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Salt Lake Community College Electrical Engineering Department EE1010 Introduction to Transistors using Simplified Circuits Original: 5/7/13 Harvey Wilson, James Quebbeman, Lee Brinton Revision 7/13/13 Harvey Wilson Latest revision 8/20/13 by James Quebbeman Introduction Of all the technological advances of the 20th Century, few had greater impact than the development of the transistor. First documented in 1947, credit for the invention of the transistor is generally given to three Bell Laboratories engineers – John Bardeen, William Shockley, and Walter Brattain. This breakthrough introduced the semiconductor revolution. Just about every electrical device or automatically controlled system today includes transistors. This lab will demonstrate the use of transistors in typical applications – controls or switches, digital logic, and current or voltage amplification. The effect of frequency and loading is included and measured. Objectives Use a transistor to control a lamp (as a switch) Use a transistor to perform digital logic functions (as a switch adapted for logic) Use a transistor to amplify smaller current into larger current (gain) Use a transistor to amplify current at various frequencies (frequency and loading effects) Equipment 2 – 2N3417 or 2 – 2N3904 NPN Transistors (or equivalent) 1-100Ω, 1-390Ω, 2-1kΩ, 1-10kΩ resistors 1– 10kΩ variable resistor (may be used as a load for final circuit) 3 LED’s of any available colors (light visible) 1 IR LED (light not visible) DMM Scope Function Generator 5 Volt source Discussion Transistors are three terminal semiconductor devices. There are two general types of transistors: 1. Bi-junction Transistors (BJT’s) 2. Metal Oxide Semiconductor Field Effect Transistors (MOSFET’s). Page 1 of 8 Although they differ fundamentally in the way they operate, both types perform similar functions. Unique characteristics of each technology determine which type of transistor to use. This lab studies BJT’s. The three terminals of a BJT are referred to as the Base, Emitter, and Collector. Figure 1 shows the location of the connections on the transistor we will use in our lab. Many transistors have the terminals in locations other than the ones shown here. Figure 1: 2N3417 NPN Transistor Terminal Locations and Schematic Symbol One way to consider the operation of a transistor is to think of it as an electrically controlled switch or valve that can be gradually enabled to allow continuously more current through the switch until it is completely conducting. For our purposes, we will consider current flowing into the collector and out the emitter. The current (or voltage) applied to the base determines the amount of current flowing. Thus, the transistor can be in one of three states: 1. The “cutoff” or “off” state (a region of operation where no current flows through the transistor). 2. The “linear”, “active”, or “partly-on” state (the region where the current flowing through the transistor is a scaled value of the current fed into the Base). 3. The saturation or “on” state (the region where the transistor switch is fully enabled allowing maximum current to flow). The base voltage (or current) of the transistor determines how far the “valve” is enabled. In other words, the Base voltage (or current) controls how much current can flow through the transistor. It takes approximately 0.6 Volts applied between the Base and the Emitter (VBE) to get the transistor to start conducting current. After that threshold voltage is attained, very small increases in VBE result in large increases in current flow. Pre-lab The transistor, when used in a voltage divider, has three sets of resistance values controlled by the voltage and current to the base terminal of the transistor. These three relative values are called “large”, medium”, and “small” and correspond to the transistor states of “cutoff”, “active”, and “on” respectively. Page 2 of 8 Given VCC = +5 Volts in Figure 2: Transistor as Controlled Voltage Divider, estimate VOUT voltage for each of these three R transistor conditions. 