﻿ Frequency Response - Amplifier And Active Filters

# Frequency Response

Wednesday, August 5th, 2020 - Analog Electronics

The frequency response of an amplifier is a plot of the way that the voltage gain varies with frequency. An example is shown in the picture below. Here, the measurement of voltage gain has been repeated at other frequencies to find the frequency range, or band, of the amplifier. Notice that such plots usually state the voltage gain in decibels, so that the scale is effectively logarithmic. The frequency is commonly plotted on a logarithmic scale too. This has the advantage that each decade (10:1 range) of frequency takes up the same amount of space, so many decades can be shown without cramping the detail at the lower frequencies. This is an example of the Bode plots. A complete description of the frequency response includes a plot of the way that the phase shift between output and input varies with frequency. This uses the same frequency scale as the voltage gain, but the phase shift is always plotted linearly.

## Frequency Response

Most amplifiers have a frequency response which is deliberately restricted to a specified frequency band appropriate for the amphfier’s intended purpose. For instance, audio amplifiers for music have a typical range of 20 Hz to 20 kHz, over which the voltage gain plot is level to within 1 or 3 dB. This exceeds the frequency response of the human ear by a margin of about 50% or so, to make sure that none of the most significant frequencies in the music are lost.

### Amplifier Frequency Response

An Amplifier Frequency Response

### Frequency Responses Of Active Filters

(a) low-pass; (b) band-pass; (c) high-pass

Some amplifiers are designed primarily to block or ‘stop’ specified frequency ranges, and ‘pass’ others. These are called ‘active filters’. They filter out unwanted frequencies and amplify the required frequencies. They have three types of frequency response namely the low-pass, the band-pass and the high-pass. The reason for these names should be obvious from the picture above. Active filters are used mainly at low frequencies, up to a few megahertz. At higher frequencies filters are made with inductors and capacitors but, at low frequencies, the size and cost of inductors make active filters a better choice for maximum frequency response.