Choosing Between Voltage Feedback (VFB) and Current Feedback (CFB) Op Amps

Current feedback and voltage feedback have different application advantages. In many applications, the difference between CFB and VFB is not significant. Many of today's high-speed CFB and VFB amplifiers are comparable in performance, but each has its own advantages and disadvantages. This guide will examine the important considerations associated with these two topologies.

DC and Operational Considerations for VFB and CFB Op Amps

VFB Operational Amplifier

• For precision, low-frequency applications that require high open-loop gain, low offset voltage, and low bias current, VFB op amps are the right choice. The input offset voltage of high-speed bipolar input VFB op amps is rarely trimmed because the offset voltage matching of the input stage is very good, typically 1 to 3 mV, and the offset temperature coefficient is 5 to 15 µV/°C. After trimming, input offset voltages below 20 µV can be achieved. Op amps with auto-zero architectures can provide offset voltages below 5 µV, but we will not consider them here. For more information on auto-zero op amps, see Tutorial MT-055.

• The input bias current on a VFB op amp (without input bias current compensation circuitry) is approximately equal at the (+) and (–) inputs and can range from 1 to 5 µA. There are FET-input op amps with input bias currents less than 200 fA, which are useful in applications such as electrometers. (e.g., the AD549).

• The output offset voltage due to input bias current can be nulled by making the effective source resistance in the inverting and noninverting inputs equal. This approach is not valid for bias current compensated VFB op amps because these amplifiers have additional current error sources at their inputs. In this case, the net input bias currents are not necessarily equal or of the same polarity.

• VFB op amps are useful in applications where the feedback network controls the overall response, such as active filter applications. However, some VFB op amps are decompensated and must be used above their minimum rated closed-loop gain.

• The simplified model of the VFB op amp is well known and is discussed in all analog electronics textbooks.

• The VFB architecture is suitable for low supply voltage applications that require rail-to-rail input and output.

CFB Operational Amplifier

• On the other hand, current feedback (CFB) op amps are less well understood and less well documented. Many designers choose VFB op amps simply because they are more familiar with them.

• The open-loop gain and accuracy of CFB op amps are generally lower than those of precision VFB op amps.

• The inverting and non-inverting input impedances of a CFB op amp are not equal, and the input bias currents of a CFB op amp are generally unequal and uncorrelated because the (+) and (–) inputs have completely different architectures. For this reason, external bias current cancellation mechanisms are also ineffective. CFB input bias currents range from 5 to 15 µA and are generally higher at the inverting input.

• Since CFB op amps are typically optimized for a fixed feedback resistor value, they have limited feedback network flexibility beyond setting the closed-loop gain. This makes CFB op amps unsuitable for most active filters, with the exception of the Sallen-Key filter, which can be designed with a suitable fixed feedback resistor. Figure 1 summarizes the DC and operational considerations for VFB and CFB op amps.

• The CFB architecture is indeed suitable for rail-to-rail input and output.

DC and Operational Considerations for VFB and CFB Op Amps

• VFB Operational Amplifier

• High open-loop gain and DC accuracy

• Provides low offset voltage (<20 µV)

• Provides low bias current (JFET, CMOS, or bias current compensation) (<200 fA)

• Balanced input impedance

• Flexible feedback network

• Provides rail-to-rail input and output

• CFB Operational Amplifier

• Lower open-loop gain and DC accuracy

• Higher offset voltage

• The inverting input impedance is low, and the non-inverting input impedance is high

• Input bias current is not as low as VFB and is not as well matched as VFB

• Optimum performance requires fixed feedback resistors

AC Considerations for VFB and CFB Op Amps

VFB Operational Amplifier

• A notable feature of VFB op amps is that they provide a constant gain-bandwidth product over a wide frequency range. In addition, there are high-bandwidth, high-slew-rate, low-distortion VFB op amps on the market that use an “H-bridge” architecture for low quiescent current (Tutorial MT-056).

• VFB op amps are suitable for various active filter architectures because their feedback networks are very flexible.

