Instituto Tecnológico y de Estudios Superiores de Occidente (), OBJECTIVES The general objective of this experiment is to work with a realworld amplifier. a) Reinforce the power analysis in electronic circuits to calculate the performance in the various amplification stages. b) Compare analytical results, simulations and measurements on power electronic circuits. c) Implement power amplifiers for practical purposes. Part II. 1. Implement the circuit in Fig. 1 with the values calculated in the first part. a) Adjust the sinusoidal output signal so it is V P =[V ] and f =1[kHz ] (with the Ra terminal connected to X). The load used must be the power resistor and not the speaker. Verify with the oscilloscope the output signal at Y. b) Perform a FFT to the output signal. Take pictures of a before and after applying the transform. SPECIFICATIONS c) Connect the feedback to node Y and verify again with the oscilloscope. Part I. Audio Amplifier. d) Again, perform the FFT to the output signal with the feedback connection. Design a class B audio amplifier (Fig. 1) that is able to output 5W on a 8 Ohms speaker when the input is 500 [mv] RMS @ 1kHZ. Take into consideration the schematic shown in Fig. 1 and also the specifications of the active components displayed on such circuit. Questions (1). What is the element/mechanism that allows us to remove the crossover distortion in the power amplifier? Explain. Switch the power resistor with an 8 Ohm speaker and use an audio signal at the input, make sure you notice the effect of the feedback at node X and Y done to the audio output. Describe and explain the effects of the distortion removal done to the audio signal. Measure the following parameters of the circuit: a) The input impedance of the output stage (X). b) The output impedance of the output stage (Y). c) The average dissipated power by each of the transistors on full load. d) The average power required on full load for each power supply (do not take into consideration the power required by the TL08 Opamp). Fig. 1: Audio amplifier schematic. 1. Calculate the input/output impedance, power calculations and efficiency of the amplifier. What is the real-world efficiency of the power amplifier? Build a table (Table II) with the gathered data obtained in the measurements, calculations and simulations. Please include the error margins.. Capture the schematic with SPICE and display the following: input/output impedance, average power and efficiency of the power stage, THD with the Ra terminal connected with the node X. THD with Ra at node Y. 3. Make a table (Table I) which sums up the calculations and simulations results. Subject: Analog Electronic Circuits - Page 1 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), Part III. DC Motor Controller. Design an electronic system that allows you to gradually change the speed of a DC Motor using a 8 bits digital word. For such design consider the block diagram (Fig, ). Some specifications to consider are that when the input byte is 00 H the speed of the motor must be zero and when the byte is FF H the speed should be maximum. Fig. : Block diagram of the DC Motor speed controller. COMPONENTS Various resistor values. 8 Ohms Power Resistor. TL08 Opamp. Complementary Bipolar Transistors (TIP31 & TIP3). 8 Ohms Speaker. Audio input plug. DAC0800LCN. Audio source. Digital multimeter. Function generator. DC Power supplies. Oscilloscope. Test probes. ####################################### TL08 Diagram. ------------------------------------------------------------------------------------------ Part IV. Implement the speed control circuit discussed in part III. Check with measurements the correct functionality required by the specifications. Questions (). ####################################### DAC0800LCN Data. ------------------------------------------------------------------------------------------ What is the load impedance (RL) of the motor? What is the power dissipated by the transistor on the worst scenario? What is the real frequency of the designed amplifier? Build up a table (Table III) with data from the previous questions. ####################################### TIP 31 & TIP3 Diagram. ------------------------------------------------------------------------------------------ ####################################### DC Motor Specifications. -----------------------------------------------------------------------------------------Impedance: 75 Ω. Vmax: 1 V. Power: 1.9 W. Subject: Analog Electronic Circuits - Page of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), ELECTRONIC AMPLIFIERS Class B Power amplification is heavily required in a big part of electronics as some applications require to do heavy-duty tasks like drilling holes on rocks, cutting