Microelectronic Circuits Output Stage and ower Amplifiers Slide 1
ntroduction Most of the challenging requirement in the design of the output stage is ower delivery to the load. ower consumption at the output stage must be as low as possible. ower dissipated in a transistor raises its internal junction temperature C (imit 150 to 00 ) High power means generally more than 1W BJTs used in the design of high power output stage called as ower transistor. ower amplifier is a simply an amplifier with a high power output. Slide
Classification (a) class A (b) class B (c) class AB (d) class C Slide 3
Class A power Amplifier Emitter follower Q1 biased with a content current supplied by transistor Q. Current must be must be greater then the largest negative load current. v v v 0 BE1 Slide 4
Class A power Amplifier v v v 0 BE1 This linear characteristic is obtained by neglecting the change in v BE1 with i. The maximum positive output is determined by the saturation of Q 1. n the negative direction, the limit of the linear region is determined either by Q 1 turning off or by Q saturating, depending on the values of and R. Slide 5
Class A power Amplifier ower Conversion Efficiency The power conversion Efficiencyof oad ower 0 4R oad ower Supply ower S 0 / 1 0 0 As 0 4 R Maximum Efficiencywill be S R R R 5% an output stage is given as Because of the low efficiency, class A output stage is rarely used in high power applications. The output voltage swing is limited to the lower value results into distortion. 0 Slide 6
Class A power Amplifier For the emitter follower = 10, = 100mA and R = 100 Ω. f the output voltage is 8 peak sinusoid, find the following: (a) ower delivered to the load (b) Average power drawn from the supplies (c) ower conversion Efficiency. Slide 7
Class A power Amplifier To improve the efficiency Transformer Coupled power Class A amplifier is used. ower DC power loss due to less resistance of primary coil. The relationship between the primary and secondary values of voltage, current and impedance are summarized as: Slide 8
Class A power Amplifier Efficiency Transformer Coupled power Class A amplifier ac power deliverd to the load dc power supplied (dc) i (ac) 0 rms rms m m ac dc ac dc 50% m m X100 Here Efficiency of transformer coupled amplifier is 50% just doubled to RC coupled class A power amplifier. m and m Slide 9
Class A power Amplifier A transformer coupled class A power amplifier supplies power to an 80Ω load connected across the secondary of the transformer having turn ratio 5:1. Determine maximum power output for 10mA collector current. oad Rsistance R N1 Turn Ratio 5 N oad on the primary side of R ' rms N1 N C R Maximum power output 5 80 80 transformer 000 out( ac ) rms R ' Slide 10
Class B power Amplifier A transformer coupled class B power amplifier n class B amplifier, the operating point is located in cut-off region and hence transistor conducts for 180 that is half cycle making the amplifier more efficient than class A power amplifier. t consist of complementary pair of pnp and npn transistors. Case 1: When vi = 0, Qp and QN cutoff and v0 = 0 Case : when vi > 0.5 QN conducts act as emitter follower and Q off Case 3: When vi < - 0.5 Q conducts act as emitter follower and QN off QN pushes the current into the load and Q pull the current from the load, Hence it is called as pushpull amplifier Slide 11
Class B power Amplifier Transfer Characteristics Slide 1
Class B power Amplifier Crossover Distortion (dead band) Slide 13
Class B power Amplifier ower Conversion Efficiency: ac power deliverd to the load dc power supplied s (ac) s- 1 R Total supply ac dc rms 1 0 R rms 0 ac 1 / R R 0 0 R Hence max 78. 5% 4 0 4 0 ac dc Slide 14
Class B power Amplifier Reducing the Crossover Distortion: Class B circuit with an op amp connected in a negative-feedback loop to reduce crossover distortion. Slide 15
Class AB power Amplifier The crossover distortion can be eliminated by AB configuration. Small BB is applied to bases of transistors. Slide 16