SUMMARY

This chapter has presented some general information that applies to all amplifiers, as well as some specific information about transistor and audio amplifiers. All of this information will be useful to you in the next chapter of this module and in your future studies of electronics.

An AMPLIFIER is a device that enables an input signal to control an output signal. The output signal will have some (or all) of the characteristics of the input signal but will generally be larger than the input signal in terms of voltage, current, or power. A basic line diagram of an amplifier is shown below.

Amplifiers are classified by FUNCTION and Frequency response. Function refers to an amplifier being a VOLTAGE AMPLIFIER or a POWER AMPLIFIER. Voltage amplifiers provide voltage amplification and power amplifiers provide power amplification. The frequency response of an amplifier can be described by classifying the amplifier as an AUDIO AMPLIFIER, RF AMPLIFIER, or VIDEO (WIDE-BAND) AMPLIFIER. Audio amplifiers have frequency response in the range of 15 Hz to 20 kHz. An rf amplifier has a Frequency response in the range of 10 kHz to 100,000 MHz. A video (wide-band) amplifier has a Frequency response of 10 Hz to 6 MHz.

THE CLASS OF OPERATION of a transistor amplifier is determined by the percent of time that current flows through the transistor in relation to the input signal.

IN CLASS A OPERATION, transistor current flows for 100% (360) of the input signal. Class A operation is the least efficient class of operation, but provides the best fidelity.

IN CLASS AB OPERATION, transistor current flows for more than 50% but less than 100% of the input signal.

IN CLASS B OPERATION, transistor current flows for 50% of the input signal.

IN CLASS C OPERATION, transistor current flows for less than 50% of the input signal. Class C operation is the most efficient class of operation.

COUPLING is used to transfer a signal from one stage to another.

DIRECT COUPLING is the connection of the output of one stage directly to the input of the next stage. This method is not used very often due to the complex power supply requirements and impedance-matching problems.

RC COUPLING is the most common method of coupling and uses a coupling capacitor and signal-developing resistors.

IMPEDANCE COUPLING uses a coil as a load for the first stage but otherwise functions just as RC coupling. Impedance coupling is used at high frequencies.

TRANSFORMER COUPLING uses a transformer to couple the signal from one stage to the next. Transformer coupling is very efficient and the transformer can aid in impedance matching.

MAXIMUM POWER TRANSFER occurs between two circuits when the output impedance of the first circuit matches the input impedance of the second circuit.

A MAXIMUM VOLTAGE INPUT SIGNAL is present when the input impedance of the second circuit is larger than the output impedance of the first circuit (mismatched).

THE COMMON-EMITTER configuration of a transistor amplifier has MEDIUM INPUT AND MEDIUM OUTPUT IMPEDANCE.

THE COMMON-BASE configuration of a transistor amplifier has LOW INPUT AND HIGH OUTPUT IMPEDANCE.

THE COMMON-COLLECTOR (EMITTER FOLLOWER) configuration of a transistor amplifier has HIGH INPUT AND LOW OUTPUT IMPEDANCE.

FEEDBACK is the process of coupling a portion of the output signal back to the input of an amplifier.

POSITIVE (REGENERATIVE) FEEDBACK is provided when the feedback signal is in phase with the input signal. Positive feedback increases the gain of an amplifier.

NEGATIVE (DEGENERATIVE) FEEDBACK is provided when the feedback signal is 180 out of phase with the input signal. Negative feedback decreases the gain of an amplifier but improves fidelity and may increase the Frequency response of the amplifier.

THE IDEAL Frequency response of an audio amplifier is equal gain from 15 Hz to 20 kHz with very low gain outside of those limits.

A PHASE SPLITTER provides two output signals that are equal in amplitude but different in phase from a single input signal. Phase splitters are often used to provide input signals to a push-pull amplifier.

A PUSH-PULL AMPLIFIER uses two transistors whose output signals are added together to provide a larger gain (usually a power gain) than a single transistor could provide. Push-pull amplifiers can be operated class A, class AB or class B.