Power Amplifiers - aast.edu

Dr. Hassan Eldib

Power Amplifiers

• To deliver power to the load efficiently

• To deliver a large current to a small load resistance e.g. audio speaker; or to deliver a large voltage to a large load resistance e.g. switching power supply;

EC 339 – Electronics 2 – Spring 2017-2018 – Lecture 6 4

Dr. Hassan Eldib

Classes of Power Amplifiers

Class A

Class AB

Class B

Class C (not covered in syllabus)

Class D (not covered in syllabus)


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• For maximum swing ( +ve and –ve), transistor is biased such that the Q point is at center of the load line.

• The transistor conducts for a full cycle of the input signal

EC 339 – Electronics 2 – Spring 2017-2018 – Lecture 6

Class A Operation

Dr. Hassan Eldib 7EC 339 – Electronics 2 – Spring 2017-2018 – Lecture 6

Class A Power Efficiency

Note:

Class A power amplifier has the lowest efficiency. 75% of power is lost. This low efficiency may be due the collector current is non-zero all of the time even when the input signal is zero.

Dr. Hassan Eldib

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

• Advantage: Better efficiency than class A


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Assuming ideal transistor;when vI = 0;both Qn & Qp are off;

when vI > 0;Qn conducts & Qp is off;

when vI < 0;Qp conducts & Qn is off


Class B: Complementary push-pull circuit

Dr. Hassan Eldib

Class B: Crossover Distortion

• Assuming cut-off region voltage of the transistors end at 0.7 V, then vO

= 0 for a range 0.7 V < vi < 0.7 V (transistors are off).

• This makes the output distorted (crossover distortion)

• This is a disadvantage over class A



Class B Maximum Power Efficiency

Maximum theoretical efficiency of class B amplifier is therefore 78.5%

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Small quiescent bias on each output transistor to eliminate crossover distortion

Small ICQ flows through each transistor in the absent of input signal


Class AB Operation


Dr. Hassan Eldib

Outline

• Introducing the Operational Amplifier (Op-amp) by providing applications:

1. Pulse width modulator

2. Buffer (voltage follower)

3. Non-inverting amplifier

4. Inverting amplifier

5. Summing amplifier

6. Subtracting amplifier

7. Multi-stage (cascade) amplifier

• You are required to write down all calculation written on the white board. In the exam, you are required to be able to replicate the same analysis methodology on different circuits.


Dr. Hassan Eldib

Amplifiers

• Differential Amplifier

– Amplifies difference between inputs

• Single-ended Amplifier

vo = A × (Input2 - Input1)vo = A × Input

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Operational Amplifier (Op-amp)

• Operational Amplifiers are represented both schematically and realistically below:

– Active component!

Op-amp IC 741


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3-stage Op-Amp


Dr. Hassan Eldib

Op-Amp Characteristics

Ideal Op-Amp Typical Op-Amp

Input Resistance infinity 106

Input Current 0 10-12 – 10-8 A

Output Resistance 0 100 – 1000

Operational Gain infinity 105 - 109

Dr. Hassan Eldib

Operational Amplifier

• Due to large op-amp gain (A), a small difference between the input voltages results in a very large amplified output voltage.– The output voltage is limited by

supply voltages

• Functions as a comparator, if NO feedback present in op-amp circuit– If V+>V- , Vout = HVS

– If V+<V- , Vout = LVS

Dr. Hassan Eldib

Application 1: Pulse Width Modulator

• Output changes when

– Vin ~= Vpot

• Potentiometer used to vary duty cycle

Vpot

Vpot

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Power Amplifiers - aast.edu