Experiment 5 - Transistors as current amplifiers

EXPERIMENT #5 PRELAB

In this lab we will be investigating another semiconductor device - the transistor. In the previous lab we studied the diode. Diodes have many applications, one of which is a switch. We can imagine that in the circuit below that we can use the variable voltage of the power supply to turn the diode on and off so that current flows or does not flow through the circuit depending on its voltage. Once the power supply voltage reaches the turnon voltage of the diode the diode will conduct opening a path for current to be supplied to the lamp.

In our circuit that drives the vehicle we need a switch that can be controlled electronically. In order to steer our car we must be able to turn the motors on and off according to what the IR sensors see when they look at the surface. Under certain circumstances the vehicle needs to steer right by either turning off the right motor or by making the right motor turn backwards while the left motor continues to run forwards. Same for the other direction. Under certain circumstances you want to completely stop. There is a serious drawback to using the diode as such a switch for this application. The load, in this case our motor (in the figure above an incandescent lamp) must be connected to the same power source as the diode. But the signals that will be used to turn the motors on and off will be computed using TTL logic gates. These gates are completely incapable of driving the motors in your cars. If you connected one up to the motor and turned it on the motor would sit serenely, quietly. So we need a switch that can be controlled by a small signal with this small signal somehow enabling a large amount of current to flow. Hence our need for the transistor. By injecting a small amount of current into the base of the transistor, a channel is open between the collector and the emitter that allows a much larger amount of current to be drawn from and independent, stronger source.

Lets see how this translates into a circuit. Look at the circuit below. This is the one that we will be building in the lab.

The input voltage Vin and base resistor are connected between the base and emitter of the transistor. Their purpose in the circuit is to put the transistor into the desired mode of which there are three: off, active, and saturation. For negative and small positive values of Vin the transistor is in the OFF state. For large positive values of Vin the transistor in SATURATION and for the input voltages in between the transistor is in the ACTIVE state. We call this part of the circuit the INPUT circuit. The part of the circuit connected between the collector and emitter is called the OUTPUT circuit. These two circuits are modeled with two independent equations written in the figure above. They were derived using Kirchoff's voltage law written around the two loops. The two equations are coupled because the base current Ib and the collector current Ic are related in the active region as .

i) Find in the data sheet (follow the link to the left 2N5192) find the values for beta (which is hfe on the data sheet), Vcesat, and Vbeon. Some of the numbers are given as a min and max value, write down both and specify. Some have two values depending upon the amount of current drawn from the collector write down both values though we will only be using the lower amperage numbers.

ii) Using the values that beta=60, Vbeon=1.2V, and Vcesat=1.2V find the input voltage Vin that just turns the transistor ON and in the ACTIVE state.

iii) Now find the input voltage that puts the transistor in saturation.

iv) In the figure below what does the second transistor do?

EXPERIMENT #5

Basic Transistor operation

We will be using transistors in our vehicle design both as current amplifiers and as switches. As current amplifiers a cascade of transistors in a configuration called a Darlington pair will be used to amplify the current delivered by digital logic and the IR receivers - both of which are too small to be useful. Without current amplification no TTL chip can provide enough current to drive the motors on the cars and without current amplification none of our circuits could hope to pick up the miniscule amount of current actually generated in the photo transistor inside our IR sensors. As switches the transistors are combined with other circuit components to build logic gates and switches with which to control the motor direction. So lets start with looking at the input and output characteristics that make this device a good current amplifier.

Transistor operation - common emitter mode

Figure 1: Circuit setup for testing a simple transistor.

i) Build the circuit shown above using a 2N5192 transistor, 10kΩ and a 1kΩ resistor, 6/25V power supply, function generator, and an oscilloscope. Be sure to properly orient the transistor in the circuit. See diagram below for explanation of transistor orientation. The easiest way is to use test boxes because the leads of the transistor are too big for the protoboard.

Figure 2: Common labeling of BJT transistors

ii) Turn on the oscilloscope and put it in XY mode. Invert channel 2. Position the displayed dot at the center of the screen.

iii) Set the voltage on the supply to the correct voltage so that the base current is 10 microAmps. How can you determine the base current since the accuracy of the multimeter is not sufficient to accurately read microAmps? Connect channel 1 of the oscilloscope across the emmitter-collector junction and connect channel 2 of the oscilloscope across the collector resistor (this is a proxy for measuring the collector current). Remember from the last lab that the grounds of BOTH channels MUST BE CONNECTED TOGETHER so the signal on channel 1 needs to be inverted to get a positive voltage on the screen. Turn on the function generator and set it to a 100 Hz sine wave. Vary the amplitude of the waveform and the DC offset from the function generator until you feel you have obtained a good image of the I-V characteristic for the transistor. Sketch this curve in your lab notebook. Repeat for several values of the base current - just vary the voltage on the power supply to obtain 3 or 4 good curves that flatten out. Be sure to save the voltage that you used for each curve and the associated base current. You will need these for part e. The autostore button on the oscilloscope might come in handy here.

NOTE: this is a difficult part of the lab to get working. Here are sume suggestions.

● BUILD THE CIRCUIT FIRST, then probe the circuit with channel 1 and 2 of the oscilloscope.
● If everything works correctly you should see on the oscilloscope a curve that looks the one below.

