Choose between voltage feedback (VFB) and current feedback (CFB) operational amplifiers
Time:2022-05-26
Views:2142
Author: ADI company
Current feedback and voltage feedback have different application advantages. In many applications, the difference between CFB and VFB is not obvious. Nowadays, many high-speed CFB and VFB amplifiers are equal in performance, but each has its advantages and disadvantages. This guide examines important considerations related to these two topologies.
DC and operation considerations of VFB and CFB operational amplifiers
VFB operational amplifier
VFB operational amplifier is the right choice for precision low-frequency applications requiring high open-loop gain, low offset voltage and low bias current. The input offset voltage of high-speed bipolar input VFB operational amplifier is rarely fine tuned, because the offset voltage matching of the input stage is very excellent, generally 1 to 3 MV, and the offset temperature coefficient is 5 to 15 µ V / ° C. After fine tuning, the input offset voltage below 20 µ V can be realized. The operational amplifier with self stabilizing zero architecture can provide offset voltage lower than 5 µ V, but we will not consider it here. For details of self stabilizing zero operational amplifier, refer to guide mt-055.
The input bias current (no input bias current compensation circuit) on the VFB operational amplifier is roughly equal at the (+) input and (–) input, ranging from 1 to 5 µ a. Some FET input operational amplifiers have an input bias current of less than 200 FA, which is suitable for electrometer and other applications. (e.g. AD549).
The output offset voltage caused by the input bias current can be reset to zero by equal effective source resistance in the inverting and in-phase inputs. This method is not effective for bias current compensated VFB operational amplifiers because there is an additional current error source at the input of such amplifiers. In this case, the net input bias current is not necessarily equal or of the same polarity.
VFB operational amplifiers are very useful in applications where feedback networks control the overall response, such as active filter applications. However, some VFB operational amplifiers are not fully compensated and must exceed their rated minimum closed-loop gain.
The simplified model of VFB operational amplifier is familiar to everyone, which is discussed in all analog electronic 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, we know little about current feedback (CFB) operational amplifiers, and there are few relevant literatures. Many designers choose VFB operational amplifier just because they know it better.
The open-loop gain and accuracy of CFB operational amplifier are generally lower than that of precision VFB operational amplifier.
The inverting and in-phase input impedances of CFB operational amplifiers are not equal, and the input bias current of CFB operational amplifiers is generally unequal and irrelevant, because the (+) input and (–) input have completely different architectures. For this reason, the external bias current cancellation mechanism does not work. The input bias current of CFB ranges from 5 to 15 µ a, which is generally high at the inverting input.
Since the CFB operational amplifier is generally optimized for a fixed feedback resistance value, the flexibility of its feedback network is insufficient in addition to setting the closed-loop gain. This makes the CFB operational amplifier not suitable for most active filters, except the Sallen key filter, because this kind of filter can be designed with appropriate fixed feedback resistance. Figure 1 summarizes the DC and operational considerations of VFB and CFB operational amplifiers.
The CFB architecture does apply to rail to rail inputs and outputs.
DC and operation considerations of VFB and CFB operational amplifiers
VFB operational amplifier
High open loop gain and dc accuracy
Provide low offset voltage (< 20 µ V)
Provide low bias current (JFET, CMOS or bias current compensation) (< 200 FA)
Balanced input impedance
CFB operational amplifier
Low open loop gain and dc accuracy
Higher offset voltage
Low inverting input impedance and high in-phase input impedance
The input bias current is not as low as VFB, and the matching degree is not as good as VFB
For optimum performance, a fixed feedback resistor is required
AC considerations for VFB and CFB operational amplifiers
Flexible feedback network
Provide rail to rail inputs and outputs
CFB operational amplifier
Low open loop gain and dc accuracy
AC considerations for VFB and CFB operational amplifiers
CFB operational amplifier
AC considerations for VFB and CFB operational amplifiers
CFB operational amplifier
Noise considerations for VFB and CFB operational amplifiers
CFB operational amplifier
Noise considerations for VFB and CFB operational amplifiers
CFB operational amplifier
summary
The following advantages can be obtained by selecting CFB operational amplifier
CFB operational amplifier
Low open loop gain and dc accuracy
Higher offset voltage
Low inverting input impedance and high in-phase input impedance
The input bias current is not as low as VFB, and the matching degree is not as good as VFB
For optimum performance, a fixed feedback resistor is required
AC considerations for VFB and CFB operational amplifiers
VFB operational amplifier
A remarkable feature of VFB operational amplifiers is that they can provide constant gain bandwidth product in a wide frequency range.
