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Transcript

POWER AMPLIFIER Concept of Power Amplifier Power BJTs Power MOSFETs Classes of Power Amplifier 1 CONCEPT OF POWER AMPLIFIER 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 2 TRANSISTOR LIMITATIONS maximum rated current, maximum rated voltage, maximum rated power, and maximum allowed temperature. 3 POWER BJTS 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) 4 Typical dc beta characteristics (hFE versus lc) for 2N3055 5 POWER BJTS Current gain is smaller in power BJT. The gain depends on IC and temperature may be related to the following: 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. 6 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 7 Instantaneous power dissipation pQ vCE iC vBE iB vCE iC The average power over one cycle 1 PQ T T 0 vCE iC dt The maximum rated power, PT VCE I C 8 •The Safe Operating Area (SOA) is bounded by IC(max); VCE(sus) and PT • The SOA of a bipolar transistor plotted on: (a) linear scales and (b) logarithmic scales 9 Example The parameters of the common emitter circuit are RL=8Ω, VCC=24 volt. Determine the required current, voltage and power ratings of the a power BJT. 10 Power MOSFETs 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) 11 PERFORMANCE CHARACTERISTIC OF POWER MOSFETs Faster switching times no second breakdown. Stable gain and response over wide temperature range. 12 Classes of Power Amplifiers Class A - The transistor conducts during the whole cycle of sinusoidal input signal 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 13 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 14 Instantaneous power dissipation in transistor is pQ vCE iC For sinusoidal input signal, the collector current; iC I CQ I p sin t And the collector emitter voltage; vCE For maximum possible swing; I p I CQ VCC V p sin t 2 VCC Vp 2 and Therefore, the instantaneous power dissipation in transistor pQ VCC I CQ 2 1 sin 2 t 15 The power conversion efficiency signalload power (PL ) supplypower (PS ) The average ac power delivered to the load VCC I CQ 1 1 VCC PL (max) VP I P I CQ 4 2 2 2 The average power supply by the VCC source PS VCC I CQ The maximum attainable conversion efficiency (max) 1 4 VCC I CQ VCC I CQ 1 0.25 25% 4 16 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. 17 Exercise The Q-point of common source circuit is VDSQ=4V Find IDQ 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. Calculate the power conversion efficiency where the signal power is the power delivered to RL. Ans: 60mA, 5V, 31.25mW, 5.2% 18