In your original post, answer the following: In your own words, explain the different classes of the power amplifiers (Class-A, Class-B, Class-AB and Class-C) and

In your original post, answer the following:

In your own words, explain the different classes of the power amplifiers (Class-A, Class-B, Class-AB and Class-C) and mention where the Q-point is for each one. Also mention about their applications.
What is the difference between a 200W stereo amplifier and a 100 W stereo amplifier in dB?
What is dBm? Where is it commonly used?
A utility company is interested in purchasing a Public Address system (audio frequencies). Because the company works near high-voltage power lines, where 60Hz interference is common, they have requested that the PA system be designed to minimize pickup from the power lines. What would be the best way to minimize this interference?

Electronic_Devices_10th_CH07.pptx

Electronic Devices

10th ed.

Chapter 7

BJT Power Amplifiers

Electronic Devices

10th ed.

◆ Explain and analyze the operation of class A amplifiers

◆ Explain and analyze the operation of class B and class AB

amplifiers

◆ Explain and analyze the operation of class C amplifiers

◆ Troubleshoot power amplifiers

Objectives:

Electronic Devices

Power Amplifiers

A power amplifier is a large signal amplifier that produces a replica of the input signal on its output. In the case shown here, the output is an inverted replica of the input.

Generally power amplifiers are defined as those in which it is necessary to consider the problem of heat dissipation (about 1 W or more).

Electronic Devices

Class-A Power Amplifiers

A class A power amplifier is a large signal amplifier that operates in the linear region. Ideally, a class A amplifier is

designed to operate in the center of the ac load line.

Electronic Devices

Class-A Power Amplifiers

A class A power amplifier is a large signal amplifier that operates in the linear region. Ideally, a class A amplifier is

designed to operate in the center of the ac load line.

Notice that a class A amplifier dissipates dc power even with no signal. The dc power dissipated is the product of ICQ and VCEQ.

Electronic Devices

Class-A Power Amplifier Efficiency

Power gain is the ratio of the power delivered to the load to the input power. The maximum signal power delivered to a load cannot be greater than 0.5 ICQVCEQ.

Electronic Devices

Class-A Power Amplifier Efficiency

Power gain is the ratio of the power delivered to the load to the input power. The maximum signal power delivered to a load cannot be greater than 0.5 ICQVCEQ.

Class A power amplifiers are not particularly efficient, so they are restricted to low power applications. The maximum theoretical efficiency for a class A amplifier is 0.25 (or 25%) and usually they are considerably less.

Electronic Devices

Class-A Power Amplifier Efficiency

Power gain is the ratio of the power delivered to the load to the input power. The maximum signal power delivered to a load cannot be greater than 0.5 ICQVCEQ.

Question:

What is the efficiency of an amplifier that delivers 200 mW to a load if the power supply is 12 V at 400 mA?

Electronic Devices

Class-A Power Amplifier Efficiency

Power gain is the ratio of the power delivered to the load to the input power. The maximum signal power delivered to a load cannot be greater than 0.5 ICQVCEQ.

Question:

What is the efficiency of an amplifier that delivers 200 mW to a load if the power supply is 12 V at 400 mA?

6.7%

Electronic Devices

Class-A Power Amplifier – Example

(a) If a 3 Vpp signal is applied to the input, what voltage do you expect to see at the speaker? (b) What power is delivered?

Example:

Electronic Devices

Class-A Power Amplifier – Example

(a) If a 3 Vpp signal is applied to the input, what voltage do you expect to see at the speaker? (b) What power is delivered?

Solution:

Example:

(a) The CC amplifier has a gain of nearly 1. The output voltage is nearly equal to the input = 3 Vpp.

Electronic Devices

Class-A Power Amplifier – Example

(a) If a 3 Vpp signal is applied to the input, what voltage do you expect to see at the speaker? (b) What power is delivered?

Solution:

Example:

(a) The CC amplifier has a gain of nearly 1. The output voltage is nearly equal to the input = 3 Vpp.

(b) The power delivered to the speaker is:

Electronic Devices

Class-A Power Amplifier – Example

(a) If a 3 Vpp signal is applied to the input, what voltage do you expect to see at the speaker? (b) What power is delivered?

Solution:

Example:

(a) The CC amplifier has a gain of nearly 1. The output voltage is nearly equal to the input = 3 Vpp.

