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: i) Internal capacitance across transistor junction and ii) 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.
- 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.
Author: Mr. Amarjeet Singh Jamwal, Assistant Loco Pilot, Ambala Division, Indian Railway.
Reference: Albert Malvino and David J Bates, “Electronic Principles”, 7th Edition, TATA McGRAW HILL.
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