Fully Differential Amplifier - The Ideal FDA

The Ideal FDA

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.

• Input voltage:
• Output voltage:
• Output common-mode voltage:

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.

Where the limitations of real devices can be ignored, an FDA can be viewed as a Black Box with gain; circuit function and parameters are determined by feedback, usually negative. An FDA as implemented in practice is moderately complex integrated circuit.