(b) The power delivered to the speaker is:

= 140 mW

Electronic Devices

Class-A Power Amplifier – Example

For the 3 Vpp input, what is the input power and what is the power gain? Assume the Darlington b = 10,000.

Follow-up:

Electronic Devices

Class-A Power Amplifier – Example

For the 3 Vpp input, what is the input power and what is the power gain? Assume the Darlington b = 10,000.

Solution:

Follow-up:

Electronic Devices

Class-A Power Amplifier – Example

For the 3 Vpp input, what is the input power and what is the power gain? Assume the Darlington b = 10,000.

Solution:

Follow-up:

= 0.183 mW

Electronic Devices

Class-A Power Amplifier – Example

For the 3 Vpp input, what is the input power and what is the power gain? Assume the Darlington b = 10,000.

Solution:

Follow-up:

The power gain is:

= 0.183 mW

Electronic Devices

Class-A Power Amplifier – Example

For the 3 Vpp input, what is the input power and what is the power gain? Assume the Darlington b = 10,000.

Solution:

Follow-up:

The power gain is:

= 0.183 mW

= 769

Electronic Devices

Class-A Power Amplifier – Example

For the 3 Vpp input, what is the input power and what is the power gain? Assume the Darlington b = 10,000.

Solution:

Follow-up:

The power gain is:

= 0.183 mW

= 769

Note that this is approximately 140 mW/0.183 mW

Electronic Devices

Class-A Power Amplifier – Multisim

The circuit in the previous example can be simulated in Multisim as a check.

Electronic Devices

Class-A Power Amplifier – Multisim

The circuit in the previous example can be simulated in Multisim as a check.

The input trace (red) and output trace (blue) are nearly on top of each other as expected.

Electronic Devices

Push-Pull Amplifiers

Push-pull amplifiers use two transistors working together. One conducts on the positive half cycle; the other conducts on the negative half cycle. This is class B operation.

Electronic Devices

Push-Pull Amplifiers

By adding a forward biased diode, the base-emitter drop of the transistors does not need to be overcome by the signal. This is class AB operation. Notice that both transistors are npn types.

Electronic Devices

Push-Pull Amplifiers

A complementary symmetry push-pull amplifier uses an npn and a pnp transistor working together on alternate half-cycles.

Electronic Devices

Push-Pull Amplifiers

A complementary symmetry push-pull amplifier uses an npn and a pnp transistor working together on alternate half-cycles.

The two diodes cause the transistors to be biased into slight conduction. Because of the slight conduction, this is also class AB operation and the transistors conduct slightly more than ½ of the input cycle.

Electronic Devices

Push-Pull Amplifiers

A complementary symmetry push-pull amplifier uses an npn and a pnp transistor working together on alternate half-cycles.

Question:

What is the advantage of class AB operation over class B operation?

Electronic Devices

Push-Pull Amplifiers

A complementary symmetry push-pull amplifier uses an npn and a pnp transistor working together on alternate half-cycles.

Question:

What is the advantage of class AB operation over class B operation?

Cross-over distortion is eliminated.

Electronic Devices

Push-Pull Amplifiers

The ac load line for the npn transistor of a complementary push-pull amplifier is shown.

Electronic Devices

Push-Pull Amplifiers

The ac load line for the npn transistor of a complementary push-pull amplifier is shown.

Notice that the Q point is near the right end of the load line.

Electronic Devices

Push-Pull Amplifiers

The ac load line for the npn transistor of a complementary push-pull amplifier is shown.

Ic(sat) is determined by the load resistor and is given by:

Notice that the Q point is near the right end of the load line.

Electronic Devices

Push-Pull Amplifiers – Example

Example:

Draw the ac load line for the npn transistor.

Electronic Devices

Push-Pull Amplifiers – Example

Example:

Draw the ac load line for the npn transistor.

Solution:

The ac load line is drawn between Ic(sat) and VCC.

+15 V

0.5 A

VCE

Electronic Devices

Push-Pull Amplifiers

The maximum peak output voltage is always less than the power supply voltage. For the amplifier shown, the maximum peak output will be about ±13 V because at the peak of the input, maximum bias current is required.

Electronic Devices

Push-Pull Amplifiers

For a given load resistor, you can enable a larger output by using smaller bias resistors, higher beta transistors (such as Darlington transistors), or increasing the power supply voltages.

