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.
The current feedback operational amplifier or CFB op-amp is a type of electronic amplifier whose inverting input is sensitive to current, rather than to voltage as in a conventional voltage-feedback (VFB) operational amplifier. The CFB op-amp was invented by David Nelson at Comlinear Corporation, and first sold in 1982 as a hybrid amplifier, the CLC103. The first patent covering a CFB op-amp is , David Nelson and Kenneth Saller (filed in 1983). The first integrated circuit CFB op-amps were introduced in 1987 by both Comlinear and Elantec (designer Bill Gross). They are usually produced with the same pin arrangements as VFB op-amps, allowing the two types to be interchanged without rewiring when the circuit design allows. In simple configurations, such as linear amplifiers, a CFB op-amp can be used in place of a VFB op-amp with no circuit modifications, but in other cases, such as integrators, a different circuit design is required. The classic four-resistor differential amplifier configuration also works with a CFB op-amp, but the common-mode rejection ratio is poorer than that from a VFB op-amp.
Referring to the schematic shown, the section marked in red forms the input stage and error amplifier. The inverting input (node where emitters of Q1 & Q2 are connected) is low-impedance and hence sensitive to changes in current. Resistors R1–R4 set up the quiescient bias conditions and are chosen such that the collector currents of Q1 & Q2 are the same. In most designs, active biasing circuitry is used instead of passive resistive biasing, and the non-inverting input may also be modified to become low impedance like the inverting input in order to minimise offsets.
With no signal applied, due to the current mirrors Q3/Q4 & Q5/Q6, the collector currents of Q4 and Q6 will be equal in magnitude if the collector currents of Q1 and Q2 are also equal in magnitude. Thus, no current will flow into the buffer's input (or equivalently no voltage will be present at the buffer's input). In practice, due to device mismatches the collector currents are unequal and this results in the difference flowing into the buffer's input resulting in an offset at its output. This is corrected by adjusting the input bias or adding offset nulling circuitry.
The section marked in blue (Q3–Q6) forms an I-to-V converter. Any change in the collector currents of Q1 and Q2 (as a result of a signal at the non-inverting input) appears as an equivalent change in the voltage at the junction of the collectors of Q4 and Q6. Cs is a stability capacitor to ensure that the circuit remains stable for all operating conditions. Due to the wide open-loop bandwidth of a CFB amplifier, there is a high risk of the circuit breaking into oscillations. Cs ensures that frequencies where oscillations might start are attenuated, especially when running with a low closed-loop gain.
The output stage (in cyan) is a buffer which provides current gain. It has a voltage gain of unity (+1 in the schematic).
VFB and CFB compared
The main reasons for choosing a CFB op-amp are to obtain a greater slew rate, and to avoid the constant gain-bandwidth product of VFB types. The bandwidth of a CFB op-amp depends only on the value of its feedback resistor. It is a common misconception that the CFB has an inherently higher bandwidth.
When CFB op-amps were first introduced, their bandwidths were huge compared to those of VFB types, making them desirable in high-frequency applications such as video and radiofrequency amplifiers adding to their reputation as high bandwidth amplifiers. Early models also had a reputation for instability, as the slightest parasitic capacitance at their inverting inputs caused them to oscillate. As the products matured, and circuit designers gained experience, the CFB op-amp became accepted as a standard circuit component. Meanwhile, the designers of VFB op-amps were forced to improve the bandwidths of their products, with the result that very fast VFB op-amps are now available.
CFB op-amps have very high slew rates, making them useful in video amplification where slew-rate limitation leads to distortion.
One of the trade-offs that designers must consider when choosing CFB op-amps is that these devices have high DC offsets. This makes them less suitable for high-precision or high-gain applications such as instrumentation amplifiers, and measuring instruments in general.
CFB op-amps also have higher current noise than VFB types, precluding their use as photodiode amplifiers. In other applications, their low voltage noise can be an advantage.
Finally, CFB op-amps are not optimised for single-supply operation, since the traditional output stage cannot swing closer than about 1.3V to the supply or ground.