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.

#### 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.