2-1 LABORATORY 2 POWER AMPLIFIER OBJECTIVES 1. To study Class B and Class AB power amplifier circuits. 2. To observe crossover distortion present in Class B power amplifiers. 3. To design and test DC biasing and frequency response of a Class AB power amplifier. 4. To simulate Class B and Class AB power amplifier circuits using MicroCap software. INFORMATION 1. Power Amplifier Class B Class B amplification involves using a dual voltage power supply along with two power transistors, an NPN, and its complementary PNP device. Such a circuit is shown in Figure 2.1 and its operation could be explained as following: • In the absence of an input signal, neither transistor conducts; both transistors are off. • On the positive half of the input cycle, once the input signal is greater than 0.7 V, Q1 will turn on and current flows as shown in Figure 2.1- a. Notice that the base- emitter voltage of Q1 causes Q2 to be held in the off state since Q2’s base-emitter is reverse biased. • As the input signal swings into the negative half of its cycle and exceeds 0.7V, Q2 is turned on and its base-emitter voltage reverse biases the base-emitter junction of Q1, turning it off. a) Positive half cycle operation b) Class B output waveforms Figure 2.1. Class B power amplifier operation + I _ I Vin Q1 B ON C Vo E OFF Vee + Vcc + RL _ _ I C1 FG Q2 2-2 Typical output waveforms for both Q1 and Q2 BJTs and a Class B amplifier output are shown in Figure 2.1-b. The time required for the input signal to move from zero volts to +0.7 V or to -0.7 V is the time during which conduction does not occur, consequently the output sits at zero volts for this interval, producing what is called crossover distortion. Crossover distortion takes its name from the dead-time distortion occurring when the input crosses over from - 0.7 V to +0.7 V or from +0.7 V to -0.7 V. Class B has a very low (almost zero) Quiescent Current, and hence low standing power dissipation and optimum power efficiency. However it should be clear that in practice Class B may suffer from problems when handling low-level signals. In the absence of an input signal, a Class B power amplifier should have zero volts dc on the output terminal with respect to ground, if the transistors are well matched. Often, they are not well matched, so the student should be aware that it is quite possible to have a dc voltage present at the output. Some output loads, such as speakers, may be damaged by dc. If such loads are to be used, they must be capacitively coupled to the output in order to block the dc. 2. Power Amplifier Class AB Crossover distortion could be eliminated in class AB power amplifiers by the addition of the diode circuitry shown in Figure 2.2a. a) Class AB circuit diagram b) Class AB output waveforms Figure 2.2.Class AB power amplifier circuit Since the diodes in Figure 2.2-a are on all the time, both Q1 and Q2 are held at the edge of the conduction mode by the diode voltages (A small but controlled Quiescent Current). When the input goes either positive or negative, very little voltage is required to put Q1 or Q2 into full conduction. Typical output waveforms for both Q1 and Q2 BJTs and a Class AB amplifier output are shown in Figure 2.2-b. D2 D1 Vee C2 Vin D R2 Q2 Q1 Vo FG Vcc I C1 RL R1 2-3 3. Transistors You will be using the TIP140_FC NPN and TIP145_FC PNP silicon Darlington pair power transistors. These transistors are a set of complimentary pair silicon power transistors. Two individual transistors connected in a Darlington configuration in each package will provide a very large short circuit current gain β which is the product of the two β’s of each internal transistor. The transistor diagrams are shown in Figure 2.3. Figure 2.3. TIP140_FC NPN and the TIP145_FC PNP circuit diagrams Note: Two resistors and a diode are integrated internally in the transistor device’s package and one of the reasons for including these components is to prevent a thermal run-away from occurring. These internal components are not shown on the circuit diagrams in Figures 2.1 and 2.2 however they should be included in the device model in your circuit simulation. 