● Some common mistakes are: i) driving the transistor too hard - the input voltage generated by your power supply should be only a few tenths of a volt, ii) getting the emitter and collector backwards, iii) not grounding the two oscilloscope channels at the same place - the ground for both channels should be connected to the collector of the transistor.
● If your signals look very ragged you may not be groundinging the channels correctly or one of your cables is bad. There have been quite a few broken cables.
● Your final plot should look something like this.

iv) Estimate from the graphs the current gain (or beta as it was discussed in lecture) of this particular transistor. Remeber beta (or hfe) is the (output current Ic)/(input current Ib). Read Ic from the flattened portion of the curves that you obtain.

v) Now disconnect the function generator and connect the collector of the transistor to your battery. Note the voltage across the terminals of the battery. This sets the collector voltage to a particular value in what is called the COMMON EMITTER configuration which is the configuration that you have exhaustively studied in class (or will). The plots that you made with the oscilloscope (in part c) show for a particular state of the transistor - as defined by the amount of current flowing into the base of the transistor - ALL POSSIBLE OUTPUT STATES (at least that were tested within the 0-10V range of the function generator). By connecting the battery, we have chosen one specific voltage rather than a varying one. Now, for each value of the base current you can measure the voltage from collector to emitter Vce, and the current Ic flowing into the collector - you can measure the current in the same way you did in part c, as a voltage, or you can measure the current directly with the multimeter. You are going to plot these points for each of the values of Ib that you plotted from part c. So set the power supply to the same voltages as you did before to set the values of Ib and measure (however you like) the voltage Vce and plot these points on top of your graph of the I-V curves of the output of the transistor. Connect them - what have you just drawn? You have drawn them before.

Transistor operation - current amplification

i) Use the same experimental setup as in part 1, but now place another transistor in the circuit by connecting the emitter of the existing transistor to the base of the new one. Connect the emitter of the new one to ground and the collectors together - refer to the figure below if you need guidance. What is the purpose of the second transistor? This configuration is called a "Darlington pair".

ii) For a few values of Ib measure Vce, and Ic - make a table. These two transistors can be considered together as a single transistor with its own characteristic parameters - Vbeon, beta, and Vcesat. These parameters are specified in all transistor data sheets for individual transistors. Lets measure these parameters considering the pair of transistors together. First estimate the effective beta of the two transistors combined. Next, estimate the voltage that needs to be supplied by the power supply to turn on BOTH transistors. Now try to measure the saturation voltage across both collector to emitter junctions. How can you do this? You must determine whether or not the transistors are in saturation and put them in that state. Could it ever happen that one transistor is operating in the active region and the other in the saturation region?

Transistor operation - TIP module

Now we will play with the module - or rather one of the two possible modules that you can use to run the motors on your car - to get a feeling for its characteristics and limitations. The schematic of the module is shown below. You can see that is just two transistors cascaded together and the entire module can be considered one big transistor with a its own Vbeon, Vcesat, and beta (we experimented with this in part b of the last section).

This module has been incorprorated into a circuit called the "current amplifier module" or simply the CA and can be found in the kits in your gray boxes. There should be two, one for the right motor and one for the left motor. The schematic for the entire module is shown below. Notice there is an extra transistor in addition to the TIP module itself. Across the base emitter junction of the exterior transistor is a jumper. In one position the exterior transistor base-emitter junction is shorted, in the other position the short is removed.

i) Build the following circuit on your protoboard with the protoboard inside the car (the numbers are pin numbers of the module). It is basically the common emitter circuit that we built at the beginning of the lab except that collector resistor has been replaced by the motor. Be sure that the jumper is in the position that shorts out the exterior transistor. Turn the variable resistor knob all the way so that the resistance is at its greatest. We are hooking up the motors to the CAs and the battery in such a way that we will be varying the speed of the motors with the variable resistor on the back of the vehicle - remember the two ways to drive a motor from lab 3? In this state is either motor running (different vehicles behave differently - some will be running and some won't)?

b) Turn the variable resistor knob on the back of the car until the motors just start running. Mark the place where you placed the knob (with a piece of tape). Is it the same place for both motors? If the answer is "no" choose one motor to track. If your vehicle's motors were running even for the highest setting of the variable resistor - record that. In other words describe how your vehicle's motors are responding to the CA when connected WITHOUT the exterior transistor.

c) Lets probe the module itself to see how the transistors are behaving. What is the voltage that needs to be applied to turn on the module? Between what two points must you measure this voltage? What is the effective beta?

d) Now change the position of the jumper so that the transistor is now included in the transistor cascade inside the module. Now there are 3 transistors amplifying the current. Now repeat parts b) and c) for this configuration.

f) These two configurations of the "current amplifier module" are the possible ways that you can modify the circuit to optimize motor performance. See if you can find a good configuration where the motor responds well to a large range of values of the variable resistor. Since the car behaves differently when loaded by having to run on a table try to put the car on one of the tables in the center of the room and let them run at different speeds using the CA with and without the additional transistor. Is there any difference that you detect when the car is running on the table?

g) What are possible purposes of the external transistor?