A fully differential amplifier, usually referred to as an 'FDA' for brevity, 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 determines the output voltage for any given input.
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 to the product of the difference of 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: Vid = (Vin+) – (Vin–)
Output voltage: Vod = (Vout+) – (Vout–) = Vid * Gain
Output common-mode voltage: Voc = ((Vout+)-(Vout–))/2
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.
Limitations of real FDAs
Finite gain — the effect is most pronounced when the overall design attempts to achieve gain close to the inherent gain of the FDA.
Finite bandwidth — all amplifiers have a finite bandwidth. This is because FDAs use internal frequency compensation to increase the phase margin.
Saturation — output voltage is limited to a peak value, usually slightly less than the power supply voltage. Saturation occurs when the differential input voltage is too high for the op-amp's gain, driving the output level to that peak value.
Limited output power — if high power output is desired, an op-amp specifically designed for that purpose must be used. Most op-amps are designed for low power operation and are typically only able to drive output resistances down to 2 kΩ.
Open-loop gain is defined as the amplification from input to output without any feedback applied. For most practical calculations, the open-loop gain is assumed to be infinite; in reality it is obviously not. Typical devices exhibit open-loop DC gain ranging from 100,000 to over 1 million; this is sufficiently large for circuit gain to be determined almost entirely by the amount of negative feedback used. Op-amps have performance limits that the designer must keep in mind and sometimes work around. In particular, instability is possible in a DC amplifier if AC aspects are neglected.
The FDA gain calculated at DC does not apply at higher frequencies. To a first approximation, the gain of a typical FDA is inversely proportional to frequency. This means that an FDA is characterized by its gain-bandwidth product. For example, an FDA with a gain bandwidth product of 1 MHz would have a gain of 5 at 200 kHz, and a gain of 1 at 1 MHz. This low-pass characteristic is introduced deliberately, because it tends to stabilize the circuit by introducing a dominant pole. This is known as frequency compensation.
Typical low cost, a general purpose FDA exhibits a gain bandwidth product of a few megahertz. Specialty and high speed FDAs can achieve gain bandwidth products of hundreds of megahertz. Some FDAs are even capable of gain bandwidth products greater than a gigahertz.
Operational amplifier applications