Electronic Devices

Push-Pull Amplifiers – Multisim

A Multisim simulation of the previous circuit shows clipping on the output when the input exceeds ±13 V. The input is the ±14 V yellow trace; the output is the blue trace and is clipped at ±13 V.

Electronic Devices

Push-Pull Amplifiers

Question:

Why is the voltage gain of the previous circuit less than 1?

Electronic Devices

Push-Pull Amplifiers

Each transistor is configured as a CC amplifier.

Question:

Why is the voltage gain of the previous circuit less than 1?

Electronic Devices

Push-Pull Amplifiers

Darlington transistors have replaced the single transistors in the previous circuit. The bias resistor are larger (less power dissipation) and two more diodes are added. The maximum output can be larger due to the Darlington transistors despite the larger bias resistors.

Electronic Devices

Push-Pull Amplifiers

Question:

Why are four diodes used in this circuit?

Electronic Devices

Push-Pull Amplifiers

Question:

Why are four diodes used in this circuit?

There are two base-emitter drops for each polarity of the input.

Electronic Devices

Push-Pull Amplifiers

The circuit shown is in the lab manual and uses a dc coupled CE amplifier stage to drive the push-pull stage. In this case, the CE amplifier is in the bias path for the push-pull stage.

Electronic Devices

Push-Pull Amplifiers

The circuit shown is in the lab manual and uses a dc coupled CE amplifier stage to drive the push-pull stage. In this case, the CE amplifier is in the bias path for the push-pull stage.

Question:

How would you adjust R5 for an optimum setting?

Electronic Devices

Push-Pull Amplifiers

The circuit shown is in the lab manual and uses a dc coupled CE amplifier stage to drive the push-pull stage. In this case, the CE amplifier is in the bias path for the push-pull stage.

Question:

How would you adjust R5 for an optimum setting?

Monitor the dc output voltage and adjust it for 0 V using R5.

0 V

Electronic Devices

Class-C Amplifiers

Class C amplifiers are biased into conduction much less than 180o. They are usually used in RF applications, such as RF oscillators and modulators.

Electronic Devices

Class-C Amplifiers

Class C amplifiers are biased into conduction much less than 180o. They are usually used in RF applications, such as RF oscillators and modulators.

The transistor is on when the input signal exceeds |VBB| + VBE. Because class C amplifiers are biased on for a small percentage of time, they can be very efficient.

Electronic Devices

Class-C Amplifiers

Class C operation is useful in oscillators. The collector circuit has a parallel resonant circuit (“tank”) and oscillations are sustained by the short pulse of collector current on each cycle.

Electronic Devices

Class-C Amplifiers

The circuit can be set up with clamping bias, where the bias resistor is connected to ground. C1 will charge (through the base-emitter diode) to 0.7 V less than the positive peak.

Electronic Devices

Class-C Amplifiers

The circuit can be set up with clamping bias, where the bias resistor is connected to ground. C1 will charge (through the base-emitter diode) to 0.7 V less than the positive peak.

Clamping action causes the transistor to be cut off except at the positive peak of the input. The R1C1 time constant needs to be long compared to the period of the signal.

Electronic Devices

Push-Pull Amplifier -Troubleshooting

If there are no faults in the push-pull amplifier shown, what DC voltage do you expect to read on meter XXM1?

Electronic Devices

Push-Pull Amplifier -Troubleshooting

If there are no faults in the push-pull amplifier shown, what DC voltage do you expect to read on meter XXM1?

You should see a forward diode drop. If diodes and transistors are matched, the point between the diodes should be at 0 V and the cathode of D2 is one diode drop less.

Electronic Devices

Push-Pull Amplifier -Troubleshooting

Assume Diode D2 is open. What do you expect to see on the meter now? Explain your answer.

Electronic Devices

Push-Pull Amplifier -Troubleshooting

Assume Diode D2 is open. What do you expect to see on the meter now? Explain your answer.

You will still see a forward diode drop. This is because there is a path from ground, through the emitter-base junction of Q2, and through R2 to VEE.

Electronic Devices

Push-Pull Amplifier -Troubleshooting

Assume Diode D2 is open. What do you expect to see on the meter now? Explain your answer.

You will still see a forward diode drop. This is because there is a path from ground, through the emitter-base junction of Q2, and through R2 to VEE.

With a signal source on, the rectified signal will cause a DC voltage to be superimposed.

Electronic Devices

Key Terms-1

Class A

Power gain

Efficiency

Class B

Electronic Devices

Key Terms-1

Class A

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