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# AE_Lectur10_PowerAmplifiers_Part2_Power Analysis of ClassB Amplifiers.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_Lecture 10_Part 2 analyzes the power conversion efficiency of capacitive coupled Emitter Followr, Class B Complementary Symmetry Amplfier, Class A transformer coupled Clas A CE Amplifier. It analyzes the working of Class B Transformer Coupled Push Pull Amplifier.

AE_LECTURE NO- 10_ Part2

CLASS B (COMPLEMENTARY CONFIGURATION)

By making ICQ=0,a fantastic improvement is achieved in power conversion efficiency[Previously Push pull configuration was used but that needed transformer coupling. Now we use Complementary Symmetry Configuration which is amenable to IC Technology]

Figure 1. Class B Complimentary Symmetry Configuration.

When vs=0, both Q1 and Q2 are off.

When vs = Positive half then

Q1 is ON and Q2 is OFF.

When vs = Negative half then

Q1 is OFF and Q2 is ON.

In the first case Q1 is sourcing current to the load and in the second half of the sinusoidal cycle,

Q2 is sinking the current from the load.

Figure 2. Complimentary Symmetry action of Q 1 and Q 2 in a Complimentary-Symmetry Amplifier.

Figure 3. Illustration of Cross-Over Distortion in Class B mode of operation.

During cross-over distortion, both Q1 & Q2 are off and it causes the Dead Zone.

Figure 4. The load line and the dynamic swing of the Q-point under signal condition.

This is Class B operation.

Q1 conducts for 180˚ of the I/P Voltage in positive half.

Q2 conducts for 180˚ of the I/P Voltage in negative half.

In class A→there is one BJT Qo. Qo conducts for 360˚ of the I/P Voltage.

CLASS B Power Conversion Efficiency.

Let |VCC|=|VEE|

Figure 5. Half Wave Rectified Voltage Wave-Form passing through individual BJT but at the load there is full sinusoidal signal..

Half wave rectified current is passing through each BJT.

Average of this half wave rectified current

But

Where

Average power supplied by each battery

Therefore 78.5% Efficiency can be achieved in full load condition.

CROSS-OVER DISTORTION is removed by Class AB operation.

Under quiescent condition , both BJT are conducting slightly so that there is no dead zone in switch over condition.

Figure 6. The Biasing Network to ensure Class AB mode of operation.

1 V bias will keep Q1 and Q2 barely conducting because 0.5 V is the cut-in voltage and 1V across R0 ensures that both transistors are barely conducting because they in cut-in stage.

If 1 V is not sufficient then we may take 1.1V to bias Q1 and Q2 so that dead zone is removed.

THE OUTPUT STAGE OF OPERATIONAL AMPLIFIER

Figure 7. The output stage of µA741 Operational Amplifier.

Q2, Q3 are short circuit protection BJT’s.

Normally Q2 & Q3 are off. But as excessive current passes through Q1 & Q4, Q2 or Q3 turn ON.

In positive half,Q2 turns ON & limits the current in Q1 by shunting IB1.

In positive half,Q3 turn ON & limits the current in Q4 by shunting IB4.

Typically if R=25Ω then maximum output current is 28 mA because 28mA X 25 Ω=0.7 V which turns on Q2 or Q3 as the case may be.

TOTAL HARMONIC DISTORTION OF CLASS B AMPLIFIER

Non linear operation causes harmonic distortion or amplitude distortion.

Large signal condition leads to nonlinear operation.

SINCE power amplifier is large signal system hence harmonic distortion is inevitable.

In high fidelity audio amplifiers Total Harmonic Distortion<0.1%.

Class B mode of operation ensures that even harmonic cancel.

Refer to Figure 2.

If isource (ωt) = IC +B0+ B1Cosωt + B2Cos2ωt + B3Cos3ωt + B4Cos4ωt ……

Then isink (ωt) = isource(ωt+π) = IC +B0 - B1Cosωt + B2Cos2ωt - B3Cos3ωt + B4Cos4ωt -……

Now net output current through the load = iout (ωt)= isource (ωt)- isink (ωt)

Therefore iout (ωt) = 2(B1Cosωt + B3Cos3ωt + B5Cos5ωt -……)

Hence overall harmonic distortion is suppressed automatically in Class B complimentary symmetry amplifier. Therefore signal fidelity is maintained inspite of large signal power amplification.

TRANSFORMER COUPLED CLASS AB PUSH PULL AMPLIFIER

Figure 8. Circuit Diagram of Transformer Coupled Class B Push-Pull Amplifier.

Below in Figure 9, the graphical interpretation of Class B Push-Pull Amplifier is given. The battery VCC in the output loop of the Class B amplifier supplies a full wave rectified current - one half is being supplied to Q1 and the other half is supplied to Q2. The two half together result in a full sine wave in the secondary coil of the output transformer as shown in Figure 9B.

Figure 9A gives the load line of one half of the circuit.

Figure 9B gives the transfer characteristics.(input to output).

Figure 9C gives the resultant sine wave generated in the secondary loop of the output transformer

Figure 9. Load Line and Q-point swing under signal condition.

TRANSFOMER COUPLED CLASS A CE AMPLIFIER

Figure 10. Circuit Diagram of Class A transformer coupled CE Amplifier.

Figure 11. Incremental Circuit of transformer coupled CE amplifier. The load is reflected as n2RL. Under no load condition there is no collector dissipation.

Where

During signal condition half of this power is delivered to the load and half power is dissipated in BJT.

The considerable improvement in Power Conversion Efficiency is due the fact that under no signal condition dc load is zero hence there is no standby power dissipation in the load circuit though there is power dissipation in the active device. Due to this the efficiency rises from 25% to 50%.

In direct coupled CE Amplifier, the power conversion efficiency is 25%. This has not been dealt with. We analyzed the capacitive coupled Load in CC Amplifier . In this case also power conversion efficiency is 25%.

Class C tuned amplifier give more than 99% power conversion efficiency. This we will deal in RF tuned amplifier.

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