ECE 3274 Common-Emitter Amplifier Project

Transcription

1 ECE 3274 Common-Emitter Amplifier Project 1. Objective The objective of this lab is to design and build three variations of the common- emitter amplifier. 2. Components Qty Device 1 2N2222 BJT Transistor 3. Introduction One of the most popular single- transistor BJT amplifier designs is the common emitter (CE) amplifier design. The CE amplifier is relatively simple to bias, delivers a high voltage gain, and is easy to understand. In this lab, you will design and build such an amplifier. The procedure for this lab is fairly straightforward. First, you ll design three versions of the amplifier for your prelab assignment. This process will involve designing the bias circuit and tailoring the frequency response of each amplifier circuit to meet the requirements. Then you ll build first the amplifier in the lab and test it to see whether it meets the requirements. You ll then test two variants of the amplifier with different gains to see the effect of emitter resistance and feedback on gain. You should refer to your lab lecture notes, your Electronics II Lecture notes, your textbook, the course website, and other reference material to determine how best to design your amplifier. This lab is intended as a design project and not as a step- by- step guide. 4. Requirements Your amplifier design must meet the following requirements. Requirement Specification Voltage Gain (Emitter Fully Bypassed) A v > 50 V/V Low Frequency Cutoff Between 100 Hz and 200 Hz High Frequency Cutoff Between 50 khz and 150 khz Input Impedance Greater than 1 kω Output Voltage Swing Greater than 1.5V pk Load Resistance 1.5 kω Emitter Resistance Two Resistors, R e1 = R e2 Power Supply Voltage 12 V dc Table 1. Common- emitter amplifier requirements. 5. Prelab Design Project You will design three amplifiers in this prelab design project. The three designs are very similar, except that the location of the emitter bypass capacitor changes between designs thus changing the gain and input impedance of the amplifier. The values of the capacitors will also change from design to design. Much of the design will not change, though, as you will see. You may use a MATLAB (or similar) program to run calculations if you would like, Revised Spring 2010/mjb 1

2 but your code and output must be included, and you must comment your code to indicate where all equations came from and any assumptions you made. Units must be included as well (it is permissible to generate a table of final values for clarity if you would prefer, but again, all work must be shown clearly somewhere). Use the following fixed component values in your circuit: Component Value R flt 100Ω R i 47Ω C byp 0.1µF C flt 4.7nF Table 2. Fixed component values. Vcc Vcc Div by 10 Rc Cbyp Vout 470 Low pass Vin Rb1 Cout Rflt Ri Cin 2N2222 Chi Rload Rgen Cflt 1 Cflt 2 Rb2 Rout Function Generater Re1 Cbyp Ce Re2 Rin Figure 1. Common- emitter amplifier circuit. 5.1 DC Bias Begin by designing the DC bias for the amplifier. Note that the location and value of the capacitors do not affect the biasing, so the values you calculate here will be valid for all three amplifier designs. Once you have designed the DC bias, use the transistor characteristics for the 2N2222 transistor to determine the transistor parameters for where you are operating. Note that there is no single correct answer and that your design may differ significantly from your colleagues. You should show all work and walk through all calculations. Revised Spring 2010/mjb 2

3 Component Values Amplifier Parameters Voltages and Currents R b1 Beta dc V ce R b2 Beta ac V be R c r π V e R e1 r o I b R e2 Table 3. DC Bias and Amplifier Parameters 5.2 AC: Full Emitter Bypass We will now design and calculate the ac characteristics for the amplifier, with the emitter capacitor fully bypassing R e1 and R e2. Table 4 shows all of the values you need to calculate. Be sure to show all work. You may use equations given in the lab lecture, class lecture, or from a textbook (i.e., you do not need to derive the voltage gain). Be sure you understand how to use the equations, though if assumptions are included, you must state these and show that you meet them. 5.3 AC: Partial Emitter Bypass Repeat section 5.2, with capacitor C e only bypassing R e2. The gain will be lower than in section 5.2, and this is acceptable. 5.4 AC: No Emitter Bypass Repeat section 5.2, with capacitor C e removed from the circuit. The gain will be significantly lower than in section 5.2 and you may need to remove the 10:1 voltage divider at the front of the circuit as a result. Component Values Amplifier Parameters Voltages, Currents, and Power C in Voltage Gain v in C out Current Gain v out C e Power Gain (in db) i in C hi Low Frequency Cutoff i out High Frequency Cutoff I c p in p out Input Resistance Output Resistance Table 4. Small Signal (ac) Amplifier Parameters 5.5 Computer-aided Analysis Once you have completed your three amplifier designs, use PSpice to analyze their performance. Generate the following plots for each amplifier design: (a) A time- domain plot of the input and output, with the output voltage of 1.5V pk or greater at 1 khz. The output should not have any distortion or clipping. Calculate the midband gain and indicate it on the plot. Compare this to your calculated values. Revised Spring 2010/mjb 3

