Exercise 2: AC Voltage and Power Gains

Transcription

1 Exercise 2: AC Voltage and Power Gains When you have completed this exercise, you will be able to determine voltage and power gains by using oscilloscope. The ac operation schematic for the COMPLEMENTARY POWER AMPLIFIER on the TRANSISTOR POWER AMPLIFIERS circuit board is shown here. A sine wave generator (GEN) provides the ac input signal (V i ), which is measured at the junction of a. C1 and R1. b. R3 and R4. The output signal (V o ) is measured at the junction of C2-C3 and load resistor R Festo Didactic P0

2 The NPN Q1 and PNP Q2 transistors have complementary symmetry: the polarity of an input signal needed for one transistor is opposite that needed for the other. the need for a. dc biasing of the transistors. b. an input signal phase splitter. Both transistors are biased to have Q-points that are very close to the cutoff points. A slight increase in V CE, caused by the input signal, puts the transistor at the a. cutoff point. b. saturation point. Each transistor is cut off during opposite half-cycles of the input signal (V i ). While one transistor is conducting, the other is at cutoff. Festo Didactic P0 191

3 The collector current (I c ) increases rapidly as the input signal decreases V CE, causing the total circuit current to a. decrease with the input amplitude. b. increase with the input signal. As the ac input signal becomes positive, NPN transitor Q1 conducts current through load resistor R8, and PNP transitor Q2 cuts off. When the ac signal (V i ) goes negative, Q2 conducts current through R8, and Q1 a. goes into saturation. b. cuts off. 192 Festo Didactic P0

4 The transistor emitter signals (outputs) are combined at the junction of emitter resistors R6 and R7. The positive half-cycle of the output signal is produced by a. Q1. b. Q2. The negative half-cycle is produced by a. Q1. b. Q2. the cycle (class AB operation). a. nondistorted signal. b. distorted signal. Festo Didactic P0 193

5 Power Calculations The input voltage, current, and power are measured at the junction of C1 and R1. The output signal (V o ) and output power (P o ) are measured across load resistor(s) a. R6 and R7. b. R8. When calculating power, use rms values of voltage and current. V rms = (V pk-pk Calculate input power (P i ) and output power (P o ) by using these equations: P i = V i(rms) I i(rms) where I i(rms) = V R1(rms) P o = V o(rms) I o(rms) where I o(rms) = V o(rms) The voltage, current, and power gains are determined by the ratios of the output to input values. The power gain is determined by the ratio of output to input power (A p = P o i ) or is the product of the voltage and current gains (A p = A v A i ). 194 Festo Didactic P0

6 The common collector circuit has a low output impedance; therefore, no transformer is needed to match the output to the low output impedance load resistor (R8 at 8.2 ). Is the output signal (V o ) in or out of phase with the input signal (V i )? a. in phase b. out of phase Two output capacitors (C2 and C3) are connected in parallel because the output coupling capacitance must be fairly large for a good low frequency response. Locate the COMPLEMENTARY POWER AMPLIFIER circuit block on the TRANSISTOR POWER AMPLIFIERS circuit board. Festo Didactic P0 195

7 Measure the supply voltage (V A ) with reference to ground. V A = Vdc (Recall Value 1) While observing the signal on channel 1 of the oscilloscope, adjust the sine wave generator for a 1 khz, 3.00 V pk-pk ac input signal (V i ) at the junction of C1 and R1. Measure the ac output signal (V o ) at the junction of C2-C3 and R8. V o = V pk-pk (Recall Value 2) 196 Festo Didactic P0

8 Calculate the voltage gain (A v ). A v = V o i A v = (Step 4, Recall Value 2 A v = (Recall Value 3) What is the phase relationship of V o to V i? a. in phase b. out of phase Observe the Q1 and Q2 emitter signals to determine the class of operation of the transistors. Use the oscilloscope ADD-INVERT method to observe the emitter signals. Festo Didactic P0 197

9 Set the oscilloscope to ADD and the channel 2 polarity to INVERT. Connect the channel 2 oscilloscope probe to the junction of emitter resistors R6 and R7. Connect the channel 1 probe to the emitter terminal of Q1, the NPN transistor, and observe the Q1 emitter signal. Why does the Q1 emitter signal have only a positive half-cycle? a. Q1 is an NPN transistor and is biased near the cutoff point. b. Q1 is a PNP transistor and is biased in the center of the load line. Does the Q1 emitter signal occur for slightly more than 180º? a. yes b. no 198 Festo Didactic P0

10 Connect the channel 1 probe to the emitter terminal of Q2, the PNP transistor, and observe the signal. Why does the Q2 emitter signal have only a negative half-cycle? a. Q2 is a PNP transistor and is biased near cutoff. b. Q2 is an NPN transistor and is biased in the center of the load line. Does the Q2 emitter signal occur for slightly more than 180º? a. yes b. no a. A b. AB c. B Festo Didactic P0 199