4. Power Amplifiers using the TIP140_FC and TIP145_FC Transistors The class B amplifier that will be simulated using the TIP140_FC and TIP145_FC transistors is shown in Figure 2.4. The DC power supply is set to ± 6 V (DC) and the load resistance is RL=R3=8Ω. Figure 2.4. Practical class B power amplifier using Darlington Transistors 2-4 Since the class AB amplifier is implemented using Darlington pairs instead of single NPN and PNP transistors, the diode compensation group should contain three diodes instead of two, as it is shown in Figure 2.5. The DC power supply is also set to ± 6 V (DC) and the load resistance remains at RL=R3=8Ω. The class AB amplifiers have a small IBIAS such that the DC quiescent operating point is just into the start of the conducting region. This will prevent a certain amount of cross over distortion. The class AB circuit must be designed at the edge of the cut-off region. For the circuit in Figure 2.5, the resistor values of R1=R2 are selected to make the diode current of Id=5mA. Figure 2.5. Class AB power amplifier circuit using Darlington Transistors. HAND CALCULATIONS 1. Class B Amplifier For the class B amplifier circuit in Figure 2.4, calculate the following: 1) When transistors Q1 and Q2 are in active mode, the voltage between the base and emitter is around VBEn=VEBp=1V, and at saturation the voltage between the collector and emitter close to VCEn=VECp=0.2V. Plot the transfer characteristic of the input voltage and output voltage of the class B amplifier. The typical transfer characteristic is shown in Figure 2.6. Figure 2.6. Typical Transfer Characteristic of the Class B Amplifier 2-5 2) The output voltage is a sinusoid with 4.2V peak amplitude, what fraction of the sinusoidal wave period does the crossover interval represent. What is the corresponding amplitude of input voltage? Repeat this analysis if the output voltage is a sinusoid with 3.2V peak amplitude. 2. Class AB Amplifier For the class AB amplifier circuit in Figure 2.5, calculate the following: 1) Assume the DC voltage drop across each diode is close to 0.6V and the β of Q1 and Q2 are very high, calculate the resistor values of R1=R2 to within two significant digits to make the diode current ID=5mA. 2) The output voltage at the load is a sinusoid with a 4.2V peak amplitude. Neglect the quiescent power dissipation and calculate the following: a) Find the average output power at the load. The average power drawn from each supply. The power conversion efficiency. b) The maximum power that each transistor must be capable of dissipating safely. Note that these calculations are also applicable to the class B amplifier for similar biasing and output voltages. 3) The output voltage at the load is a sinusoid with a 3.2V peak amplitude. a) Find the average output power at the load. The average power drawn from each supply. The power conversion efficiency. b) The maximum power that each transistor must be capable of dissipating safely. Note that these calculations are also applicable to the class B amplifier for similar biasing and output voltages. MICROCAP SIMULATION 1. Individual Process Variation The class B and class AB amplifier circuits of Figure 2.4 and 2.5 will be simulated modeling the process variation of manufacturing the amplifier. Based on the last digit of your student number let all the Microcap model parameters for the TIP140_FC be perturbed by a factor of X and for TIP145_FC be perturbed by a factor of Y. This includes perturbing the model parameters of transistors Q1 and Q2, the diode parameters of D and resistor values of R1 and R2 (see Figure 2.3). This step is similar to what was done in Lab 1 for the BJT 2N3904 transistor. See the tips section for additional help. If the last digit of your student number is 0 then X=0.975 and Y=0.977 If the last digit of your student number is 1 then X=0.980 and Y=0.982 If the last digit of your student number is 2 then X=0.985 and Y=0.987 If the last digit of your student number is 3 then X=0.990 and Y=0.992 If the last digit of your student number is 4 then X=0.995 and Y=0.987 If the last digit of your student number is 5 then X=1.005 and Y=1.003 If the last digit of your student number is 6 then X=1.010 and Y=1.008 If the last digit of your student number is 7 then X=1.015 and Y=1.013 If the last digit of your student number is 8 then X=1.020 and Y=1.018 If the last digit of your student number is 9 then X=1.025 and Y=1.023 2-6 Based on the second to last digit of your student