Voltage amplifiers have various uses, but are usually used for boosting smaller voltages to larger ones. Suppose you have a sensor that has a voltage range of 10mv to 50mv. Few microcontrollers can measure this tiny amount! So what you would need to do is amplify this range to say 1V to 5V (since digital logic is usually 5V).
I would like to stress that voltage amplifiers are for amplifying low current/voltage signals (such as from force and torque sensors), not powering things like motors. It is for low current applications only. If you would like to power motors, consider getting power amps. Now that we are clear on uses, lets talk about how to make an amplifier. To make a basic voltage amplifier circuit, all you usually need are two resistors and an op-amp (Operational Amplifier). Op-amps are cheap IC chips available at any electronics distributer, often coming as dual packages and having other neat features.
Op-Amps have a special power source requirement. Typically it is 20V+ and 20V- to the positive and negative pins, respectively. However, your power source does not actually need to exceed your desired voltage output. For example, suppose your desired output is between the range of 0V and 5V. This is typical of any digital circuit for a microcontroller. You would then only need to apply 0V (ground) to the negative pin and 5V to your positive pin. You would usually not actually need a negative voltage unless you were to use an inverting amplifier (described below).
If you DO however need a negative voltage source, you can do this by placing batteries in series, and using the center connection as ground (shown here).
Non-Inverting Amplifier Circuit
Inverting Amplifier Circuit
Voltage Follower (Buffer)
Voltage Difference Amplifier
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