11 Input Power When you calculate power gains, convert peak-to-peak voltages to rms voltages to calculate average current and power. V rms = (V pk-pk V i is set at 3.0 V pk-pk. Calculate the rms input voltage (V i(rms) ). V i(rms) = (V pk-pk V i(rms) V i(rms) = V rms (Recall Value 4) The input power is the product of V i(rms) and I i(rms) at what point (junction)? a. R3 and R4 b. C1 and R1 200 Festo Didactic P0

12 Connect the channel 1 oscilloscope probe at the junction of C1 and R1. Connect the channel 2 oscilloscope probe at the junction of R1, R3, and R4. Using the oscilloscope ADD-INVERT method, measure the peak-to-peak voltage across R1 (V R1(pk-pk) ). V R1(pk-pk) = V pk-pk (Recall Value 5) Festo Didactic P0 201

13 Calculate the rms value of V R1(pk-pk). V R1(rms) = (V R1(pk-pk) V R1(rms) = ( [Step 19, Recall Value 5 V R1(rms) = V rms (Recall Value 6) i(rms) ). I i(rms) = V R1(rms) I i(rms) = ( V rms [Step 20, Recall Value 6 I i(rms) = ma rms (Recall Value 7) Calculate the input power. P i = V i(rms) I i(rms) P i = V rms (Step 20, Recall Value 4) ma rms (Step 21, Recall Value 7) P i = mw (Recall Value 8) 202 Festo Didactic P0

14 You measured V o to be V pk-pk (Step 4, Recall Value 2). Calculate the rms value of the peak-to-peak output voltage (V o ). V o(rms) = (V pk-pk V o(rms) = ( [Step 4, Recall Value 2 V o(rms) = V rms (Recall Value 9) o(rms) ). I o(rms) = V o(rms) I o(rms) =( V rms [Step 23, Recall Value 9 I o(rms) = ma rms (Recall Value 10) Calculate the output power (P o ). P o = V o(rms) I o(rms) P o = ( V rms [Step 23, Recall Value 9]) ( ma [Step 24, Recall Value 10]) P o = mw (Recall Value 11) Festo Didactic P0 203

15 Power Gain Calculate the power gain (A p ). A p = P o i A p =( mw [Step 25, Recall Value 11 mw [Step 22, Recall Value 8]) Current Gain A p = mw (Recall Value 12) Calculate the current gain (A i ). A i = I o(rms) i(rms) A i = ( ma [Step 24, Recall Value 10 ma [Step 21, Recall Value 7]) A i = (Recall Value 13) Is the current gain (A i at [Step 27, Recall Value 13]) considerably greater than the voltage gain (A v [Step 5, Recall Value 3])? a. yes b. no If necessary, adjust the input signal to 3.0 V pk-pk. 204 Festo Didactic P0

16 Place CM switch 13 in the ON position to increase the value of load resistor R8 from 8.2 to 18. Measure V o. V o = V pk-pk (Recall Value 14) You measured V o to be V pk-pk (Step 30, Recall Value 14). Calculate the rms value (V o(rms) ) of the peak-to-peak output voltage (V o ). V o(rms) = (V pk-pk V o(rms) = ( [Step 30, Recall Value 14 V o(rms) = V rms (Recall Value 15) Calculate the output power (P o = V o(rms)2 P o =( [Step 31, Recall Value 15]) 2 P o = mw (Recall Value 16) Festo Didactic P0 205

17 Compare your calculated output powers with load resistor R8 at 8.2 and at 18 when CM switch 13 is in the ON position. R8 P o 8.2 (Step 25, Recall Value 11) 18 (Step 32, Recall Value 16) Did the output power increase or decrease with an increase in load resistance? a. increase b. decrease Make sure all CMs are cleared (turned off) before proceeding to the next section. NPN transistor Q1 conducts during the positive half-cycle of the input signal. PNP transistor Q2 conducts during the negative half-cycle of the input signal. crossover distortion. The output signal is in phase with the input signal. The high power gain is the product of a very high current gain and a voltage gain that is less than 1.0. Increasing the value of the load resistor increases the output power. 206 Festo Didactic P0

18 1. Locate the COMPLEMENTARY POWER AMPLIFIER circuit block, and connect the circuit shown. Adjust V i for a 1 khz, 3.0 V pk-pk sine wave. Place CM switch 6 in the ON position to change the value of R3 from 1 k to 100. Observe the output signal (V o ). The crossover distortion of the output signal is caused by the a. transistors being biased at cutoff when the ac input signal is near 0 V pk-pk. b. transistors operating at saturation. c. Q-point being in the middle of the dc load line. d. input impedance being decreased. Festo Didactic P0 207

19 2. a. low current gain and a high voltage gain. b. a very high current gain and a voltage gain that is slightly above 1.0. c. a very high current gain and a voltage gain that is less than 1.0. d. no input or output transformers. 3. The output signal (V o a. usually distorted at the peak. b. usually distorted at the valley. c. in phase with the input signal. d. out of phase with the input signal. 4. a. the transistors are biased to amplify in-phase signals. b. the circuit has two matched NPN and PNP transistors connected in series. c. one signal is inverted by the voltage divider circuit. d. the output signal is in phase with the input signal. 5. load because a. b. there is no input transformer. c. d. Make sure all CMs are cleared (turned off) before proceeding to the next section. 208 Festo Didactic P0