number, perturb all resistors of Figure 2.4 and 2.5 by a factor of Z: If the second to last digit of your student number is 0 then Z=0.975 If the second to last digit of your student number is 1 then Z=0.980 If the second to last digit of your student number is 2 then Z=0.985 If the second to last digit of your student number is 3 then Z=0.990 If the second to last digit of your student number is 4 then Z=0.995 If the second to last digit of your student number is 5 then Z=1.005 If the second to last digit of your student number is 6 then Z=1.010 If the second to last digit of your student number is 7 then Z=1.015 If the second to last digit of your student number is 8 then Z=1.020 If the second to last digit of your student number is 9 then Z=1.025 All capacitor values are set to 47μF. 2. Simulations, Plots, Calculations, and Questions Class B Amplifier 1) To obtain the transfer characteristics of the class B amplifier of Figure 2.4, plot the output voltage at the load as a function of the input voltage level. To do this using MicroCap perform a DC sweep on the SG voltage source (Figure 2.4) from -7.5V to +7.5V using a 50mV increment. See the tips section for additional information on setting up the analysis. Compare the transfer characteristic obtained using Microcap with the one calculated. 2) Simulate the circuit of Figure 2.4 and obtain the following DC voltages and currents when no AC or DC input signal is applied to the circuit (only the bias voltages). This is done in MicroCap by performing a Dynamic DC (Analysis>Dynamic DC) or Probe AC (Analysis>Probe AC) simulation in MicroCap. Note that you will have to do some simple calculations since MicroCap only displays the nodal voltages. Calculate DC voltages and currents of Table 2.1 with the assumptions made in the hand calculation section and compare with the simulation results. Q1 Q2 VCE [V] VBE [V] IC [A] VCE [V] VBE [V] IC [A] Simulations Calculations Table 2.1. Class B power amplifier DC biasing 3) Set the frequency of the AC input voltage to f=1kHz. Find the AC amplitude of the input voltage to make the output voltage at the load have a sinusoidal peak amplitude of 4.2V. Perform the transient analysis when the output voltage at the load has a sinusoidal peak amplitude of 4.2V at f=1kHz. Simulate the time domain response for five periods and plot the following results: 2-7 Note: for parts a, b, and c you may want to create copies of your circuit file to accommodate the different transient analysis plots produced in each part. a) To obtain the instantaneous power dissipated by the load resistor RL, plot the voltage at the load Vo, the current through the load IL and the product Vo*IL. The average power dissipation at the load can be computed by adding the maximum and minimum amplitudes of instantaneous power Vo*IL and by dividing by two. Alternatively, average power dissipation at the load can be obtained by using the build-in computational facilities of the probe by taking the running average of the instantaneous power Vo*IL using the AVG command in the MicroCap transient plot limits. Figure 2.7 shows the typical voltage, current, instantaneous power and average power dissipated at the load resistor. The running average command produces some sort of transient behavior in early time and eventually settles into a quasi-constant steady-state value. For your plot remember to show 5 cycles and set your output voltage to 4.2V. Use the parameters shown in the Transient Limits box below. Remember that you will have to match the parameter R1 with the name of the load resistor in your circuit. Figure 2.7. The upper graph displays the voltage at the load Vo. The middle graph displays the current through the load IL. The lower graph displays the typical instantaneous and average power dissipated at the load. 2-8 b) To obtain the instantaneous power supplied by the positive DC voltage VCC, plot the collector voltage VC1 of transistor Q1, the collector current IC1 of transistor Q1 and the product VC1*IC1. The average power supplied by the DC voltage VCC=V1 can be obtained using the MicroCap probe by taking the running average of the instantaneous power VC1*IC1. Figure 2.8a shows the typical voltage, current, instantaneous power and average power supplied by the positive DC voltage source. Repeat the same