Amplifiers have one or more of the following properties.

  • The output voltage may be bigger than the input voltage
  • The output current may be bigger than the input current
  • The output power may be bigger than the input power
  • High input impedance (AC equivalent of resistance)
  • Low output impedance (AC equivalent of resistance)

 

Voltage Gain = Vout / Vin

 

Power Gain = Powerout / Powerin

 

Current Gain = Currentout / Currentin

 

Examples

Microphone Pre-amplifier

This has a high input impedance, a low output impedance and a useful voltage gain. The gain depends on the type of microphone and on whether the power amplifier sub-system provides any additional voltage gain.

Voltage Follower used as a Voltage Regulator

A zener diode is used to provide a reference voltage. The voltage regulator provides any required output current up to its rated maximum without the output voltage significantly changing. The voltage gain is 1.

Power Amplifier (audio or radio frequency)

This sub-system can have a voltage gain as low as 1 but the output current is generally much larger than the input current.

Radio Frequency Amplifier

This has a high input impedance, a lower output impedance and may provide voltage or power gain. It is used to

  • boost weak radio signals before they are processed by later sub-systems in a receiver
  • increase the output power of a transmitter (possibly up to megawatt levels(interesting electronics here))

Push Pull Amplifier

One MOSFET or transistor amplifies the positive half of the input signal. A second MOSFET or transistor amplifies the negative half of the input signal. The outputs from both devices are combined. This is also called Class B amplification.