PHYSICS 5620 LAB 4: TRANSISTORS

Transcription

1 PHYSICS 5620 LAB 4: TRANSISTORS Objective The objectives of this laboratory experiment are to familiarize the student with the basic characteristics of zener diodes, field effect transistors (FET s), and bipolar junction transistors (BJT s). Introduction The transistor is the device around which all modern amplifiers and electronic switches are constructed. Transistors fall into one of two major classes: bipolar junction transistors (BJT) and field effect transistors (FET). In this experiment, you will study: A Zener diode is a special type of diode that exhibits reverse breakdown at a roughly fixed voltage called the Zener voltage. The Zener voltage for the diodes we will use is about 5 V. You will measure the I-V characteristics of these diodes. A Field Effect Transistor (FET) is a three terminal device. An FET is ideal for use as a switch. A voltage applied at the gate of the FET can control the current flow from source to drain. A Bipolar Junction Transistor (BJT) is also a three terminal device. You will build a simple amplifier circuit with a BJT and investigate some of the properties of the amplifier. Experiments 4.1. Zener Diode R 0-12V Zener Diode Figure 4.1: Zener diode test circuit.. 1

2 Construct the circuit shown in Figure 4.1. The resistor in the circuit should be chosen so that it limits the current through the diode to less than 50 ma. Ask your instructor if you are unsure of an appropriate value for R. HINT: Consider the maximum voltage which can be applied across the diode and resistor. The 12 V variable DC voltage source on the NI ELVIS board is used in this circuit. Measurement 1: Using the DMM, measure the voltage drop across the diode and the current flowing through the diode. Note: It is almost always more convenient to measure a voltage than a current. The current through the diode is equal to the current through the resistor, which is proportional to the voltage across the resistor. Measure a sufficient number of points in both the forward-biased and the reverse-biased directions so that you can accurately plot the I-V curve. Present your data in a table and an I-V graph. Question 1: Approximately what is the Zener voltage for your diode? Don t confuse the forward voltage drop of about 0.7 V with the Zener voltage Field Effect Transistor (FET) Figure 4.2: MOSFET circuit to measure transconductance. FET s are three terminal devices, with the terminals called the source (S), gate (G), and drain (D). The instructor will tell you the arrangement of S, G, and D for the FET s you will be using. FET's are characterized by their transconductance, g m, defined in the small signal limit as the drain current divided by the gate-source voltage, gm = id vg s. (The small signal limit is roughly the derivative gm = Id V gs.) You will determine g m for your device using the circuit shown in Figure 4.2. Use the variable 12 V DC supply of the NI ELVIS system to generate the gate-source voltage. The gate current in a MOSFET is extremely small, so the source current equals the drain current. Choose a value for R S so that the maximum drain ( source) current doesn t exceed ~30-50 ma with the 15 V supply voltage. (Note that when the MOSFET is on its drain to source resistance is approximately 7.5 Ω.) Measurement 2: Measure I d for V gs between 0 and about 2 V. Make a table of your results and plot them. 2

3 Question 2: From the plot, determine g m in the linear region, near V gs = 2 V Characteristics of the Common Emitter Amplifier Assemble the common emitter amplifier circuit shown in Figure 4.3. Use an NPN transistor: 2N4123, 2N4124, 2N2222 or 2N3904. V CC = 15V R B1 56k R L 3.9k V out V in 2N3904 R B2 12k R E 1.0k C E Figure 4.3: Common emitter amplifier circuit Quiescent State An amplifier circuit must be powered in order to operate. The state of the circuit (voltages and currents) with power and without an input signal is called the quiescent state. Measurement 3: Measure the quiescent voltages and currents for this circuit, V CC, V C, V E, V B, V CE, V BE, I C, I E. Present your measurements in a table Small Signal Response I Use a 1 khz, 10 mv pp sinusoidal signal from the function generator as the input signal to the circuit. A resistor across the output of the function generator will help attenuate the signal to the desired level. Remember to AC couple the oscilloscope in order to measure the AC signals. 3

4 Note: It is often helpful to place a resistor (~680 kω) across the output (between the output terminal and ground) to prevent the DC blocking capacitor on the output from developing a DC bias. This is necessary because the NI ELVIS oscilloscope inputs have a very high input impedance and so cannot provide a current path for any stored charge on this capacitor. Measurement 4: Measure the small signal values: v in, v out, v E, the measured gain A = v v, and the phase of the output with respect to the input. v out in Measurement 5: Sketch and label the waveforms of the AC voltages, v in, v out, v E Small Signal Response II Turn off the dc power. Remove the emitter capacitor C E from the circuit. Reapply the dc power. Measurement 6: Measure v out and v E when a 1 khz, 100 mv pp signal is applied to the input. Measurement 7: Examine the small signal performance by measuring: v in, v out, v E, the small signal gain Av = vout vin, and the phase of the output signal with respect to the input signal. Measurement 8: Sketch and label the waveforms of the AC voltages, v in, v out,and v E. Question 3: Calculate a theoretical value of the small signal gain A v = v out v in. Show your calculations. Compare with the measured value. Don t forget to include uncertainties! Measurement 9: Measure the peak value of v E. Compare with the calculated value of v E. Show your calculations Bandwidth To determine the bandwidth of this amplifier circuit, replace capacitor C E. Measurement 10: Using a 10 mv pp input signal, measure v in and v out and the phase shift between v in and v out for frequencies of 3 Hz, 10 Hz, 30 Hz, 100 Hz, 300 Hz, 1 khz, 3 khz, 10 khz and 30 khz. Be sure that the signal generator amplitude remains constant. Plot the gain and phase shift versus frequency, as you did for the high and low pass filter circuits of the second lab. Question 4: What is the low frequency 3 db point for your amplifier? Distortion 4

5 Distortion occurs when the output signal is no longer an amplified copy of the input signal. The exact definition of distortion depends on the application of the circuit, for instance distortion in an audio amplifier may mean something different from distortion of a radar signal. For our purposes, we will say that distortion occurs when a sinusoidal input no longer gives a sinusoidal output. Measurement 11: Using a 1 khz sinusoidal input signal, increase the input signal amplitude until the output signal begins to show some signs of distortion. Note the amplitudes at the onset max max of distortion, v in and v out. Sketch input and output waveforms in which some distortion is observed in the output. 5