analysis with the negative DC voltage supply VEE. Use the transient limits shown below. Ensure that your NPN transistor is called Q1 and the PNP transistor is called Q2 in your circuit or the limits will not work. When performing the analysis for VEE and Q2, change the Y expression for the IC current to VC(Q2.Q2)*(IC(Q2.Q2)+IC(Q2.Q1)-I(Q2.D1)), and change the Y expression for the VC voltage to VC(Q2.Q2) as shown below in Figure 2.8b. Note how the conducting period is shifted and the waveform inverted for the Q2 transistor. Figure 2.8a. The upper graph displays collector voltage VC1 of transistor Q1. The middle graph displays the collector current IC1 of transistor Q1. The lower graph displays the typical instantaneous and average power supplied by the positive DC voltage source. 2-9 Figure 2.8b. Plots and limits for Q2 transistor c) Another, important quantity is the power dissipated by the two transistors Q1 and Q2. To obtain the instantaneous power dissipated by transistor Q2, plot the voltage between emitter and collector VCE2 of transistor Q2, the collector current IC2 of transistor Q2 and the product VCE2*IC2. The average power dissipation of transistor Q2 can be obtained by taking the running average of the instantaneous power VCE2*IC2. Figure 2.9 shows the typical voltage, current, instantaneous power and average power waveforms for the power dissipated by transistor Q2. Due to nonlinear distortion, the instantaneous power waveform may not be sinusoidal. Repeat the same analysis for transistor Q1. Compare the maximum and average power dissipated by each transistor with the hand calculations. Use the transient limits shown below. Ensure that your NPN transistor is called Q1 and the PNP transistor is called Q2 in your circuit or the limits will not work. When performing the analysis for Q1, edit the expressions appropriately as was shown in part b) above. 2-10 Figure 2.9. The upper graph displays the voltage between emitter and collector VCE2 of transistor Q2. The middle graph displays the collector current IC2 of transistor Q2. The lower graph displays the typical instantaneous and average power waveforms for the power dissipated by transistor Q2. 4) Repeat the analysis of part 3 a) for an input voltage level which results in a 3.2Vpk output voltage. Include the plots. Calculate the power conversion efficiency, power gain, and voltage gain using the values obtained from MicroCap and compare with the hand calculations. Use the set of equations shown below and fill out the table for two input voltages. Remember that PDC is calculated for the power dissipated across the voltage supply. See the tips section for instructions on how to determine the parameters in the table. 2-11 AC input measurements AC output measurements DC input measurements Calculations Vin [V] Iin [A] Pin [W] Vo(rms) [V] Po [W] VDC [V] PDC [W] Av [dB] Ap [dB] n (efficiency percentage) Table 2.2. Class B power amplifier measurements Figure 2.10. Gain and efficiency equations 5) Set the input voltage to have a sinusoidal peak amplitude of 8.0V. Plot the output voltage at the load and comment on and explain the observed waveform. Using your observation and any additional analysis, determine if your amplifier can deliver 500mW of output audio power without distortion. 6) Perform AC analysis and plot the magnitude and phase of the voltage gain Vo/Vi from 10Hz to 50kHz. Class AB Amplifier 1) To obtain the transfer characteristics of the class AB amplifier of Figure 2.5, plot the output voltage at the load as a function of the input voltage level. To do this using MicroCap perform a DC Analysis sweep on the SG voltage source (Figure 2.5) from -7.5V to +7.5V using a 50mV increment. See the tips section for help on setting up the analysis. Compare this transfer characteristic with the class B amplifier. Plot the voltage gain Vo/Vi when a DC sweep on the SG voltage source is from -7V to +7V using a 50mV increment. Comment on how this distorts the output signal. 