The current-feedback operational amplifier (CFOA or CFA) is a type of

electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...

whose inverting input is sensitive to

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

, rather than to

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

as in a conventional voltage-feedback

operational amplifier An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...

(VFA). The CFA was invented by David Nelson at Comlinear Corporation, and first sold in 1982 as a hybrid amplifier, the CLC103. An early patent covering a CFA is , David Nelson and Kenneth Saller (filed in 1983). The

integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

CFAs were introduced in 1987 by both Comlinear and Elantec (designer Bill Gross). They are usually produced with the same pin arrangements as VFAs, allowing the two types to be interchanged without rewiring when the circuit design allows. In simple configurations, such as linear amplifiers, a CFA can be used in place of a VFA 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 CFA, but the

common-mode rejection ratio In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both i ...

is poorer than that from a VFA.

Operation

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.

Resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...

s R1–R4 set up the quiescent 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 to minimise offsets. With no signal applied, due to the

current mirror A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and ...

s 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 (equivalently, no voltage will be present at the buffer's input). In practice, due to device mismatches, the collector currents are unequal, resulting in the difference flowing into the buffer's input and 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. ''C''_s is a stability capacitor to ensure that the circuit remains stable for all operating conditions. Due to the wide open-loop bandwidth of a CFA, there is a high risk of the circuit breaking into oscillations. ''C''_s ensures that frequencies, where oscillations might start are attenuated, especially when running with a low closed-loop gain. The output stage (in magenta) is a buffer that provides current gain. It has a voltage gain of unity (+1 in the schematic).

Voltage-feedback amplifier comparison

Internally compensated VFA bandwidth is dominated by an internal dominant pole compensation capacitor, resulting in a constant gain/bandwidth limitation. CFAs also have a dominant pole compensation capacitor, but due to using current feedback instead of voltage feedback, the resulting open loop response is different. VFA stability depends on the ratio of open loop gain to feedback gain; CFA stability depends on the ratio of open loop transimpedance to feedback resistance. VFAs have a gain/bandwidth dependence; CFAs have a transimpedance/feedback resistance dependence. In VFAs, dynamic performance is limited by the gain-bandwidth product and the slew rate. CFAs use a circuit topology that emphasizes current-mode operation, which is inherently much faster than voltage-mode operation because it is less prone to the effect of stray node-capacitances. When fabricated using high-speed complementary bipolar processes, CFAs can be orders of magnitude faster than VFAs. This is largely due to most VFAs being compensated for stability at unity gain. Decompensated VFAs can be just as fast as CFAs. With CFAs, the amplifier gain may be controlled independently of bandwidth. This constitutes the major advantages of CFAs over conventional VFA topologies. Disadvantages of CFAs include poorer input offset voltage and input bias current characteristics. Additionally, the DC loop gains are generally smaller by about three decimal orders of magnitude. CFAs have much higher inverting input current noise. CFA circuits must use a specific value of feedback resistance to achieve maximum performance. A lower value of feedback resistance can make the amplifier oscillate. CFA circuits must never include a direct capacitance between the output and inverting input pins as this often leads to oscillation. CFAs are ideally suited to very high speed applications with moderate accuracy requirements. Development of faster VFAs is ongoing, and VFAs are available with gain-bandwidth products in the low UHF range at the time of this writing. However, CFAs are available with gain-bandwidth products more than an octave higher than their VFA cousins and are also capable of operating as amplifiers very near their gain-bandwidth products.

References

{{Reflist
"Current Feedback Amplifiers"
by Erik Barnes of Analog Devices Inc.
"Op Amps for Everyone Design Guide (Rev. B)"
by Ron Mancini of

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Inc. Electronic amplifiers

Operation

Voltage-feedback amplifier comparison

See also

Further reading

References