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POWER AMPLIFIER CHAPTER 4 1 Outcome of Chapter 4 Ability to perform simple DESIGN and EVALUATE A, B and AB classes of BJT and FET amplifiers, in terms of their frequency response, equivalent circuit, termal management (power dissipation) and gain. 2 Outline Introduction Concept of Power Amplifier Power Amplifier Classification BJTs/ MOSFETs Power Amplifier Class A Power Amplifier 3 Introduction Power amplifiers are used to deliver a relatively high amount of 4 power, usually to a low resistance load. Typical load values range from 300W (for transmission antennas) to 8W (for audio speaker). Although these load values do not cover every possibility, they do illustrate the fact that power amplifiers usually drive lowresistance loads. Typical output power rating of a power amplifier will be 1W or higher. Ideal power amplifier will deliver 100% of the power it draws from the supply to load. In practice, this can never occur. The reason for this is the fact that the components in the amplifier will all dissipate some of the power that is being drawn form the supply. Concept of Power Amplifier Provide sufficient power to an output load to drive other power device. To deliver a large current to a small load resistance e.g. audio speaker; To deliver a large voltage to a large load resistance e.g. switching power supply; To provide a low output resistance in order to avoid loss of gain and to maintain linearity (to minimize harmonic distortion) To deliver power to the load efficiently 5 Power Amplifier Power Dissipation VCC The total amount of power being dissipated by the amplifier, Ptot , is I CC I1 Ptot = P1 + P2 + PC + PT + PE I CQ P1 = I12R1 The difference between this total value and the total power being drawn from the supply is the power that P2 = I22R2 actually goes to the load – i.e. output power. R1 PC = I2CQR C PT = I2TQ R T I EQ R2 I2 6 RC RE PE = I2EQ R E Power Amplifier Efficiency A figure of merit for the power amplifier is its efficiency, . Efficiency ( ) of an amplifier is defined as the ratio of ac output power (power delivered to load) to dc input power . By formula : ac output power P (ac) 100% o 100% dc input power Pi (dc) As we will see, certain amplifier configurations have much higher efficiency ratings than others. This is primary consideration when deciding which type of power amplifier to use for a specific application. 7 Power Amplifiers Classification Class A - The transistor conducts during the whole cycle of sinusoidal input signal 8 Class AB - The transistor conducts for slightly more than half a cycle of input signal Class B - The transistor conducts during one-half cycle of input signal Class C - The transistor conducts for less than half a cycle of input signal Efficiency Ratings The maximum theoretical efficiency ratings of class-A, B, and C amplifiers are: 9 Amplifier Maximum Theoretical Efficiency, max Class A 25% Class B 78.5% Class C 99% BJT Power Amplifier Comparison of the characteristics and maximum ratings of a small-signal and power BJT Small-signal BJT (2N2222A) Power BJT (2N3055) Power BJT (2N6078) VCE (max) (V) 40 60 250 IC (max) (A) 0.8 15 7 PD (max) (W) 1.2 115 45 35 – 100 5 – 20 12 – 70 300 0.8 1 Parameter fT (MHz) 10 Typical dc beta characteristics (hFE versus Ic) for 2N3055 11 BJTs Power Amplifier Current gain is smaller in power amplifier BJT. The gain depends on IC and temperature may be related to the following: 12 maximum current that connecting wires can handle at which current gain falls below a stated value current which leads to maximum power dissipation. maximum voltage limitation