* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download power amp - classification
Survey
Document related concepts
Voltage optimisation wikipedia , lookup
Power factor wikipedia , lookup
Standby power wikipedia , lookup
Pulse-width modulation wikipedia , lookup
History of electric power transmission wikipedia , lookup
Buck converter wikipedia , lookup
Opto-isolator wikipedia , lookup
Wireless power transfer wikipedia , lookup
Mains electricity wikipedia , lookup
Power over Ethernet wikipedia , lookup
Electric power system wikipedia , lookup
Distribution management system wikipedia , lookup
Electrification wikipedia , lookup
Alternating current wikipedia , lookup
Power engineering wikipedia , lookup
Transcript
EMT 112/4 ANALOGUE ELECTRONICS 1 Power Amplifiers Syllabus Power amplifier classification, class A, class B, class AB, amplifier distortion, class C and D, transistor power dissipation, thermal management. POWER AMPLIFIERS Part II Classification of Power Amplifiers POWER AMPLIFIER – Classification They are grouped together based on their Q-points on the DC load line. POWER AMPLIFIER – Classification In class-A; the transistor conducts during the whole cycle of sinusoidal input signal POWER AMPLIFIER – Classification In class-B; the transistor conducts during one-half cycle of input signal POWER AMPLIFIER – Classification In class-AB; the transistor conducts for slightly more than half a cycle of input signal POWER AMPLIFIER – Classification In class-C; the transistor conducts for less than half a cycle of input signal POWER AMPLIFIER – Class-A Operation For maximum swing ( +ve and –ve), transistor is biased such that the Q point is at centre of the load line. The transistor conducts for a full cycle of the input signal POWER AMPLIFIER – Class-A Operation Instantaneous power dissipation in transistor is; pQ vCE iC For sinusoidal input signal; iC I CQ I p sin t And; vCE VCC V p sin t 2 POWER AMPLIFIER – Class-A Operation For maximum possible swing; I p I CQ And; VCC Vp 2 Therefore; pQ VCC I CQ 2 1 sin 2 t POWER AMPLIFIER – Class-A Operation When the input signal = 0, the transistor must be capable of handling a continuous power of; VCC I CQ 2 Efficiency; PL PS PL = average ac power to the load PS = average power supplied by the source (VCC) POWER AMPLIFIER – Class-A Operation For maximum possible swing; 1 1 VCC PL V p I p 2 2 2 VCC I CQ I CQ 4 Power supplied by the source; PS VCC I CQ The efficiency; VCC I CQ 4 Maximum theoretical efficiency of class A amplifier is therefore 25% VCC I CQ 0.25 POWER AMPLIFIER – Class-B Operation 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 Both devices are off when the input is zero (See Figure) POWER AMPLIFIER – Class-B Operation When vI = 0, both A and B are OFF and therefore vO = 0. The complementary pair POWER AMPLIFIER – Class-B Operation When vI > 0, A is ON and B is OFF and vO > 0. POWER AMPLIFIER – Class-B Operation When vI < 0, A is OFF and B is ON and vO < 0. POWER AMPLIFIER – Class-B Operation The transfer characteristic of the complementary pair POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit Assuming ideal transistor; when vI = 0; both Qn & Qp are off and vO = 0 POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit vO = vI vBEn When vI > 0; Qp is off & Qn conducts and becomes an emitter follower, sourcing the current IL to RL. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit When vI < 0; Qn is off & Qp conducts and becomes an emitter follower, sinking the current IL from RL. POWER AMPLIFIER – Class-B Operation Approximate Class-B : Complementary push-pull circuit The problem with ClassB: transistors are not ideal. POWER AMPLIFIER – Class-B Operation Crossover Distortion Assuming cut-in voltage of transistor is 0.6 V, vO = 0 for a range 0.6 V < vI < 0.6 V. The transfer characteristic becomes non-linear (See Figure). The range where both transistors are simultaneously off known as the dead band. POWER AMPLIFIER – Class-B Operation Crossover Distortion The output will be distorted – crossover distortion (Figure) POWER AMPLIFIER – Class-B Operation Crossover Distortion Crossover distortion can be eliminated by biasing the transistor with small quiescent current – class-AB POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency VCC V V POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The Q-point is at the cutoff point of both transistors (zero collector current) POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The output voltage of the idealized class-B is; vO V p sin t The maximum possible value of Vp is VCC The instantaneous power dissipation in Qn is; pQn