Fully differential amplifier

A fully differential amplifier (FDA) is a DC-coupled high-gain electronic voltage amplifier with differential inputs and differential outputs. In its ordinary usage, the output of the FDA is controlled by two feedback paths which, because of the amplifier's high gain, almost completely determine the output voltage for any given input.

In a fully differential amplifier, common-mode noise such as power supply disturbances is rejected; this makes FDAs especially useful as part of a mixed-signal integrated circuit.

An FDA is often used to convert an analog signal into a form more suitable for driving into an analog-to-digital converter; many modern high-precision ADCs have differential inputs.

For any input voltages the ideal FDA has infinite open-loop gain, infinite bandwidth, infinite input impedances resulting in zero input currents, infinite slew rate, zero output impedance and zero noise.

In the ideal FDA, the difference of the output voltages is equal the difference between the input voltages multiplied by the gain. The common mode voltage of the output voltages is not dependent of the input voltage. In many cases, the common mode voltage can be directly set by a third voltage input.

A real FDA can only approximate this ideal, and the actual parameters are subject to drift over time and with changes in temperature, input conditions, etc. Modern integrated FET or MOSFET FDAs approximate more closely to these ideals than bipolar ICs where large signals must be handled at room temperature over a limited bandwidth; input impedance, in particular, is much higher, although the bipolar FDA usually exhibit superior (i.e., lower) input offset drift and noise characteristics.

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