associated with avalanche breakdown in reverse-biased collector-base junction. second breakdown in BJT operating at high voltage and current. VCE(sus) =115 volt at which these curve merge and the minimum voltage necessary to sustain the transistor in breakdown. The breakdown voltage, VCE0 ~130 volt when the base terminal is open circuited, IB=0 13 Instantaneous power dissipation pQ vCE iC vBE iB vCE iC The average power over one cycle T 1 PQ T 0 vCE iC dt The maximum rated power, PT VCE I C 14 MOSFETs Power Amplifier Characteristic of two power MOSFETs Power MOSFET 2N6757 Power MOSFET 2N6792 150 400 ID (max) (A) (at T = 25C) 8 2 PD (max) (W) 75 20 Parameter VDS (max) (V) 15 Performance Characteristic of MOSFETs Power Amplifier Faster switching times no second breakdown. Stable gain and response over wide temperature range. 16 Class A Amplifier vin Av vout output waveform same shape input waveform + phase shift. The collector current is nonzero 100% of the time. inefficient, since even with zero input signal, ICQ is nonzero (i.e. transistor dissipates power in the rest, or quiescent, condition) 17 Basic Operation Common-emitter (voltage-divider) configuration (RC-coupled amplifier) +VCC I CC I1 I CQ R1 RC RL v in R2 RE 18 Typical Characteristic Curves for Class A Operation Configuration : No inductor @ transformer are used (a) Common-emitter amplifier, (b) dc load line (the Q point is at centre of the load line) (c) instantaneous power dissipation versus time in the transistor 19 DC Input Power +VCC The total dc power, Pi(dc) , that an amplifier draws from the power supply : I CC I1 I CQ R1 RC Pi (dc) VCC I CC RL I CC I CQ I 1 I CC I CQ ( I CQ I 1 ) v in R2 RE Pi ( dc ) VCC I CQ Note that this equation is valid for most amplifier power analyses. We can rewrite for the above equation for the ideal amplifier as Pi (dc) 2VCEQ I CQ 20 AC Output Power AC output (or load) power, Po(ac) 2 Po (ac) ic ( rms) vo ( rms) ic vo vo ( rms) RL Above equations can be used to calculate the maximum possible value of ac load power. HOW?? vce vin rC RC//RL R1//R2 Disadvantage of using class-A amplifiers is the fact that their efficiency ratings are so low, max 25% . Why?? A majority of the power that is drawn from the supply by a class-A amplifier is used up by the amplifier itself. 21 IC(sat) = VCC/(RC+RE) IC(sat) = ICQ + (VCEQ/rC) DC Load Line ac load line IC IC (mA) VCE(off) = VCC VCE(off) = VCEQ + ICQrC VCE VPP2 VCEQ I CQ 1 Po ( ac) VCEQ I CQ 8 RL 2 2 2 ac load line IC VCE Q - point dc load line VCE 22 Po ( ac ) Pi ( dc ) 1 VCEQ I CQ 2 100% 100% 25% 2VCEQ I CQ Limitation 23 Example +VCC = 20V Calculate the input power [Pi(dc)], output power [Po(ac)], and efficiency [] of the amplifier circuit for an input voltage that results in a base current of 10mA peak. VCC VBE 20V 0.7V 19.3mA RB 1k ICQ I B 25(19.3mA) 482.5mA 0.48 A IBQ VCEQ VCC ICRC 20V (0.48 A)( 20) 10.4V V 20V I c ( sat) CC 1000mA 1A RC 20 VCE ( cu to ff ) VCC 20V IC ( p ea k) Ib ( p ea k) 25(10mA peak ) 250mA peak Po ( ac) Pi ( dc) 24 I C2 ( peak ) 250 10 A) 3 2 RC (20) 0.625W 2 2 VCC I CQ (20V )(0.48 A) 9.6W Po ( ac) Pi ( dc) 100% 6.5% Vi RB 1k IC RC 20 Vo 25 Example The common source circuit parameters are VDD=10V, RD=5kΩ and the transistor parameters are Kn=1mA/V2, VTN=1V and =0. Assume the output voltage swing is limited to the range between the transition point and vDS=9V to minimize nonlinear distortion. Calculate the actual efficiency of a class A output stage. 