From Electronics I Laboratory Manual

General Amplifier Concepts

An amplifier can be represented as a two-port device.  The two ports are called input and output.  Each port has two terminals, and in general one of the terminals is ground.  When a voltage (vi) is applied at the input terminals, an output voltage (vo) is generated at the output terminal.  The voltage vi is normally generated by a signal generator or source that has an internal resistance Rs.  Normally, the output voltage of the amplifier is applied to a load which is represented by a resistor RL.  The general characteristics of any amplifier are related to the following three parameters:

The no–load voltage Gain: This parameter is defined as

 

(14.1)

The Input Impedance: Zi, defined by

 

(14.2)

and

The Output Impedance: Zo, defined by

 

(14.3)
                                                                                                                                                   

Note:

The no–load voltage gain, Avnl, as its name implies, is calculated by determining or measuring the input and output voltages when there is no load connected to the output of the amplifier.  Avnl is the ratio of the output voltage to the input voltage. 


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Power amplifiers deliver a specific amount of AC power to a load. They are used in audio frequency and radio frequency applications.
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Unijunction transistors (UJT) are three-terminal devices that exhibit a negative resistance characteristic.
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