MALVINOElectronicPRINCIPLES

Chapter 11Power Amplifiers

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.

tICtICtICtICISATABCD

Types of couplingDirect (dc amplifier)Capacitive (ac amplifier)Untuned transformer (ac amplifier)Tuned transformer (narrowband ac amplifier)

CapacitiveDirectTransformerTunedtransformeracacacdc

+VCCRER2RCR1RLvinThis amplifier has two load lines.

0246810121416182468101214VCE in VoltsIC in mAIC(sat) =VCC RC + REThe dc load lineVCE(cutoff) = VCCThe ac load line has a higher slope.Q

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)

0246810121416182468101214VCE in VoltsIC in mAQ

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.

Push-pull Class B amplifiervinT1T2+VCCQ1Q2On the positive half-cycle of the input, Q1 is on.

vinT1T2+VCCQ1Q2On the negative half-cycle of the input, Q2 is on.

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.

+VCCRBRLvinLCfrfAAmaxClass C amplifier

VCCVCE IC dc load lineClass C amplifier dc and ac load lines

+VCCRBRLLCNegative clamping at the base causes the collector current to flow in brief pulses.tIC

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

Duty cycle and conduction angleD ==f = 180of = 90o

PDIdcThe effects of conductionangle in a Class C amplifier

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