A differential amplifier has two inputs, one inverting and one non-inverting. Modern diff amps usually sit on a single chip. Inside the microchip or pair of discrete transistors, the positive and negative signals are added and the resultant becomes the single output. The difference between these two input voltages is amplified, while any common-mode voltage is rejected and does not become part of the output. The relevant formula is:
Vout = A(V+in –V–in )
Where A is the differential gain, inherent in the device.
This equation applies to an ideal differential amplifier, but there may be a certain amount of common-mode gain in this imperfect world.
A principle application is to eliminate noise, which appears as a common-mode fluctuating voltage. Because common-mode may be held at a low level, the differential amplifier is quite effective in eliminating noise as well as undesirable bias voltages that are present in both inputs.
A differential amplifier may be configured to operate as a single-ended amplifier simply by grounding one of the inputs.
The first differential amplifiers were built in the 1930s using vacuum tubes. In the following decade transistors emerged and, like other semiconductors, they quickly became the dominant technology. Solid-state differential amplifiers were built using a pair of transistors. This arrangement was called a long-tailed pair because of the high-ohm resistors attached. Currently a convenient way to implement the circuit is by means of an IC.
An operational amplifier is a type of differential amplifier that has high differential-mode gain, high input impedance and low output impedance. These are desirable qualities because they help minimize inter-stage loading. However, the astronomical gain makes for instability. To improve the situation, negative feedback is employed and the gain is reduced to a sensible level.
Like all amplifiers, the op amp has less than infinite bandwidth. In other words, the gain falls off as the frequency rises. In conjunction with the high dc gain, the op amp functions as a low-pass filter.
One result of the limited bandwidth is that input and output are not altogether in phase, making for oscillation in some implementations. (Because of the phase delay, a 180° out-of-phase feedback signal adds to the amplified signal to the point where sine-wave oscillation takes over.
Use of a higher-bandwidth op amp can help mitigate this problem. Extreme cases might make use of a current-feedback op amp.
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