Electronic PRINCIPLES MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION

Chapter 12 Power Amplifiers

Topics Covered in Chapter 12 Amplifier terms Two load lines Class A operation Class B operation Class B push-pull emitter follower

Topics Covered in Chapter 12 (Continued) Biasing Class B/AB amplifiers Class B/AB driver Class C operation Class C formulas Transistor power rating

Classes of operation Class 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 AB amplifiers have collector current between 180° and 360° Class C means that current flows for less than 180o of the ac cycle.

IC IC A B t t IC IC ISAT C D t t

AB Class AB is biased near but not at cutoff

Types of coupling Direct (dc amplifier) Capacitive (ac amplifier) Untuned transformer (wideband ac amplifier) Tuned transformer (narrowband ac amplifier)

dc ac Capacitive Direct ac ac Tuned transformer Transformer

Load lines Every amplifier has a dc and an ac equivalent and therefore has two load lines The Q point location for a small signal amplifier is not critical The Q point for a large signal amplifier must be at the center of the ac load line

This amplifier has two load lines. +VCC RE R2 RC R1 RL vin vout

VCC IC(sat) = The dc load line VCE(cutoff) = VCC RC + RE 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 Q 4 20 mA 2 0 mA 2 4 6 8 10 12 14 16 18 VCE in Volts The ac load line has a steeper slope.

AC load line

Large Signals When 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 = 2MP MP = ICQrc or VCEQ (whichever is smaller)

Class A operation Power gain equals ac output power divided by ac input power The power rating of a transistor must be greater than the quiescent power dissipation Efficiency of the amplifier equals ac output power divided by dc input power times 100 percent

100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA Q 6 4 20 mA 2 0 mA 2 4 6 8 10 12 14 16 18 VCE in Volts pout = vpp2 8RL pout(max) = MPP2 8RL

Efficiency The dc power supplied to an amplifier is Pdc = VCCIdc Efficiency = 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.

Class B Collector current flows for 180° of the ac cycle Q point is located at cutoff on both the dc and ac load lines An advantage is lower current drain and higher stage efficiency

Push-pull Class B amplifier Q1 T2 T1 +VCC vin Q2 On the positive half-cycle of the input, Q1 is on.

Q1 T2 T1 +VCC vin Q2 On the negative half-cycle of the input, Q2 is on.

Class B push-pull operation Each 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 and can be eliminated by using a push-pull emitter follower.

Class B push-pull emitter follower Complementary npn and pnp transistors are used

Class AB The transistor is biased slightly above cutoff The conduction angle is slightly greater than 180° Crossover distortion is reduced

Biasing Class B/AB amplifiers Crossover distortion is reduced by setting the Q point slightly above cutoff in Class AB With voltage-divider bias (VDB), the Q point is unstable and may result in thermal runaway Diode bias is preferred

Class B/AB driver Direct rather than capacitive coupling can be used The driver’s collector current establishes the quiescent current through complementary diodes

Class AB Q point

Class C operation Most Class C amplifiers are tuned RF amplifiers The input signal is negatively clamped which produces narrow collector current pulses The tank circuit is tuned to the fundamental frequency

AV Class C amplifier AVmax 2p LC 1 fr = +VCC C L f fr RL RB vin

Class C amplifier dc and ac load lines IC dc load line VCC rc Q VCE VCC

Negative clamping at the base causes the collector current to flow in brief pulses. IC +VCC C L t -2 VP + VP - VP RL RB

Class C formulas BW = f2 - f1 = fr/Q QL = 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 angle f 360o W T D = = f = 180o f = 90o

The effects of conduction angle in a Class C amplifier PD f The effects of conduction angle in a Class C amplifier MPP2 40rc 180o Idc f h 180o 100% f 180o

Transistor power rating The power rating of a transistor decreases as temperature increases Data sheets list derating factors or show graphs Heat sinks can remove heat more rapidly and produce a higher power rating

Derating factor Power devices generate heat PD(max) rating good up to 25 oC Decrease in rating = DP Derating factor = D (W/oC) TA = ambient temperature (oC) DP = D(TA - 25 oC) PD(safe) = PD(max) - DP