Amplifier Bandwidth

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Analogue Bandwidth

Measured in Hertz / Kilohertz / Megahertz / Gigahertz

Voltage Bandwidth is the range of frequencies over which the output voltage is at least 0.7 times the maximum output voltage.
In the image below, the 70% output voltage bandwidth goes from 2 or 3 Hz up to about 20000Hz.
This is a 20 kHz (19998 Hz) bandwidth. The red arrow heads show this.
This would be typical for an audio amplifier with a good bass response.
Human hearing works for frequencies between 20 and 20,000 Hz.
AM radio limits the bandwidth to 100 - 4000 Hz.
Land-line phones limit the bandwidth to 300 - 3000 Hz.
Bandwidth tells you the frequency range over which a circuit or transmission medium will operate.
Bandwidth is measured in Hz, kHz, MHz, GHz, etc.

Power Bandwidth is the range of frequencies over which the output power is at least 0.5 times the maximum output power.
On the Y Axis the Power is 50%.

Normal human hearing covers the range from 20 to 20000 Hz. The upper limit drops with age and damage due to exposure to very loud sounds.
AM radio limits the bandwidth to 4000 Hz. This allows more stations to share the radio band but the sound quality is less good.
Old-style telephones limit the bandwidth to 3000 Hz. This is just sufficient for intelligible speech. Mobile phones do a bit better.

Measured in bits per second.

Example: An Internet connection.

using a modem 56kb/s is possible - this allows text, poor quality audio and slow image downloads.
using ADSL Broadband, the bandwidth is between 512 Kb/s to 8Mb/s - this allows good quality audio, fast image downloads and video in a window the size of a postage stamp.
faster versions of ADSL up to 100Mb/s are being rolled out (2011) - this will allow full screen high quality video.
Note that b = bit and B = Byte. These are frequently confused.