materials with lasers or even pumping electrical engines for instance. Also referred as push-pull (Fig. 4) amplifiers because of their design. Only half of the signal is used per active device, each transistor conducting alternatively. As you can clearly imagine all those require big loads of energy in short periods of time (power), and as we know the electrical power depends mainly on two factors: 1) Potential (V). ) Current (I). The term power amplifier relates to the amount of power that is delivered to the load. Typically power amplification stages are the last in the transmission chain (output stage). The power amplification stage requires special attention and care in the efficiency. This leads us to a division of amplifier classes defined by their efficiency and derived according to their mode of operation. These power amplifier circuits are classified (Fig. 3) as A, B, AB, C, for analog designs. This classification was chosen based on the conduction angle (angle of flow Θ). Class Conduction Angle (Θ) [Deg] A 360 B 180 AB 180 < Θ << 360 C < 50 Fig. 4: Push-pull amplifier with Bipolar Transistors. Push-pull amplifiers have a very distinctive effect called crossover distortion, this effect is produced as none of the transistors will conduct for a period of time as a result of the active elements being switched (loss of signal). Class AB With each active device conducting more than half of the time (but not necessarily all of the time). This class includes different methods for correcting the crossover distortion by forcing (Fig. 5) the devices to operate. There are other critical factors on an output stage other than the delivered power and conduction angle which requires special care on the design such as: input/output impedance and total harmonic distortion (THD). I. Class A. II. Class B. Fig. 3: Conduction Angle. III. Class AB. Class A As we could see this amplifier uses 100% of the signal. It is very linear but offers a maximum efficiency of 5%. This types of amplifier are usually used in small-signal stages or for low-power applications. Fig. 5: Crossover distortion correction As different methods for biasing the active devices are used, there will be other factors accounting to the switching time ( represented as crossover distortion) such as capacitance or temperature. Therefore the choice on analog amplifier classes should be made mainly by considering the application specifications, cost and efficiency. Subject: Analog Electronic Circuits - Page 3 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), DESIGN And about the load and transistors 1 V o 1 8.95 P L= = = 4.95[W ] RL 8 Part I. Audio Amplifier. As the required specifications is 5W on 8 Ohms then P Q= o V =P L V o= P L R L = 6.35 [V ] RL As we know the input will be 500 [mv] RMS @ 1kHZ that means the maximum amplitude of a test sinusoidal signal would be RL =.85 [W ] Thereby the calculated efficiency of our design is supposed to be = V op =0.5 700 [ mv ] V CC P L 4.95 = = 46.34 % P S 10.68 Therefore meaning our gain must be of at least Part II. DC Motor Controller. AV = 6.35 = 1.78 [V /V ] 0.700 Taking into consideration the electrical specifications of our motor and digital inputs, we start out with the next schematic (Fig. 6). According to the schematic (Fig. 1) the gain of the non-inverter amplifier is given by AV = so by defining Ra 1 = 1.78[V /V ] Rb Rb=10 [k ] Ra 10[ k ] then. For the impedance calculations we have Ro=r e // r e = QP The Early voltages ( R o QN VA VT I Q and Ri = ) are 100 V and.05 = 1.01 [ ] I Q V BE V E IB IQ = 1.041 [ma]. Ri = 1.431[ K ] And the power analysis given by V CC V ORMS 15 8.95 PS= = = 10.68[W ] RL 8 Fig. 6: DC Motor Controller Schematic ( VCC = 15 & -VEE = -15 ) Since by design 00 H will make the DAC output 0 V and FF H will have 5 V, then to operate the motor at maximum speed we need a gain in the preamplification stage of: AV = 1 =.4 5 And as the same non-inverter amplifier was used then Rb=33[k ] and Ra =47 [k ]. Subject: Analog Electronic Circuits - Page 4 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), TESTING: AUDIO AMPLIFIER The following figure (Fig. 8) shows a FFT applied to the output. As we could clearly see by calculations and experimentations the gain of the non-inverter amplifier implemented with the opamp reduces our crossover distortion. V o A V i ± 0.7 / A After various tests with (Fig. 8) and without feedback (Fig. 7) (therefore with and without crossover distortion) we were able to appreciate the audible effect of distortion. By analyzing the signal in the frequency spectrum (FFT) we can clearly notice the distortion in a 1 KHz pure sinusoidal signal. Fig. 8: Corrected with the non-inverter amplifier.. With those simulations we can clearly see how the cross-over distortion is reduced with the feedback (Fig. 9). Please note that the graphs are semi-logarithmic. 