Amplifier Teaching Aid (DED Philippinen, 86 p.)
 Lesson 10 - Large Signal Amplifier
 Lesson Plan
 (introduction...) Large signal amplifier Classes Class A power amplifier Class B power amplifier Class AB power amplifier

### (introduction...)

Title: Large Signal Amplifier

Objectives:

- Understand the importance of amplifier efficiency
- Know the most common classes of power amplifier and their basic characteristics

Figure

### Large signal amplifier

The early stages of amplifier systems are dealing with small signals. These stages are designed to give good voltage gain. Small signal transistors have a power rating of less than half a watt and power transistors have a power rating of more than half a watt.

The later stages of an amplifier system have much larger collector currents, because the load impedances are much smaller (i.e.: Loudspeaker 1W, 4W, 8W, 16W).

Efficiency is most important when large amounts of signal power are required:

### Classes

This refers to how the amplifying device is biased. Amplifier can be biased for class A, B or AB operation.

### Class A power amplifier

The amplifiers we have discussed have been class A amplifiers. Class A amplifier operate in the center of the load line. This gives the best possible output swing without clipping.

Efficiency: low, maximum 50%

Even when no signal is applied, a high current is flowing (100 mA) and there is a power dissipation in the load.

Distortion: low

Applications: Few audio amplifier (high quality)

Fig. 10-1: Class A amplifier

### Class B power amplifier

The class B amplifier is biased at cutoff. No current will flow until an input signal provides the bias to turn on the amplifier.

Fig. 10-2: Q point of a class B amplifier

Only one half of the input signal is amplified. Two transistors can be operate in class B together in one circuit, one transistor for the positive portion of the signal and one transistor for the negative portion of the signal (Push Pull):

Fig. 10-3: Push Pull amplifier

Fig. 10-4: Signal swing of a. push pull amplifier, Class B operation

Problem: Crossover distortion, the emitter diode takes 0,7V to turn on.

Fig. 10-5: Crossover distortion

Efficiency: 78.5%

Distortion: High

Application: High power stages, not used in audio applications.

### Class AB power amplifier

Solution to the crossover distortion:

Provide some forward bias for the base emitter junction.

Fig. 10-6: Class AB amplifier

Fig. 10-7: Complementary push pull amplifier

Two complementary transistors are used (NPN + PNP) , so no transformer is needed any more.

Efficiency: between A and B

Distortion: Moderate

Application: High power stages in audio and radio-frequency applications.