Chapter 13 Output Stages in addition to Power Amplifiers 13.1 General Considerations 13.2 E

Chapter 13 Output Stages in addition to Power Amplifiers 13.1 General Considerations 13.2 E www.phwiki.com

Chapter 13 Output Stages in addition to Power Amplifiers 13.1 General Considerations 13.2 E

Stout, Allen, Midday On-Air Talent has reference to this Academic Journal, PHwiki organized this Journal Chapter 13 Output Stages in addition to Power Amplifiers 13.1 General Considerations 13.2 Emitter Follower as Power Amplifier 13.3 Push-Pull Stage 13.4 Improved Push-Pull Stage 13.5 Large-Signal Considerations 13.6 Short Circuit Protection 13.7 Heat Dissipation 13.8 Efficiency 13.9 Power Amplifier Classes Why Power Amplifiers Drive a load with high power. Cell phone needs 1W of power at the antenna. Audio system needs tens to hundreds Watts of power. Ordinary Voltage/Current amplifiers are not equipped as long as such applications CH 13 Output Stages in addition to Power Amplifiers Chapter Outline CH 13 Output Stages in addition to Power Amplifiers

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Power Amplifier Characteristics Experiences small load resistance. Delivers large current levels. Requires large voltage swings. Draws a large amount of power from supply. Dissipates a large amount of power, there as long as e gets “hot”. CH 13 Output Stages in addition to Power Amplifiers Power Amplifier Per as long as mance Metrics Linearity Power Efficiency Voltage Rating CH 13 Output Stages in addition to Power Amplifiers Emitter Follower Large-Signal Behavior I As Vin increases Vout also follows in addition to Q1 provides more current. CH 13 Output Stages in addition to Power Amplifiers

Emitter Follower Large-Signal Behavior II However as Vin decreases, Vout also decreases, shutting off Q1 in addition to resulting in a constant Vout. CH 13 Output Stages in addition to Power Amplifiers Example: Emitter Follower CH 13 Output Stages in addition to Power Amplifiers Linearity of an Emitter Follower As Vin decreases the output wave as long as m will be clipped, introducing nonlinearity in I/O characteristics. CH 13 Output Stages in addition to Power Amplifiers

Push-Pull Stage As Vin increases, Q1 is on in addition to pushes a current into RL. As Vin decreases, Q2 is on in addition to pulls a current out of RL. CH 13 Output Stages in addition to Power Amplifiers I/O Characteristics as long as Large Vin For positive Vin, Q1 shifts the output down in addition to as long as negative Vin, Q2 shifts the output up. CH 13 Output Stages in addition to Power Amplifiers Overall I/O Characteristics of Push-Pull Stage However, as long as small Vin, there is a dead zone (both Q1 in addition to Q2 are off) in the I/O characteristic, resulting in gross nonlinearity. CH 13 Output Stages in addition to Power Amplifiers

Small-Signal Gain of Push-Pull Stage The push-pull stage exhibits a gain that tends to unity when either Q1 or Q2 is on. When Vin is very small, the gain drops to zero. CH 13 Output Stages in addition to Power Amplifiers Sinusoidal Response of Push-Pull Stage For large Vin, the output follows the input with a fixed DC offset, however as Vin becomes small the output drops to zero in addition to causes “Crossover Distortion.” CH 13 Output Stages in addition to Power Amplifiers Improved Push-Pull Stage With a battery of VB inserted between the bases of Q1 in addition to Q2, the dead zone is eliminated. CH 13 Output Stages in addition to Power Amplifiers

Implementation of VB Since VB=VBE1+VBE2, a natural choice would be two diodes in series. I1 in figure (b) is used to bias the diodes in addition to Q1. CH 13 Output Stages in addition to Power Amplifiers Example: Current Flow I CH 13 Output Stages in addition to Power Amplifiers Example: Current Flow II CH 13 Output Stages in addition to Power Amplifiers

Addition of CE Stage A CE stage (Q4) is added to provide voltage gain from the input to the bases of Q1 in addition to Q2. CH 13 Output Stages in addition to Power Amplifiers Bias Point Analysis For bias point analysis, the circuit can be simplified to the one on the right, which resembles a current mirror. The relationship of IC1 in addition to IQ3 is shown above. VA=0 Vout=0 CH 13 Output Stages in addition to Power Amplifiers Small-Signal Analysis Assuming 2rD is small in addition to (gm1+gm2)RL is much greater than 1, the circuit has a voltage gain shown above. CH 13 Output Stages in addition to Power Amplifiers

Output Resistance Analysis If is low, the second term of the output resistance will rise, which will be problematic when driving a small resistance. CH 13 Output Stages in addition to Power Amplifiers Example: Biasing CH 13 Output Stages in addition to Power Amplifiers Problem of Base Current 195 µA of base current in Q1 can only support 19.5 mA of collector current, insufficient as long as high current operation (hundreds of mA). CH 13 Output Stages in addition to Power Amplifiers

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Modification of the PNP Emitter Follower Instead of having a single PNP as the emitter-follower, it is now combined with an NPN (Q2), providing a lower output resistance. CH 13 Output Stages in addition to Power Amplifiers Example: Input Resistance CH 13 Output Stages in addition to Power Amplifiers Additional Bias Current I1 is added to the base of Q2 to provide an additional bias current to Q3 so the capacitance at the base of Q2 can be charged/discharged quickly. CH 13 Output Stages in addition to Power Amplifiers

Example: Minimum Vin CH 13 Output Stages in addition to Power Amplifiers HiFi Design Using negative feedback, linearity is improved, providing higher fidelity. CH 13 Output Stages in addition to Power Amplifiers Short-Circuit Protection Qs in addition to r are used to “steal” some base current away from Q1 when the output is accidentally shorted to ground, preventing short-circuit damage. CH 13 Output Stages in addition to Power Amplifiers

Example: Three-Input NAND CH 15 Digital CMOS Circuits NMOS in addition to PMOS Duality In the CMOS philosophy, the PMOS section can be obtained from the NMOS section by converting series combinations to the parallel combinations in addition to vice versa. CH 15 Digital CMOS Circuits

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