FIGURE 28.8 Complementary symmetry power amplifier. Source: C J. Savant, M. Roden, and G. Carpenter, Electroni Design, Circuits and Systems, 2nd ed, Redwood City, Calif. Benjamin-Cummings, 1991, P. 248. with permission.) Power Amplifiers Emitter followers can be used as power amplifiers. Even though they have less than unity voltage gain they can rovide high current gain. Using the standard linear EF amplifier for a maximum output voltage swing provides less than 25% efficiency(ratio of power in to power out). The dc current carrying the ac signal is where the loss of efficiency occurs. To avoid this power loss, the Q point is placed at Ico equal to zero, thus using the majority of the power for the output signal. This allows the efficiency to increase to as much as 70%. Full signal amplification requires one transistor to amplify the positive portion of the input signal and another transistor to amplify the negative portion of the input signal. In the past, this was referred to as push-pull operation. A better system is to use an NPn transistor for the positive part of the input signal and a PnP transistor for the negative part. This type of operation is referred to as Class B complementary symmetry operation(Fig. 28. 8) In Fig. 28.8, the dc voltage drop across R, provides the voltage to bias the transistor at cutoff. Because these are power transistors, the temperature will change based on the amount of power the transistor is absorbing This means the base-emitter junction voltage will have to change to keep Ico=0. To compensate for this change in temperature, the R, resistors are replaced with diodes or transistors connected as diodes with the same turn on characteristics as the power transistors. This type of configuration is referred to as the complementary compensated(CSDC)amplifier and is shown in Fig. 28.9. To avoid crossover distortion, small resistors can be placed in series with the diodes so that Ico can be raised slightly above zero to get increased amplification in the cutoff region. Another problem that needs to be addressed is the possibility of thermal runaway. This can be easily solved by placing small resistors in series with the emitters of the power transistors. For example, if the load is an 8-Q2 speaker, the resistors should not be greater than 0.47 2 to avoid output To design this type of amplifier, the dc current in the bias circuit must be large enough so that the diodes remain on during the entire input signal. This requires the de diode current to be equal to or larger than the zero to peak current of the input signal,or D≥l(0 to peak) (VcC2-VBE/R2=IB(O ) V(O to peak)/R
© 2000 by CRC Press LLC Power Amplifiers Emitter followers can be used as power amplifiers. Even though they have less than unity voltage gain they can provide high current gain. Using the standard linear EF amplifier for a maximum output voltage swing provides less than 25% efficiency (ratio of power in to power out). The dc current carrying the ac signal is where the loss of efficiency occurs. To avoid this power loss, the Q point is placed at ICQ equal to zero, thus using the majority of the power for the output signal. This allows the efficiency to increase to as much as 70%. Full signal amplification requires one transistor to amplify the positive portion of the input signal and another transistor to amplify the negative portion of the input signal. In the past, this was referred to as push-pull operation. A better system is to use an NPN transistor for the positive part of the input signal and a PNP transistor for the negative part. This type of operation is referred to as Class B complementary symmetry operation (Fig. 28.8). In Fig. 28.8, the dc voltage drop across R1 provides the voltage to bias the transistor at cutoff. Because these are power transistors, the temperature will change based on the amount of power the transistor is absorbing. This means the base-emitter junction voltage will have to change to keep ICQ = 0. To compensate for this change in temperature, the R1 resistors are replaced with diodes or transistors connected as diodes with the same turnon characteristics as the power transistors. This type of configuration is referred to as the complementary symmetry diode compensated (CSDC) amplifier and is shown in Fig. 28.9. To avoid crossover distortion, small resistors can be placed in series with the diodes so that ICQ can be raised slightly above zero to get increased amplification in the cutoff region. Another problem that needs to be addressed is the possibility of thermal runaway. This can be easily solved by placing small resistors in series with the emitters of the power transistors. For example, if the load is an 8-W speaker, the resistors should not be greater than 0.47 W to avoid output signal loss. To design this type of amplifier, the dc current in the bias circuit must be large enough so that the diodes remain on during the entire input signal. This requires the dc diode current to be equal to or larger than the zero to peak current of the input signal, or ID ³ Iac (0 to peak) (VCC /2 – VBE)/R2 = IB (0 to peak) + VL (0 to peak)/R2 FIGURE 28.8 Complementary symmetry power amplifier. (Source: C.J. Savant, M. Roden, and G. Carpenter, Electronic Design, Circuits and Systems, 2nd ed., Redwood City, Calif.: Benjamin-Cummings, 1991, p. 248. With permission.)
VRn< R2 D, C? D2 RL R2 FIGURE 28.9 Complimentary symmetry diode compensated power amplifier. Source: C.J. Savant, M. Roden, and G Carpenter, Electronic Design, Circuits and Systems, 2nd ed, Redwood City, Calif. Benjamin-Cummings, 1991, P. 251. With Nonconducting RI R R2 BZL -B FIGURE 28.10 AC equivalent circuit of the CSDC amplifier. Source: C J. Savant, M. Roden, and G. Carpenter, Electronic Design, Circuits and Systems, 2nd ed, Redwood City, Calif. Benjamin-Cummings, 1991, P 255. With permission. When designing to a specific power, both Ig and Vi can be determined. This allows the selection of the value of R, and the equivalent circuit shown in Fig. 28. 10 can be developed. Using this equivalent circuit, both the input resistance and the current gain can be shown. R is the forward resistance of the diodes Rin=(r+ R2)//[R+(r2//Beta ruI P。= ICma r2/2 The power rating of the transistors to be used in this circuit should be greater than d(pIrI) C1=1/(2Pif。wR2) C2=10/2Pif。(Rn+R) where R, is the output impedance of the previous stage and fow is the desired low frequency cutoff of the amplifier Related Topics 24.1 Junction Field-Effect Transistors. 30.1 Power Semiconductor Devices c 2000 by CRC Press LLC
© 2000 by CRC Press LLC When designing to a specific power, both IB and VL can be determined. This allows the selection of the value of R2 and the equivalent circuit shown in Fig. 28.10 can be developed. Using this equivalent circuit, both the input resistance and the current gain can be shown. Rf is the forward resistance of the diodes. Rin = (Rf + R2)//[Rf + (R2 //Beta RL)] Po = ICmaxRL/2 The power rating of the transistors to be used in this circuit should be greater than Prating = V2 C C/(4Pi2RL) C1 = 1/(2Pi flowRL ) C2 = 10/[2Pi flow(Rin + Ri )] where Ri is the output impedance of the previous stage and f low is the desired low frequency cutoff of the amplifier. Related Topics 24.1 Junction Field-Effect Transistors • 30.1 Power Semiconductor Devices FIGURE 28.9 Complimentary symmetry diode compensated power amplifier. (Source: C.J. Savant, M. Roden, and G. Carpenter, Electronic Design, Circuits and Systems, 2nd ed., Redwood City, Calif.: Benjamin-Cummings, 1991, p. 251. With permission.) FIGURE 28.10 AC equivalent circuit of the CSDC amplifier. (Source: C.J. Savant, M. Roden, and G. Carpenter, Electronic Design, Circuits and Systems, 2nd ed., Redwood City, Calif.: Benjamin-Cummings, 1991, p. 255. With permission.)