and enter the measured values in the last column of the table  "Measured Value". Transfer this table to your write-up. 
  • For the last value of hFE, determine the estimate using your estimated IC and IB values. The Measured value should reflect the actual measurements of IC and IB.
  • Sketch the final circuit with all measured values in your write-up.

Conclusions:

  1. State briefly, but clearly, what you have learned from this session. In particular, why are the output characteristics of a transistor important. 
  2. What was the most difficult aspect of the lab?
  3. State one thing you enjoyed about the session.
  4. State one thing you disliked about the session.
  5. Add any final comment of your own.
There is no template document for this laboratory as you have seen what is required in the previous laboratories; however, any of the tables can be cut-and-pasted into your final report.

Please submit your laboratory write up to: http://moodle.dcu.ie/mod/assign/view.php?id=27754

Ċ
Derek Molloy,
27 Nov 2012, 11:48
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