Frequency response of an amplifier

Fig. 9-1: Amplifier output voltage in terms of frequency

Fig. 9-1 shows the typical frequency response of an amplifier. At low frequencies the output voltage decreases because of coupling and bypass capacitors. At high frequencies, the output voltage decreases because of transistor and stray wiring capacitance.

Critical frequencies:

Where the output voltage is 0.707 of V_max.
Two critical frequencies -> f₁, f₂

Midband:

Is the band of frequencies between 10 * f₁ and 0.1 * f₂.
The midband is where an amplifier is supposed to be operated.

Ex: Find the midband of an amplifier with f₁ = 5 Hz and f₂ = 100 KHz.

10 * f₁ = 10 * 5 Hz = 50 Hz -- > lower end

0.1 * f₂ = 0.1 * 100 KHz = 10 KHz -- > upper end

Midband: 50 Hz - 10 KHz