2-12 2) Simulate the circuit of Figure 2.5 when no AC or DC input signal is applied to the circuit. Use the Dynamic DC or Probe AC Analysis in MicroCap the same way as was done for the Class B amplifier above. What is the DC current Id passing through the diodes? If the current Id is not close to 5mA (i.e. between 4.5mA to 5.5mA), then adjust R1=R2 to make Id within the range of 4.5mA to 5.5mA. Once you are satisfied your circuit is biased correctly, obtain the DC voltages and currents of Table 2.2 from the simulation. Compare the simulated values with your calculations. Calculate the quiescent power of the circuit from these simulations. Q1 Q2 VCE [V] VBE [V] IC [A] VCE [V] VBE [V] IC [A] Simulations Calculations Table 2.3. Class AB power amplifier DC biasing 3) Perform the steps of section 3 to 5 of the class B amplifier for the class AB amplifier. Use the following table when completing part 4. AC input measurements AC output measurements DC input measurements Calculations Vin [V] Iin [A] Pin [W] Vo(rms) [V] Po [W] VDC [V] PDC [W] Av [dB] Ap [dB] n (efficiency percentage) Table 2.4. Class AB power amplifier measurements LAB REPORT Prepare a lab report which includes all schematics with DC nodal voltages and currents shown, and all plots as specified above. Also include written submissions for all questions and calculations. Compile all plots and written submissions into one file. MICROCAP SIMULATION TIPS • To provide a power supply to the circuit use two “Battery” sources from the MicroCap library. Connect them as Vcc and Vee voltage sources with common ground and set them to a 6VDC. • To obtain the values of all the bias currents and voltages on your schematic from Analysis menu choose the Dynamic DC mode and click on Node Voltages and Currents icons on the toolbar. • For a sine wave signal source use a 1MHz Sinusoidal Source from the Micro–Cap library. Set the AC Amplitude to A= 4(V) in the model description area of the signal 2-13 source. Note that A=4V corresponds to Vp=4V. Note that you will have to adjust the AC amplitude to match the output voltages required. • Run “TRANSIENT ANALYSIS” to obtain an input and output waveforms. Note: Set the parameter P to plot separate diagram for each curve. • Run “AC ANALYSIS” to obtain the gain and phase frequency response plots for this circuit for frequency range from 10 Hz to 100 kHz. Note: Set the parameter P to plot separate diagram for each curve. • To insert the Darlington transistors, browse to Component> Analog Library > BJT > Darlington > T. See below for demo. Figure T-1 Darlington Transistors • When perturbing the values for the Darlington Transistors, edit the parameters directly in the SPICE netlist shown when you click on the transistors you placed in your schematic layout, as shown below. Note that you will have to implement the process variation for each transistor on each of your classB and classAB amplifiers. To simplify this work, once you edit the parameters for each of the TIP140 and TIP145 netlists in your classB amplifier layout, you can copy the netlist into the transistor models in your classAB amplifier layout. MicroCap saves each edited netlist locally for each schematic file so you will have to edit the parameters twice. 2-14 Figure T-2 Edit model parameters in netlist • When performing the DC Analysis for the transfer characteristics of the amplifier remember to remove the input capacitor or your analysis will not work. To run the DC Analysis, select Analysis>DC from the top menu and use the DC Limits parameters shown below for the Class B amplifier. Edit the parameters to fit your circuit’s node names. Edit the parameters for the Class AB simulation. Figure T-3 DC Analysis limits for transistor transfer characteristic • For the diodes in the class AB amplifier, use the GENERIC diode model and change the diode voltage to 0.6V as shown below. 2-15 Figure T-4 Diode Model • For part 4, this tip will help you determine which values to use when filling out the table. For the DC input measurements, use the VCC voltage (battery) as your VDC. For PDC, use the current across the battery source from your Dynamic DC analysis for your IDC value. For the AC output measurements, use the rms value of the output voltage waveform (neglecting DC) for your Vo. For Po, use the RMS value of the steady state average in your last plot in part 3 a). For the AC output measurements, you will have to perform a transient analysis plot to determine the input current IIn. Plot the current waveform across your input source and IIn will be the RMS value of the input waveform.
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