* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Power Amplifier Circuits
Wireless power transfer wikipedia , lookup
Electrical ballast wikipedia , lookup
Power factor wikipedia , lookup
Solar micro-inverter wikipedia , lookup
Negative feedback wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Nominal impedance wikipedia , lookup
Electric power system wikipedia , lookup
Scattering parameters wikipedia , lookup
Electrical substation wikipedia , lookup
Power inverter wikipedia , lookup
Transformer wikipedia , lookup
Public address system wikipedia , lookup
Voltage optimisation wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Electrification wikipedia , lookup
Current source wikipedia , lookup
Mains electricity wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
History of electric power transmission wikipedia , lookup
Three-phase electric power wikipedia , lookup
Amtrak's 25 Hz traction power system wikipedia , lookup
Power electronics wikipedia , lookup
Wien bridge oscillator wikipedia , lookup
Power engineering wikipedia , lookup
Transformer types wikipedia , lookup
Resonant inductive coupling wikipedia , lookup
Alternating current wikipedia , lookup
Two-port network wikipedia , lookup
Opto-isolator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Zobel network wikipedia , lookup
Audio power wikipedia , lookup
Section F3: Power Amplifier Circuits - Class A Operation As mentioned in the previous section, our studies of BJT and FET amplifiers to date have been of the Class A designation since the output signal is an amplified version of the entire input. What we’re going to look at now are some of the useful circuit configurations for Class A power amplifiers, where the category is determined by the coupling mechanism. Please note that the configurations we will be discussing are by no means the only configurations available. Whatever strategy we take; remember that we want to maximize power delivered to the output. This involves a large current gain and impedance matching to the load at the output stage. There is some difficulty in this chapter with the orientation of the polarized capacitors. I have attempted to make appropriate changes, both in the notes and the assigned homework, but please be aware of this problem in case I miss something (but let me know if I do, ok?) Inductively Coupled Amplifier An example of an inductively coupled amplifier is presented to the right and is based on Figure 8.6a of your text. This schematic is our old friend the common emitter BJT amplifier with an inductor replacing the collector resistor RC. Recall that the effective load for a CE amplifier was RC||RL, and that the effective load was less than either RC or RL. Using a properly sized inductor instead of a resistor in the collector leg will allow us to increase the output voltage and, as we’ll see a little later, increase the efficiency of the amplifier circuit. By analyzing the expression for the impedance of an inductor Z L = Rcoil + jωL = Rcoil + j2πfL , we can see that the impedance consists of the internal resistance of the coil, Rcoil, and an inductive reactance term that is dependent upon the frequency of operation, ωL. At dc (ω=0), ZL=Rcoil, while at high frequencies the ωL term dominates and becomes very large. Harking back to circuit days, remember that we modeled an ideal inductor as a short circuit to dc and an open circuit to large frequencies. To make effective use of this configuration, we want to approximate the behavior of the ideal inductor as closely as possible. Therefore, for dc we will require Rcoil << Rload and Rcoil << RE , (Equation 8.2) and for the lowest input signal frequency ωL=2πfL (which will yield the minimum reactive impedance) ω L L >> Rload . (Equation 8.1) If the above conditions are met, we may approximate the ac and dc equivalent resistances as Rdc = Rcoil + RE ≅ RE and Rac = Z L || Rload ≅ Rload . Figure 8.6b, reproduced to the right, illustrates the load lines for the inductively coupled amplifier. Note that it is assumed in this figure (from the slopes of the load lines) that RE<< Rload. To observe the effect on the load lines of replacing RC with L, please review Section C8 and, in particular, Figure 4.17. Choosing the Q-point for maximum swing (as we have done so many times before) and using the approximations for ac and dc resistances above, we get I CQ = VCC VCC = . Rac + Rdc Rload + RE (Equations 8.3 & 8.4) By using the assumption implied in the load lines above, RE << Rload, the voltage drop across both Rcoil and RE may be considered negligible and we may make the approximation VCEQ ≈ VCC. If all this works out, the expression for ICQ may be further simplified as I CQ ≅ VCC V ≈ CE . Rload Rload (Equation 8.5, Modified) Now, realizing that the Q-point is in the center of the load line, we may project V’CC to be equal to 2VCC. This is actually pretty cool – the inductor stores energy in its magnetic field during the conducting cycle and essentially acts like a second VCC source in series with the dc power supply. So…by using an inductor in the amplifier circuitry, we have created an available voltage swing that is equivalent to doubling