A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs and one output. The formula that describes the behavior of the circuit is Vout = A * (Vin+ - Vin-), where A is the gain of the amplifier. Differential amplifiers are useful in electrically noisy environments where a low amplitude electrical signal can be easily corrupted by the effect of unwanted external noise. They are used at the front end of analog systems being used to detect a low amplitude electrical signal, such as an ECG monitor to detect heart rate where the electrical signal may be of the order of only a few millivolts. The purpose of the differential amplifier is to increase the amplitude of the heart signal to a level where it can be converted into a digital form. Differential amplifiers are also found in many circuits that utilize series negative feedback, where one input is used for the input signal, and the other is used for the feedback signal. They are used as the input stage emitter coupled logic gates and as a switch. Differential amplifiers are an essential building block in a wide range of electronic systems, from audio equipment to data acquisition systems.