Download - Electronic principles, powe amplifiers
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MALVINOElectronicPRINCIPLES
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Chapter 11Power Amplifiers
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Classes of operationClass A means that the transistor collector current flows for 360o of the ac cycle.Class B amplifiers have collector current for 180o of the ac cycle.Class C means that current flows for less than 180o of the ac cycle.Class D amplifiers switch on and off and spend no time in the linear mode.
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tICtICtICtICISATABCD
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Types of couplingDirect (dc amplifier)Capacitive (ac amplifier)Untuned transformer (ac amplifier)Tuned transformer (narrowband ac amplifier)
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CapacitiveDirectTransformerTunedtransformeracacacdc
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+VCCRER2RCR1RLvinThis amplifier has two load lines.
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0246810121416182468101214VCE in VoltsIC in mAIC(sat) =VCC RC + REThe dc load lineVCE(cutoff) = VCCThe ac load line has a higher slope.Q
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Large SignalsWhen the Q point is at the center of the dc load line, the signal cannot use all of the ac load line without clipping.MPP < VCC (max. peak-to-peak output)MPP = 2MPMP = ICQrc or VCEQ (whichever is smaller)
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0246810121416182468101214VCE in VoltsIC in mAQ
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EfficiencyThe dc power supplied to an amplifier is Pdc = VCCIdcEfficiency = h = pout/Pdc x 100%The maximum efficiency for Class A amplifiers with a dc collector resistance and a separate load resistance is 25%.Class A is usually not acceptable when watts of power are required.
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Push-pull Class B amplifiervinT1T2+VCCQ1Q2On the positive half-cycle of the input, Q1 is on.
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vinT1T2+VCCQ1Q2On the negative half-cycle of the input, Q2 is on.
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Class B operationEach transistor conducts for half a cycle.There is no bias so each transistor is at cutoff with no input signal.The maximum efficiency is 78.5 percent.Transformers are bulky and expensive.The transformers can be eliminated.
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+VCCRBRLvinLCfrfAAmaxClass C amplifier
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VCCVCE IC dc load lineClass C amplifier dc and ac load lines
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+VCCRBRLLCNegative clamping at the base causes the collector current to flow in brief pulses.tIC
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Class C formulasBW = f2 - f1 = fr/QQL = XL/RS (inductor only)RP = QLXL (equiv. parallel resistance)rc = RP RL (ac resistance)Q = rc/XL (overall circuit)PD = MPP2/40rc
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Duty cycle and conduction angleD ==f = 180of = 90o
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PDIdcThe effects of conductionangle in a Class C amplifier
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Derating factorPower devices generate heatPD(max) rating good up to 25 oCDecrease in rating = DPDerating factor = D (W/oC)TA = ambient temperature (oC) DP = D(TA - 25 oC)PD(safe) = PD(max) - DP