How a Differential Amplifier Works

Transistors

• No amplifier circuit would be able to work without transistors. A transistor is a circuit component that works kind of like a variable switch. It has three connections, or leads: the base, collector and emitter. When there is a small amount of voltage supplied to the base, current flows from base to emitter, and to a much larger extent, from collector to emitter. The collector-emitter current is around 100 times the base-emitter current. You could think of the transistor as a kind of current amplifier.

Simple Differential Amplifier

• In a simple differential amplifier, the power source sends current through two parallel branches. Each branch is connected to one of the two output leads and goes through the collector and emitter of a transistor. The bases of the transistors are connected to the two input voltages. As the input voltage on each branch changes, it creates an amplified current change -- and therefore voltage change -- through the transistor. In this way, the voltage difference between the two output leads is the amplified difference in voltage between the inputs.

Operational Amplifiers

• This simple differential amplifier is the backbone behind a whole class of amplifier circuits known as operational amplifiers, or op-amps for short. On its own, the basic op-amp will amplify the input voltage difference way too much, so more circuit components are usually added to keep it under control. There are lots of ways to add feedback loops, and other structures using more resistors and transistors to create amplifiers that are suitable for practical purposes.

Practical Differential Amplifier

• The term ̶0;differential amplifier̶1; is typically used to describe a kind of op-amp, rather than the basic circuit utilized in all op-amps. Many op-amps use only one input, with the other connected to ground, so that the output is just the amplified input voltage. In a differential amplifier, two different input signals are connected to the inputs, so that it amplifies their difference. Also, a wire with a resistor connects the output to one of the inputs, creating a feedback loop that limits the output voltage to a practical range.