9.0V 1.0V Fig. 7: No feedback 10mV 0Hz After connecting the crossover-distortion correction mechanism (Fig. 8) we can clearly notice how the unwanted harmonic distortion gets reduced. By comparison, the relationships of amplitudes from fundamental against the biggest harmonic. Without feedback With feedback Fundamental 58 db 5 db Biggest Harmonic 58 db 13 db V(RL:) 5KHz 10KHz 15KHz 0KHz Frequency Fig. 9: Crossover distortion (FFT). Here a simulation (Fig. 10) after implementing the correction clearly proves how the distortion is reduced below our window levels. 10V 1.0V The numbers speak alone; with the correction the biggest distortion is 177.8 times lower in amplitude than the fundamental. 100mV 0Hz V(Ra:) 0KHz 40KHz 60KHz 80KHz Frequency Fig. 10: With the correction mechanism (FFT). Subject: Analog Electronic Circuits - Page 5 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), For the output impedance measurements we built the circuit below (Fig. 11) and measured the current required. By using the same method described earlier we must build the circuit shown below (Fig. 14) to get the input impedance. Fig. 11: Measuring the impedance @ output. Fig. 14: Measuring the impedance @ input. So after implementing the circuit we took the following capture (Fig. 1). And with our simulation software we obtain the impedance curve (Fig. 15). 936 95 Fig. 1: Current (ma). Voltage of the input. Meaning that the output impedance is: 1.11[ ]. The next SPICE simulation (Fig. 13) clearly shows that the approximation is valid. 900 880 0Hz 100Hz V(V8:)/I(V8) 1.0KHz 10KHz Frequency Fig. 15: Input impedance. 11.705 11.7000 11.6975 11.6950 0Hz 100Hz V(Q3:e)/I(V5) 1.0KHz 10KHz Frequency Fig. 13: Output impedance. Subject: Analog Electronic Circuits - Page 6 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), For the efficiency measurements of our amplifier we looked at the supply (Fig. 16) power requirements (ignoring the opamp power consumption). Looking at Fig. 18, the average power on the load is given by P L= I L R L = 0.71 8 = 4.03[W ] Meaning our real world efficiency is = PL = 37.84 % PS Fig. 16: DC Power supply meters. Now to take a look to the power dissipated (Fig. 19) by each transistor. By looking at the voltage-current relationship we get P S = 15.355 = 10.65[W ] And according to the simulation in SPICE the average power is shown in the(fig. 17). Fig. 19: Base-collector voltage on the transistor and output current. 0W Meaning the average power dissipation from each transistor is P Q= 6.68.690 = 4.609[W ] -4W Which goes by with our simulation values (Fig. 0). -8W 8.0W -1W 0s 0.5ms AVG(W(V4)W(V7)) 1.0ms 1.5ms.0ms Time Fig. 17: Average power given by the DC supplies. And we can clearly see average power is approximately of 10 W once we look at the stabilized portion. Now we measure the average power on the load by measuring the I orms flowing at the load. 4.0W 0W -4.0W 0s AVG(W(Q1)) 0.5ms 1.0ms 1.5ms.0ms Time Fig. 0: Average power dissipation on each transistor. Fig. 18: Average load current on full load. Subject: Analog Electronic Circuits - Page 7 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), TOTAL HARMONIC DISTORTION = 5.31558E-0 PER- One important and meaningful value in our design would be the THD (total harmonic distortion) as the amplifier is expected to be used for audio. With help of SPICE software we show the values below. CENT ### Without feedback ################################# ----------------------------------------------------- OVERVIEW: AUDIO AMPLIFIER FOURIER COMPONENTS OF TRANSIENT RESPONSE V(R_RL) DC COMPONENT = HARMONIC NO ############################################### ####### Here we show a table (Table I) summing up all the important values after the tests and simulations were performed. -5.1718E-0 FREQUENCY (HZ) FOURIER NORMALIZED PHASE NORMALIZED COMPONENT COMPONENT (DEG) PHASE (DEG) Calculated Simulation Implemented Zin (Ω) 1,430 99 -- Zout (Ω) 1.01 11.70 1.11 1 1.000E03 7.977E00 1.000E00 1.799E0 0.000E00 PQAVG (W).85 3.9 4.6.000E03 6.586E-03 8.57E-04 9.719E01 -.65E0 PSAVG (W) 10.68 10.60 10.65 3 3.000E03.891E-01 3.64E-0-5.19E-01-5.401E0 4 4.000E03 4.931E-03 6.18E-04 8.704E01-6.34E0 Eff. 46.34 46.08 37.84 5 5.000E03 1.649E-01.067E-0-6.089E-01-8.999E0 6 6.000E03 4.170E-03 5.8E-04 8.454E01-9.947E0 7 7.000E03 1.10E-01 1.404E-0-8.467E-01-1.60E03 8 8.000E03 3.768E-03 4.74E-04 8.41E01-1.357E03 9 9.000E03 8.48E-0 1.034E-0-1.13E00-1.60E03 10 1.000E04 3.516E-03 4.408E-04 8.030E01-1.718E03 11 1.100E04 6.351E-0 7.961E-03-1.444E00-1.980E03 1 1.00E04 3.334E-03 4.180E-04 7.83E01 -.080E03 TOTAL HARMONIC DISTORTION = (%) THD Without feedback