Amplifier Frequency Effects


Frequency response of an amplifier is the graph of its Gain versus the Frequency.

  • The middle range of frequency is where amplifier is normally operated.
  • At low frequencies coupling and bypass capacitor acts as high capacitive reactance circuit so most of the AC signal is droped resulting loss of voltage gain.
  • At high frequencies voltage gain reduces because of:
    1. Internal capacitance across transistor junction and 
    2. Strong wiring capacitance.
  • Internal capacitance provide bypass paths for the ac signal as the frequency increases, the capacitive reactance become low enough to prevent normal transistor action.
  • Any connecting wire in a transistor circuit acts like one plate of capacitor and the charging ground acts like the other plate.
  • The strong winding capacitance that exists between this wire and ground is unwanted.
  • At higher frequencies, its low capacitive reactance prevents the ac current from reaching the load resistor and voltage gain drops OFF.

Cut off Frequency

  • The frequencies at which the voltage gain equals 0.707 of its maximum value are called the Cut Off Frequencies.
  • Cut Off Frequencies are also called half power frequencies because the load power is half of its maximum value at these frequencies.

Impedance Matching

In many communication system (Microwave, TV and Telephone) all impedance are matched i.e. Ra=Rin=Rout=R. Because this produces maximum power transfer.

  • Microwave Communication: R = 50 Ω
  • TV Coaxial Cable: R = 75 Ω
  • TV Twin Lead: R = 300 Ω
  • Telephone: R = 600 Ω
Note: If the Power Gain doubles, the decibel power gain increases by 3 dB and if the Voltage Gain doubles, the decibel voltage gain increases by 6 dB.

Reference:

Albert Malvino and David J Bates, “Electronic Principles”, 7th Edition, TATA McGRAW HILL.

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