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Transcript
DMT 231 / 3 ELECTRONICS II Lecture I Power Amplifiers (Class A & Class B) 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 Assuming ideal transistor; when vI = 0; both Qn & Qp are off and vO =0 when vI > 0; Qn conducts & Qp is off and vO vI when vI < 0; Qp conducts & Qn is off and again vO vI. 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 The collector current is; iCn Vp RL sin t for 0 t and iCn 0 for t 2 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 PQn 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 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%