vCEniCn POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency For 0 t , the collector current is; iCn Vp RL sin t For t 2 the collector current is; iCn 0 POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The collector-emitter voltage is; vCEn VCC V p sin t Therefore the instantaneous power dissipation in Qn is; pQn vCEniCn VCC Vp V p sin t sin t RL for 0 t POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency And; pQn 0 for t 2 Therefore, the average power dissipation in Qn is; PQn VCCV p RL Vp 2 4 RL POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Plotting PQn versus Vp as given by the equation; PQn VCCV p RL Vp 2 4 RL gives us the following curve: POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Because of symmetry; Differentiating PQn PQn PQp VCCV p RL Vp 2 4 RL with respect to Vp, for maximum PQn gives us; 2 VCC PQn max 2 RL which occurs when Vp 2VCC POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Since each power source supplies half sinewave of current, the average value is; IS Vp RL The total power supplied by the two sources is; Vp PS 2VCC I S 2VCC RL POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The power delivered to the load is; PL 2 O rms V RL The efficiency is; V p / 2 RL 2 V 2 p 2 RL PL V p PS 4VCC Maximum efficiency occurs when V p VCC POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency Under this condition; 4 0.785 Maximum theoretical efficiency of class B amplifier is therefore 78.5% The efficiency obtained in practice is less than the maximum value because of other circuit losses and because the peak output voltage must remain less than VCC to avoid transistor saturation which can cause distortion in the output signal. POWER AMPLIFIER – Class-B Operation Idealized Power Efficiency The maximum transistor power dissipation occurs at; Vp 2VCC Substituting in the expression for efficiency; V p 4VCC 2VCC 1 4VCC 2 50% POWER AMPLIFIER Crossover distortion can be virtually eliminated by applying small quiescent bias on each transistor (See Figure) If Qn and Qp are matched, each emitter-base junction is biased with VBB/2 when vI is zero. Hence vO is also zero. The quiescent collector currents are; iCn iCp I S eVBB / 2VT – Class-AB Operation POWER AMPLIFIER – Class-AB Operation As vI increases, the voltage at the base of Qn increases and vO increases. Qn operates as an emitter follower supplying current to RL. The output voltage is given by; VBB vO vI vBEn 2 The collector current of Qn is; iCn iL iCp (Neglecting the base currents) POWER AMPLIFIER Since iCn must to supply the load current, vBEn increases which causes vBEp to decrease because VBB is constant. The decrease in vBEp results in a decrease in iCp. – Class-AB Operation POWER AMPLIFIER When vI goes negative, the base voltage of Qp decreases followed by a decrease in vO. Qp operates as emitter follower, sinking the load current. As iCp increases vEBp increases causing a decrease in vBEn and iCn. – Class-AB Operation POWER AMPLIFIER – Class-AB Operation Transfer characteristics (vO versus vI) POWER AMPLIFIER – Class-AB Operation POWER AMPLIFIER – Class-AB Operation – Class-AB Operation POWER AMPLIFIER iCn iCp relationship vBEn vEBp vBB Using the relationship I C I S e can be written as; VBE / VT , the above expression iCn iCp I CQ VT ln VT ln 2VT ln IS IS IS Hence; iCniCp I 2 CQ The produc of iCn and iCp is constant, therefore if iCn increases iCp decreases but does not to zero POWER AMPLIFIER – Class-AB Operation Example 8.8 Mn and Mp are matched transistors with the following parameters; VT 1 V; K 0.20 A/V 2 If VDD = 10 V and RL = 8 , find the bias voltage VBB/2 for IDQ = 0.05 A. Find also VGSn, VSGp andvI if vO = 5 V. POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution iD K vGS VT 2 Since the MOSFETs are matched, at quiescent point; iD I DQ and vGS VGSQ VSGQ Hence; I DQ VBB 2 VBB K VT 2 2 POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution (cont’d) Substituting values; VBB 0.05 0.2 1 2 which yields; 2 VBB 1 .5 V 2 From the expression iDn K vGSn VT we have; 2 iDn vGSn VT K POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution (cont’d) When vO 5 V; iDn vO iL RL 5 0.25 A; 20 and iDn 0.25 vGSn VT 1 2.12 V K 0.2 POWER AMPLIFIER – Class-AB Operation Example 8.8 – Solution (cont’d) Since; VBB vGSn vSGp then; vSGp VBB vGSn 3 2.12 0.88 V And; VBB 3 vI vO vGSn 5 2.12 5.62 V 2 2 POWER AMPLIFIER – Class-C Operation Transistor conducts for less than half a cycle of input signal • • • Tuned circuit is required. Used for RF amplifier. Efficiency > 78.5% B – E junction is reverse-biased to obtain Q-point beyond cut-off.