25 Exercise The Q-point of common source circuit is VDSQ=4V a) Find IDQ b) Determine the max peak to peak amplitude of a symmetrical sinusoidal output voltage if the min value of instantaneous drain current must be no less than 0.1IDQ and the min value of instantaneous drain source voltage must be no less than vDS=1.5V. c) Calculate the power conversion efficiency where the signal power is the power delivered to RL. Ans: 60mA, 5V, 26 31.25mW, 5.2% Design of Class A C-E Amplifier To find R1, R2, RE, RC use the DC analysis and design formula; VCC VCC RE VEQ ; VCQ ; R2 10 2 10 I C I CQ given in data sheet. If have R L ; I C 10 I L To find Zi, Zo, Av, Ai use AC analysis (without loading effect) To find Zi, Zo, Avs, Ai use AC analysis (with loading effect if have Ri and RL 27 Example & Exercise Will be given in our class. 28 Class B Power Amplifier Consists of complementary pair electronic devices One conducts for one half cycle of the input signal and the other conducts for another half of the input signal When the input is zero, both devices are off, the bias currents are zero and the output is zero. Ideal voltage gain is unity 29 For input larger than zero, A turn ON and supplies current to the load. For input less than zero, B turn ON and sinks current from the load 30 Complimentary Push-Pull Circuit 31 CROSSOVER DISTORTION DEAD BAND 32 The Ideal Class B i Cn iCn Vp RL sin t iCn Vp RL sin t vo V p sin t i Cp vo VP sin t 33 • Maximum possible value of Vp is VCC vCEn VCC V p sin t • The instantaneous power in Qn is; pQn vCEniCn Vp pQn VCC V p sin t ) sin t RL 34 The average power in Qn is PQn VCCV p RL Vp 2 4 RL Differentiating for maximum PQn with respect to Vp equal to zero gives us dPQn VCC 2V p 0 dV p RL 4 RL then VP 2VCC Maximum average power dissipation; 2 VCC PQn max ) 2 RL 35 The average power delivered to the load is PL Power source supplies half sinewave of current, the average value is; 2 V 1 p 2 RL IS Vp RL Vp PS Ps VCC I S VCC RL The total power supplied by the two sources is Vp PS 2VCC I S 2VCC RL 36 The efficiency is 2 V 1 P 2 RL VP CC RL PL PS 2V maximum 4 ) V p 4VCC efficiency when VP VCC 0.785 78.5% 37 Class AB Power Amplifier Small quiescent bias on each output transistor to eliminate crossover distortion 38 Class C Power Amplifier 39 Class AB Voltage Transfer Curve 40 Collector Currents & Output Current iCn iL iCp 41 Example The parameters are VDD=10V, RL=20Ω. The transistor are matched and K=0.2A/V2, VT=1V, IDQ=0.05 when vo=5V. Determine the required biasing in a MOSFET class AB output stage. 42 Inductively Coupled Amplifier 43 The maximum possible average signal power delivered to the load PL (max) 1 1 VCC 2 I CQ RL 2 2 RL 2 The possible average signal power supply by VCC PS VCC I CQ VCC RL 2 The maximum possible power conversion efficiency 2 1 VCC 2 RL L P (max) (max) PS 44 VCC 2 RL 1 0.5 50% 2 Transformer Coupled Amplifier 45 • The theoretical maximum efficiency of a basic RCcoupled class-A amplifier is limited to 25%. • In practical circuit, the efficiency is less than 25%. • Used for output power of about 1 W only. • Transformer coupling can increase the maximum efficiency to 50% • Disadvantage of transformer coupling – expensive & bulky. Neglecting transformer resistance and assuming RE is small; VCEQ VCC 46 For ideal transformer; iL aiC and v2 v1 a N1 a turn ratio N2 iC v2 v1 / a RL iL aiC v1 1 RL 2 iC a 47 v1 RL ' iC RL ' a 2 RL Turn ratio is designed for maximum symmetrical swing, hence; 2VCC VCC RL ' a 2 RL 2I CQ I CQ The maximum average power delivered to load equals maximum average power delivered to the primary of the transformer 1 PL max ) VCC I CQ 2 48 (VCC and ICQ are maximum possible amplitudes of signal) The average power supplied by the VCC source is; PS VCC I CQ The maximum possible efficiency is; PL max ) 0.5 50% PS 49