4 (b) An FFT of your time- domain waveform. Circle and indicate the height of any strong harmonics, in db relative to your fundamental frequency at 1 khz. (c) A frequency sweep of the amplifier from 10 Hz to 1 MHz. Indicate the high and low frequencies on the plot (these should correspond to the half- power, or - 3dB points). Compare these to your calculated values. 5.6 Prelab Question How could you vary the gain of the amplifier using a potentiometer of a value equal to R e1 + R e2 without affecting the Q point? Draw a circuit. 6. Lab Procedure 6.1. Construct the CE amplifier shown in Figure 1. Remember that R src is internal to the function generator and is not in your circuit. Also remember to use two emitter resistors of approximately the same value, as shown in the schematic. Record the values of the bias network resistors and the capacitors you used in the circuit Measure the following values: (a) Q- point: Vce, Vbe, Ve, Ib, and Ic. (b) Voltage, current, and power gains. (c) Maximum undistorted peak- to- peak output voltage. (d) Input and output resistance. (e) Low and high cutoff frequencies (half power point). Recall that input impedance is given by R in = v in /i in, output impedance is given by R out = (v oc v load )/i out, voltage gain is given by A v = v out /v in, and current gain is given by A i = i out /i in. Additionally, plot the following: (a) Input and output waveform at the maximum undistorted value. (b) FFT showing the fundamental and first few harmonics. (c) Frequency response from 10 Hz to 1 MHz (set the input voltage to a value that does not cause distortion across the entire passband of the amplifier) Replace the load resistor, RL, with a 100 Ohm and a 10k resistor, and measure the maximum output swing and voltage gain without clipping. Comment on the loading effect, and remember to change back to a 1.5k load resistor after this step Modify your circuit so that the emitter capacitor only bypasses R e2. Remember to change the capacitor values to maintain the correct frequency response, and remember to increase the input voltage so that you obtain an output voltage swing equal to or greater than 1.5 V pk. Repeat step Modify your circuit to remove the emitter capacitor entirely. Remember to change the capacitor values to maintain the correct frequency response. You may need to remove the 10:1 divider at the front of the circuit so that you can obtain a large enough output signal. Repeat step 6.2 again. Revised Spring 2010/mjb 4

5 ECE 3274 Common Emitter Amplifier Lab Data Sheet Name: Lab Date: Grade: /100 Partner: Remember to include units for all answers and to label all printouts. There are a total of nine (9) printouts in this lab. Only one set of printouts is required per group Component Values R b1 : R b2 : R c : R e1 : R e2 : R L : 6.2. Common- emitter amplifier with a fully bypassed emitter. There are three printouts here. Capacitor Values: C in : C out : C e : C hi : Q-Point: V ce : V be : V e : I b : I c : Resistance: Input Output Frequency Response: Low: High: 6.3. Common- emitter amplifier with a variable load resistor. There are no printouts here. 100Ω Resistor: 10kΩ Resistor: 6.4. Common- emitter amplifier with a partially bypassed emitter. There are three printouts here. Capacitor Values: C in : C out : C e : C hi : Q-Point: V ce : V be : V e : I b : I c : Revised Spring 2010/mjb 5

6 Resistance: Input Output Frequency Response: Low: High: 6.5. Common- emitter amplifier with a fully unbypassed emitter. There are three printouts here. Capacitor Values: C in : C out : C hi : Q-Point: V ce : V be : V e : I b : I c : Resistance: Input Output Frequency Response: Low: High: Revised Spring 2010/mjb 6