With feedback 4.59% 0.053% Table I. 4.593631E00 PER- CENT ### With feedback #################################### ----------------------------------------------------FOURIER COMPONENTS OF TRANSIENT RESPONSE V(R_RL) DC COMPONENT = HARMONIC NO.893855E-04 FREQUENCY (HZ) FOURIER NORMALIZED PHASE NORMALIZED COMPONENT COMPONENT (DEG) PHASE (DEG) 1 1.000E03 8.943E00 1.000E00 1.799E0 0.000E00.000E03.413E-03.698E-04-1.18E0-4.779E0 3 3.000E03 6.335E-04 7.083E-05 5.19E01-4.874E0 4 4.000E03 1.69E-03 1.419E-04-9.805E01-8.175E0 5 5.000E03 1.6E-03 1.411E-04 8.40E01-8.153E0 6 6.000E03 1.491E-03 1.667E-04-1.003E0-1.179E03 7 7.000E03 1.48E-03 1.395E-04 7.53E01-1.184E03 8 8.000E03 1.4E-03 1.590E-04-1.01E0-1.541E03 9 9.000E03 1.59E-03 1.408E-04 7.48E01-1.544E03 10 1.000E04 1.470E-03 1.643E-04-1.058E0-1.904E03 11 1.100E04 1.36E-03 1.38E-04 6.88E01-1.910E03 1 1.00E04 1.440E-03 1.610E-04-1.08E0 -.67E03 Subject: Analog Electronic Circuits - Page 8 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), RET IMPLEMENTING: MOTOR CONTROLLER We used the 8051 Microcontroller to send the digital data depending by up/down buttons. Implementing the following assembly code (Attachment 1) and connecting the schematic shown in Fig. 6. END ----------------------------------------------------After implementing it on the lab (Fig. 1) we did some basic testing to see if our system worked flawlessly. ### Assembly Code #################################### ----------------------------------------------------; DC Motor Controller for the 8051. ; Omar X. Avelar // Omar de la Mora // Diego I. Romero. ; 16:7 on 10/06/08 ; ------------------------------------------------------------------------------; Aliases ----------------------------------------------------------------------STEP_U EQU P.7 STEP_D EQU P.6 ; Inputs O_DATA EQU P0 ; Outputs DTIME EQU 41H ; Delay Parameter ; --------------------------------------------------------------------------------; Main -------------------------------------------------------------------------ORG 0 A,#7FH O_DATA,A KEYPRESS JMP RERUN ; Initialized at half the speed RERUN: ; Looks for a keypress ; Routines ---------------------------------------------------------------------; ------------------------------------------------------------------------------; ------------------------------------------------------------------------------KEYPRESS: DTIME,#00d DELAY ; Delay is now 80ms per " DELAY" DELAY DELAY DELAY DELAY R7,O_DATA JNB STEP_U,NOT_UP INC A JNB STEP_D,NOT_DWN DEC A ; Decrements speed O_DATA,A ; Outputs change ; Backup of the current value @ R7 NOT_UP: ; Increments speed NOT_DWN: RET ; ------------------------------------------------------------------------------; ------------------------------------------------------------------------------DELAY: DRSRT: R6,DTIME R,#00d DJNZ R,$ ; 400 us DJNZ R6,DRSRT ; Time Multiplier Subject: Analog Electronic Circuits - Page 9 of 11 Fig. 1: Implementation of the DC Motor controller
Instituto Tecnológico y de Estudios Superiores de Occidente (), TESTING: MOTOR CONTROLLER By analyzing the power supply current flow, and potential (Fig. ) we are able to calculate the power being drawn from them. Fig. : Power taken from the supplies with the DC motor at max speed. P S = 15 10x10 3 = 1.8[W ] Therefore the load of the motor is R L 15[ ] OVERVIEW: MOTOR CONTROLLER According to the electrical characteristics of our motor and the circuit (Page ) we can build up a comparative table (Table II). Measurements Nominal Power (w) 1.8 1.9 Impedance (Ω) 15 75 Table II. Subject: Analog Electronic Circuits - Page 10 of 11
Instituto Tecnológico y de Estudios Superiores de Occidente (), CONCLUSIONS A convenient design method for amplifier implementations is to have the preamplification (voltage in our case) stage separate from the power stage. For example our digital processing stage (which hardly uses energy in this situation) from the more energy hungry stage that was feeding the motor load. The feedback loop at the output/input had a considerable effect on our design as it clearly reduced the THD by removing the dead-zone in the amplification produced by our components switching. With all the types of different analog amplifiers topologies it is a wise choice to analyze the specifications required for the design such as efficiency and distortion. BIBLIOGRAPHY A.R. Hambley, Electronics: A Top-Down Approach to ComputerAided Circuit Design, Englewood Cliffs, NJ : Prentice Hall, 000. R.C. Jager, Microelectronic Circuit Design, New York, NY: McGraw Hill, 1997. Malvino, Albert, Electronic Principles, 6th Edition, McGraw Hill, 1999. A. S. Sedra and K. C Smith, Microelectronic Circuits. New York, NY: Oxford University Press, 003. Subject: Analog Electronic Circuits - Page 11 of 11