Notes_concept_of_input_output_impedance

Notes_concept_of_input_output_impedance - ELEC 2507 COURSE...

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ELEC 2507 COURSE NOTES Analog Electronics : Concept of Input imepdance, Ouput impedance, and Voltage Gain Prof. Ram Achar February 4, 2011 Acknowledgements : Venu Reddy 1
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ELEC 2507 COURSE NOTES From circuit theory, an electrical circuit which provides signal amplification while preserving other input signal characteristics such as shape, frequency or time is called as an amplifier. If the amplitude of a voltage signal is amplified then it is called as a voltage amplifier. The most important design parameters of a voltage amplifier that are to be considered are input impedance, output impedance, and gain. Input impedance: Input impedance ( Z i ) is measured as the ratio of the voltage ( V i ) and current ( I i ) at the input terminals of an amplifier. The input impedance is defined as the equivalent impedance looking into the input terminals of an amplifier circuit with all other voltage sources short circuited and current sources open circuited. Ideally, the input impedance of a voltage amplifier should be infinity. Figure 1: Amplifier Equivalent Circuit As illustrated in Figure 1, input impedance is computed as, Z i = V i I i Output impedance: Output impedance ( Z o ) is measured as the ratio of voltage ( V o ) and current ( I o ) at the output terminals of an amplifier. The output impedance is defined as the equivalent impedance looking from the output terminals into the amplifier with all other voltage sources short circuited and current sources open circuited. Ideally, the output impedance of a volt- Page 2 of 8
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ELEC 2507 COURSE NOTES age amplifier should be zero. As illustrated in Figure 1, output impedance is computed as, Z o = V o I o Voltage gain: Voltage gain is measured as the ratio of output voltage ( V o ) to input voltage ( V i ) at their respective terminals of an amplifier.
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