Fully Differential Operational Amplifiers. Properties of Fully Differential Amplifiers. Small-Signal Models for Balanced Differential Amplifiers. Common-Mode Feedback. Common-Mode Feedback at Low Frequencies. Stability and Compensation Considerations in a CMFB Loop. CMFB Circuits. Fully Differential Op Amps. Unbalanced Fully Differential Circuits. Bandwidth of the CMFB Loop.
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.
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 input resistance — this puts an upper bound on the resistances in the feedback circuit.
* Nonzero output resistance — important for low resistance loads. Except for very small voltage output, power considerations usually come into play first. (Output impedance is inversely proportional to the idle current in the output stage — very low idle current results in very high output impedance.)
* Input bias current — a small amount of current (typically ~10 nA for bipolar FDAs, or picoamperes for CMOS designs) flows into the inputs. This current is mismatched slightly between the inverting and non-inverting inputs (there is an input offset current). This effect is usually important only for very low power circuits.
* Input offset voltage — the FDA will produce an output even when the input pins are at exactly the same voltage. For circuits which require precise DC operation, this effect must be compensated for.
* Common mode gain — A perfect operational amplifier amplifies only the voltage difference between its two inputs, completely rejecting all voltages that are common to both. However, the differential input stage of an FDA is never perfect, leading to the amplification of these identical voltages to some degree. The standard measure of this defect is called the common-mode rejection ratio (denoted, CMRR). Minimization of common mode gain is usually important in non-inverting amplifiers (described below) that operate at high amplification.
* Temperature effects — all parameters change with temperature. Temperature drift of the input offset voltage is especially important.
* Finite bandwidth — all amplifiers have a finite bandwidth. This is because FDAs use internal frequency compensation to increase the phase margin.
* Input capacitance — most important for high frequency operation because it further reduces the open loop bandwidth of the amplifier.
* Common mode gain — See DC imperfections, above.
* Noise - all real electronic components (except superconductor) generate noise. You can find devices with 0.8 to several hundreds nv/rtHz noise performance.
* 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.
* Slewing — the amplifier's output voltage reaches its maximum rate of change. Measured as the slew rate, it is usually specified in volts per microsecond. When slewing occurs, further increases in the input signal have no effect on the rate of change of the output. Slewing is usually caused by internal capacitances in the amplifier, especially those used to implement its frequency compensation, particularly using pole splitting.
* Non- linear transfer function — The output voltage may not be accurately proportional to the difference between the input voltages. It is commonly called distortion when the input signal is a waveform. This effect will be very small in a practical circuit if substantial negative feedback is used.
* 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.
* Limited output current — the output current must obviously be finite. In practice, most op-amps are designed to limit the output current so as not to exceed a specified level thus protecting the FDA and associated circuitry from damage.
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.