1. large >> R1 2. medium or about = R1 3. small << R1. Figure 2: Transistor as Controlled Voltage Divider Experiment 1. Transistor as a Control or Switch. a. Connect the circuit shown in Figure 3 using a jumper wire to act as a switch and a lamp or LED (provided by lab instructor) as the indicator. Use a fixed approximately 5 Volt DC source such as the BK power supply. Observe that closing the switch activates the transistor, enabling current flow through the indicator, illuminating it, switching it to the “on” condition. Page 3 of 8 R2 39 0ohm V1 5V LED1 LED_r e d J1 R1 Ke y = Spac e Q1 2N3904 1k ohm Figure 3: Transistor as Control 2. Digital logic gates: NOT, AND and OR gates As mentioned earlier, transistors are the basic building block of digital computing devices. In this role, transistors operate as voltage controlled switches. Digital circuits operate at two voltage levels. The low voltage level (usually something close to 0 Volts) represents a binary “off” or a “0”. The high voltage (in our case something close to 5 Volts) represents a binary “on” or a “1”. The basic logical operations of NOT, AND, and OR are easily accomplished using transistors. The NOT gate (a.k.a. INVERTER) simply changes the state from 0 to 1 or 1 to 0. The AND and OR operations are defined by their Truth Tables below: Table 1: AND Truth Table A B A&&B 0 0 0 0 1 0 1 0 0 1 1 1 Table 2: OR Truth Table A B A||B 0 0 0 0 1 1 1 0 1 1 1 1 In this lab, we will represent the state of the output gate with an LED. A binary “0” will be represented with the LED “off”. A binary “1” will be represented with the LED glowing. a. Construct the NOT gate shown in Figure 4. Verify the operation by demonstrating when the input is “0” (i.e. the switch is open) the output is a “1” and the LED glows. Closing the switch (applying a “1” to the input results in a “0” output and the LED is “off”. Page 4 of 8 R2 39 0ohm V1 5V J1 R1 Ke y = Spac e Q1 LED1 2N3904 LED_gr een 1k ohm Figure 4: Transistor as NOT Gate b. Construct the AND gate shown in Figure 5. Verify the operation of the gate by testing each of the four possible combinations of inputs (open and closed switches SWA and SWB) and demonstrating the correct output. R2 39 0ohm LED1 LED_r e d V1 5V SW A Ke y = A SW B Ke y = B R3 Q2 2N3904 1k ohm R1 Q1 2N3904 1k ohm Figure 5: Transistor as AND Gate c. Construct the OR gate shown in Figure 6. Verify the operation of the gate by testing each of the four possible combinations of inputs (open and closed switches SWA and SWB) and demonstrating the correct output. Page 5 of 8 R2 39 0ohm LED1 LED_r e d V1 5V R3 SW A Ke y = A 1k ohm Q1 R1 SW B Q3 2N3904 2N3904 Ke y = B 1k ohm Figure 6: Transistor as OR Gate 3. Current Gain a. Connect the circuit shown in Figure 7. Measure the current passing through the Base of the transistor through R1 and the current passing through the Collector of the transistor through R2 by measuring the voltage changes, then using Ohms Law to calculate the currents. Notice how much larger the Collector current is than the Base current. The Gain of this transistor circuit is the ratio of Collector current divided by Base current. Gain values of between 50 and 200 are normal. XM M 2 R2 10 0ohm V1 5V LED1 LED_r e d XM M 1 J1 R1 Ke y = Spac e Q1 2N3904 10 kohm Figure 7: Transistor as Current Amplifier (Gain) Page 6 of 8 4. Current Gain Affected by Frequency and Loading a. Connect the circuit shown in Figure 8. Observe the voltage waveform displayed on the scope. Adjust the Function Generator and Scope controls to about as shown in Figure 9 to obtain a smooth sine wave display at about 10kHz. Observe how the size and shape of the output waveform degrades as frequency is increased near 5MHz. If the LED indicator may be interfering, placing a jumper across the LED may help. The use of a probe, first in the 1x setting, then in the 10x setting may help identify whether the transistor or the probe or both caused the upper frequency limitation. The probe in the10x setting provides less loading. XSC1 G R2 A B T 10 0ohm V1 5V LED1 LED_r e d XFG 1 R1 Q1 2N3904 10 kohm Figure 8: Transistor Current Gain Affected By Frequency Page 7 of 8 Figure 9: Setup for Figure 8 Conclusions Record any final observations and check off as usual. Page 8 of 8