Fully differential amplifier

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

• Input voltage: ${\displaystyle V_{\mathrm {id} }=V_{\mathrm {in+} }-V_{\mathrm {in-} }}$[5]
• Output voltage: ${\displaystyle V_{\mathrm {od} }=V_{\mathrm {out+} }-V_{\mathrm {out-} }=V_{\mathrm {id} }\times \mathrm {Gain} }$[5]
• Output common-mode voltage: ${\displaystyle V_{\mathrm {oc} }={\frac {(V_{\mathrm {out+} })+(V_{\mathrm {out-} })}{2}}}$[5]

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.

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 workaround. 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.

A typical low-cost general-purpose FDA will have 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.

• Active filter
• Analog computer
• Current-feedback operational amplifier
• Instrumentation amplifier
• Operational amplifier applications
• Operational transconductance amplifier

The Wikibook Electronics has a page on the topic of: Op-Amps#fully_differential_amplifier

• Fully-Differential Amplifiers (TI Application Report)
• Fully Differential Amplifiers (TI Analog Applications Journal)