CFB Operational Amplifier

• The CFB topology is primarily used in applications where high bandwidth, high slew rate, and low distortion are extremely important. For a detailed discussion of the AC characteristics of CFB op amps, refer to Tutorial MT-057.

• For a given complementary bipolar IC process, CFB generally produces a higher FPBW (and therefore lower distortion) than VFB for the same amount of quiescent current. This is because CFB has virtually no slew rate limitations. For this reason, its full-power bandwidth and small-signal bandwidth are about the same. However, the “H-bridge” architecture used in high-speed VFB op amps is nearly equivalent in performance to CFB op amps (Tutorial MT-056).

• Unlike VFB op amps, CFB op amps have very low inverting input impedance. This is an advantage when using the op amp as an I/V converter in inverting mode because it is less sensitive to inverting input capacitance than VFB.

• The closed-loop bandwidth of a CFB op amp is determined by the value of the internal capacitors and the external feedback resistors and is relatively independent of the gain setting resistors (i.e., the resistor from the inverting input to ground). This makes CFB op amps ideal for programmable gain applications that require gain-independent bandwidth.

• Because CFB op amps must be used with a fixed feedback resistor for optimum stability, their applications as active filters are very limited, except for Sallen-Key filters.

• In a CFB op amp, small values ​​of stray capacitance across its feedback resistor may cause instability.

AC Considerations for VFB and CFB Op Amps

• VFB Operational Amplifier

• Constant gain-bandwidth product

• Provides high slew rate and wide bandwidth

• Low distortion version available

• Flexible feedback network

• Suitable for active filters

• CFB Operational Amplifier

• The bandwidth is relatively constant at various closed-loop gains

• Gain-Bandwidth Product is not constant

• Slightly higher slew rate and bandwidth (compared to VFB) for specific processes and power consumption

• Low distortion version available

• Optimum performance requires fixed feedback resistors

• Stray feedback capacitance causes instability

• Difficult to use with non-Sallen-Key type active filters

• Low inverting input impedance reduces input capacitance effects in I/V converter applications

Noise Considerations for VFB and CFB Op Amps

VFB Operational Amplifier

• Some precision VFB op amps on the market have input voltage noise of less than 1 nV/√Hz. Most JFET or CMOS input VFB op amps have input current noise less than 100 fA/√Hz, and some have less than 1 fA/√Hz. However, the total output noise depends not only on these values, but also on the closed-loop gain and the actual value of the feedback resistor (Tutorial MT-049).

• For VFB op amps, the inverting and noninverting input current noises are generally equal and almost always uncorrelated. Typical values ​​for wideband bipolar VFB op amps range from 0.5 pA/√Hz to 5 pA/√Hz. The input current noise of a bipolar input stage improves when input bias current compensation circuitry is added because their current noises are uncorrelated and thus add (in an RRS manner) to the intrinsic current noise of the bipolar stage. However, bias current compensation is rarely used in high speed op amps.

CFB Operational Amplifier

• The input voltage noise in a CFB op amp is generally lower than that of a VFB op amp with similar bandwidth. This is because the input stage in a CFB op amp is generally operated at a higher current, which reduces the emitter resistance and, as a result, reduces the voltage noise. Typical values ​​for CFB op amps range from 1 to 5 nV/√Hz.

• However, the input current noise of CFB op amps is generally greater than that of VFB op amps because their bias currents are generally higher. The inverting and non-inverting current noise of CFB op amps are usually different because they use a unique input architecture and are expressed as separate specifications. In most cases, the inverting input current noise is the larger of the two. The typical input current range for CFB op amps is 5 to 40 pA/√Hz. This can often dominate, except at very high closed-loop gains where voltage noise dominates.

The best way to calculate the noise is to write a simple spreadsheet calculation program to automatically perform the calculation, including all noise sources. The equations discussed in Tutorial MT-049 can be used for this purpose.

Noise Considerations for VFB and CFB Op Amps

• VFB Operational Amplifier

• Provides low voltage noise ( < 1 nV/√Hz)

• Provides low current noise (JFET and CMOS inputs)

• Inverting and non-inverting input current noise are equal and uncorrelated

• The feedback network and external resistor values ​​must be considered when calculating the total noise