the supply voltage. The remainder of the design for this amplifier configuration follows the standard process developed for the common emitter amplifier in Section D2 with the appropriate modifications arising from the assumptions as to the inductive impedance; i.e., |ZL|≈0 for dc and |ZL|>>Rload for all input frequencies. Specifically, Rin = RB || rπ AV = − Rload re Rout = Rcoil Ai = − RB RB β . + re To define the conversion efficiency (η), we develop the ratio of ac power delivered to the load to the dc power supplied by the source (please review Section C7 for our discussion of Power Considerations from last semester). Keeping our assumption that RE << Rload, so that VCEQ ≈ VCC and ICQ ≈ VCC/Rload, we may derive the power supplied by the voltage source as follows (your author skipped a few steps) Pin (dc) = 2 VCC 2 (Rcoil + RE ) + VCEQ I CQ + I CQ R1 + R2 2 ⎛V VCC = + (VCC )⎜⎜ CC R1 + R2 ⎝ Rload ⎛ 1 1 2 ⎜ = VCC ⎜R + R + R load 2 ⎝ 1 2 ⎞ ⎟⎟ (Rcoil + RE ) . ⎠ R + RE ⎞ ⎟ + coil 2 Rload ⎟⎠ ⎞ ⎛ VCC ⎟⎟ + ⎜⎜ ⎠ ⎝ Rload So…recalling that R1 and R2 are pretty huge when compared to the other resistances and we’ve defined Rcoil and RE to be much less than Rload, the second term in the parentheses is much larger than the other two. Therefore, Pin (dc) = 2 VCC Rload , (Equation 8.6) while the ac power delivered to the load, assuming the current is sinusoidal with a maximum amplitude Iloadmax (and the Q-point is in the center of the ac load line so Iloadmax=ICQ), is Pout (ac) = Pload = 2 VCC 1 2 1 2 . I load max Rload = I CQ Rload = 2 2 2Rload (Equation 8.7) Note that the expressions above are in keeping with the CE derivations of Section C7. Specifically, if RE is negligible, then VCEQ ≈ VCC and the average power dissipated will range between Pout(ac) and Pin(dc) as defined in Equations 8.6 and 8.7. It is worth noting that the true conversion efficiency must take into account the power dissipated in the bias circuitry which would effectively increase Pin(dc). Through our assumptions, we have neglected these losses and the conversion efficiency below is an absolute maximum. Using Equation 4.37, we may define the conversion efficiency (in percent) as 2 Pout (ac) VCC /(2Rload ) η(%) = * 100 = * 100 = 50% . 2 Pin (dc ) VCC / Rload (Equation 8.8) Your author states that if the inductor is replaced with a resistor, that the maximum efficiency of the amplifier will be 25%. This is actually somewhat optimistic. He has assumed that the load resistance and collector resistance are equal so that the current is divided equally. This is okay - it will halve the maximum power to the load and therefore halve the amplifier conversion efficiency, all else constant. The problem with this approach is that he does not consider the effect on the dc input power (even if we can still neglect RE, we must include the losses in RC) and the effect on the load lines (VCEQ will no longer be VCC, etc.). So… take it for what it’s worth… but 25% is probably pretty large! Transformer Coupled Power Amplifier The other Class A power amplifier configuration we’re going to be considering is the transformer coupled circuit shown to the right (a modified version of Figure 8.7 in your text). This figure illustrates an EF (CC) amplifier that uses a transformer to couple the load for ac operation. Note that the ac and dc resistances are seen at the primary side of the transformer (please see Section D7 for a review on transformer coupled amplifiers). For Rdc, all we have is the resistance of the primary coil. This is usually quite small and will be considered negligible for our purposes, yielding Rdc = R primary ≅ 0 . For ac operation, the resistance seen at the emitter is the reflected load impedance. Recall that the relationship between a resistance seen at the primary to a resistance at the secondary is determined by the turns ratio: ⎛N R1 = R2 ⎜⎜ 1 ⎝ N2 ⎞ ⎟⎟ ⎠ 2 , and a in the above figure is equal to N1/N2. We may therefore express Rac as Rac = a 2 RL . (Equation 8.9) The load lines for the transformer-coupled amplifier are illustrated in the figure to the right (a modified version of Figure 8.7b). This representation is extremely similar to that for the inductively coupled amplifier and, except for the value of the ac resistance, will proceed in exactly the same manner. Designing for maximum output swing, we place the Q-point in the center of the load line by I CQ = VCC V ≅ 2CC . Rac + Rdc a RL (Equation 8.10) The remainder of the amplifier design follows the process developed for emitter follower (common collector) amplifiers in Section D4. Following the procedure discussed for the inductively coupled amplifier above, the maximum power conversion efficiency of the transformer coupled power amplifier is also 50%. Note that even though the EF configuration has a voltage gain near unity, the turns ratio of the transformer determines the voltage gain to the load through ⎛N v 2 = v 1 ⎜⎜ 2 ⎝ N1 ⎞ v1 ⎟⎟ = a ⎠ . Finally, note that the transformer-coupled amplifier offers a significant advantage in that the transformer can match the load impedance to the amplifier output impedance for maximum power transfer by choosing the proper turns ratio.