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ES110 Transistor Current Amplifiers Sonoma State University Department of Engineering Science Fall 2016 ES-110 Laboratory Introduction to Engineering & Laboratory Experience Transistors and Transistor Applications Introduction Transistors are divided into two general categories: Bipolar Junction Transistors (BJT) and Field-Effect Transistors (FET). The latter is divided into several different sub categories (JFET, MOSFET, etc.). Each type is manufactured in many different forms and sizes and one chooses a transistor based on the required parameter, which include current amplification, voltage amplification, switching speed, frequency response, power ratings, cost, etc. In this laboratory we will primarily use BJT Transistors and refer to other types when appropriate. Transistor can be packaged in single transistor or multiple transistor packages. Transistors are also commonly and abundantly used in many analog and digital integrated circuits. Here we will start with a single transistor and then combine two or more transistors in our circuits. Transistors may be used for voltage amplification, current amplification, power amplification, or for switches. Specific transistors are used for each application. Transistors are manufactured in different packages and some of the packages are shown in figure below. From left: TO-92; TO- 18; TO-220, and TO-3 (TO stands for Transfer Outline). Transistors in general have three pins, identified as Collector, Base and Emitter. The TO-3 type only has two pins (Base and Emitter) and its metal casing acts as a Collector. The three pins of field-effect transistors are called Source, Gate and Drain. There is no universal agreement on the arrangement of the pins and in order to identify transistor pins one must view the manufacturers' pin diagrams, which are easily accessible on the web. In general, one applies an input signal to a transistor with the goal of obtaining an output signal which is different and more pronounced (amplified) than the input. The input and output signals 1 Sonoma State University Department of Engineering Science Fall 2016 must have a common ground. For example, if the emitter of a transistor is common between the input and the output signals, the arrangement is called a "common emitter" arrangement. Two other configurations are "common base" and "common collector". In this lab we will use a "common emitter" configuration. Finally, there are two types of BJT transistors, NPN and PNP, which refer to the type of the semiconducting material of the collector, base, and emitter. Here we will use NPN transistors in which the collector and emitter are constructed of n-type semiconductors, and the base is made of a p-type semiconductor. The symbol for NPN and PNP transistors are shown below. The principle of operation of a common emitter transistor amplifier is to apply a small current/voltage to its base and recover an amplified current/voltage at its collector. The transistor acts as a switch when a small base current turns on a much larger collector-emitter current. For example, an audio amplifier has two stages of amplification. In the first step, which is called the preamplifier step, the input voltage is amplified, and in the second stage the current is amplified. We will build an audio preamplifier in lab 8. In this lab we will focus on the switching and current amplification features of transistors. Output Input Preamp Amplifier P = I V P = I V P = I V The basic amplification nature of a transistor amplifier lies in the fact that the small base current (IB) and the large collector current (IC) are connected to each other as described in the following diagram. AC base and collector currents are referred to as ib and ic. Note that the source supplying current to the collector is often different and stronger than the source supplying IB. For transistor switches, IC is chosen in its plateau region (saturation). The switch is off when the 2 Sonoma State University Department of Engineering Science Fall 2016 base current is too small or zero, and the switch is on when the base current is large. For voltage amplification, the base current is kept in its active range. Part 1: A Simple Transistor Switch The main idea in this section is to use a transistor as a switch: Apply a small current to the base of the transistor and observe that it allows a larger current from the 5-V power supply to go through the LED to the ground. The size and type of the transistor is chosen depending on the voltage and the current (recall that power = I V). We will use a low-power 2N3904 (or 2N2222) NPN transistor. When viewing the flat side of the transistor, the pins from the left are E, B, and C. We supply a small current to the base in µA-range, and observe a current in the mA range turns on the LED. The current gain of the transistor is defined to be β = IC / IB which could be around 100. Note that the transistor gain for inexpensive transistors could vary significantly from one transistor to the other. For this transistor, the power dissipated in the transistor must be less than 500 mW: IC VCE < 500 mW. Construct the circuit by supplying V1 from the power supply and V2 from the fixed 5-V power supply of your Discovery Scope. Make sure your circuit has one common ground. Vary the base voltage V1 from zero to about 2 volts and observe the voltage at which the LED turns on brightly. Next disconnect V1 and touch the left end of the base resistor R1 and observe that the LED shines dimly. Your touch could supply about 5 µA to the base, 3 Sonoma State University Department of Engineering Science Fall 2016 which in turn can amplify to a collector current of 0.5 mA with a current gain of 100. You could use slightly different resistor values if you do not have the exact resistances shown in the diagram. R2 V2 200Ohm_5% 5 V LED1 R1 B C Q1 E 2N3904 V1 20kOhm_5% 1.2 V Analysis Record the value of V1 for which the LED turns on brightly. Using a multimeter, measure the voltage across R1, VBE, voltage across R2, voltage across the LED, and VCE. 1. Verify that V1 = VR1 + VBE 2. Verify that V2 = VR2 + VLED +VCE 3. Using Ohm's law, calculate the current through resistors R1 and R2. These are IB and IC, respectively. 4. Calculate the current gain of this transistor, β = IC / IB. 5. Calculate the power dissipated in the transistor P = IC VCE and verify that it is less than 0.5 W, which is the rated power of the 2N3904 transistor. Part 2 (Optional 1): A Transistor Touch Sensor Note: Two or more transistors required! In part 1 you observed that a few µA base current can switch LED which requires much higher current to turn on. In this part of the experiment we wish to increase the sensitivity of the transistor switch so the LED can turn on at much less base current. The idea is to construct a sensor which can sense your hand when it gets close to it, or when you touch a piece of 4 Sonoma State University Department of Engineering Science Fall 2016 insulating paper or table. In order to increase the sensitivity of the circuit, we can connect several transistors in a formation called Darlington pair connections. This is somewhat similar to overlaying two or more magnifiers on top of each other. Ideally, the current gain of a Darlington transistor pair is equal to the product of the gains of each transistor. However, the actual gain may be less than the product of two gains. Connecting the third and the fourth transistors in Darlington configuration will further increase the sensitivity of the circuit, which means the LED will turn on (perhaps dimly) in response to the extremely low base current. In a Darlington combination, we simply connect the emitter of the first transistor to the base of the second transistor. This combination will act as a single transistor as shown below. C Q1 B 2N3904 Q2 2N3904 E Combine four transistors in Darlington configuration. We can treat this combination as a single transistor and connect the LED and resistors similar to part 1. Bring your finger close to the sensor tip and observe the LED light dimly. Touch the sensor tip and observe the intensity of the light. Place the sensor tip on a piece of paper and touch the other side of the paper and observe the LED light. If interested, create your own sensor applications! 5 Sonoma State University Department of Engineering Science Fall 2016 V1 R1 5 V 200Ohm_5% LED1 C Sensor Tip R2 B Q1 2N3904 20kOhm_5% Q2 2N3904 Q3 2N3904 Q4 2N3904 E Caution: Clearly each transistor in this cascading configuration draws more current than the previous transistor. Q4 carries the maximum current. Therefore, it is important not to allow too much current to go through the lower transistors. Exceeding the power rating of these transistors will destroy them. If instead of the diode you need to turn a motor or use a device that requires large current, then appropriate higher power transistors must be used. Part 3 (Optional 2) Each of the following two circuits uses a single transistor. One is a "Light" detector and the other is a "Dark" detector. A photo resistor is used to control the base current. The location of the photo resistor is chosen depending on whether it is desired to turn the transistor switch on in light or dark conditions. In each case the LED turns on when there is light or dark, respectively.
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