In addition, there are VFB operational amplifiers with high bandwidth, high slew rate and low distortion on the market, which adopts the "H-bridge" architecture for low quiescent current (Guide mt-056).
VFB operational amplifier is suitable for all kinds of active filter architectures because its feedback network is very flexible.
CFB operational amplifier
CFB topology is mainly used in occasions with high requirements for high bandwidth, high slew rate and low distortion. For a detailed discussion of the AC characteristics of CFB operational amplifiers, refer to guide mt-057.
For a given complementary bipolar IC process, CFB can generally produce higher fpbw (and therefore lower distortion) than VFB at the same amount of quiescent current. This is because there is almost no yaw rate limit for CFB. Therefore, its full power bandwidth and small signal bandwidth are about the same. However, the "H-bridge" architecture used in high-speed VFB operational amplifier is almost equivalent to CFB operational amplifier in performance (Guide mt-056).
Unlike VFB operational amplifiers, CFB operational amplifiers have very low inverting input impedance. This is an advantage when the operational amplifier is used as an I / V converter in inverting mode, because its sensitivity to inverting input capacitance is lower than VFB.
The closed-loop bandwidth of CFB operational amplifier is determined by the value of built-in capacitance and external feedback resistance. Relatively speaking, it is independent of the gain setting resistance (i.e. the resistance from the inverting input to the ground). This makes the CFB operational amplifier an ideal choice for programmable gain applications requiring gain independent bandwidth.
Since CFB operational amplifiers must be used with a fixed feedback resistor to achieve the best stability, their application as active filters is very limited except Sallen key filters.
In CFB operational amplifier, the small stray capacitance on the feedback resistance may lead to instability.
AC considerations for VFB and CFB operational amplifiers
VFB operational amplifier
Constant gain bandwidth product
Provide high slew rate and high bandwidth
Low distortion version available
Flexible feedback network
Suitable for active power filter
CFB operational amplifier
The bandwidth under various closed-loop gains is relatively constant
Gain bandwidth product is not constant
Slightly higher slew rate and bandwidth (compared to VFB) for specific process and power consumption
Low distortion version available
For optimum performance, a fixed feedback resistor is required
Stray feedback capacitance leads to instability
It is difficult to be used in non Sallen key active power filter
Low inverting input impedance reduces the influence of input capacitance in I / V converter applications
Noise considerations for VFB and CFB operational amplifiers
VFB operational amplifier
The input voltage noise of some precision VFB operational amplifiers on the market is less than 1 NV / √ Hz. The input current noise of most JFET or CMOS input VFB operational amplifiers is less than 100 fa / √ Hz, and some are less than 1 fa / √ Hz. However, the total output noise depends not only on these values, but also on the actual values of closed-loop gain and feedback resistance (Guide mt-049).
For VFB operational amplifiers, inverting and in-phase input current noise are generally equal and almost always uncorrelated. The typical value range of broadband bipolar VFB operational amplifier is 0.5 PA / √ Hz to 5 PA / √ Hz. When the input bias current compensation circuit is added, the input current noise of the bipolar input stage will increase because their current noise is not related, so the endogenous current noise of the bipolar stage will be increased (in RRs mode). However, bias current compensation is rarely used in high-speed operational amplifiers.
CFB operational amplifier
The input voltage noise of CFB operational amplifier is generally lower than that of VFB operational amplifier with similar bandwidth. The reason is that the input stage of CFB operational amplifier generally works under high current, which reduces the emitter resistance and leads to the reduction of voltage noise. The typical value range of CFB operational amplifier is 1 to 5 NV / √ Hz.
However, the input current noise of CFB operational amplifier is generally greater than that of VFB operational amplifier because its bias current is generally high. The inverse current noise and in-phase current noise of CFB operational amplifier are usually different, because they adopt a unique input architecture, and they are expressed as independent specification parameters. In most cases, the inverse input current noise is the larger of the two. The typical input current range of CFB operational amplifier is 5 to 40 PA / √ Hz. This may often dominate, except at ultra-high closed-loop gains where voltage noise dominates.
The best way to calculate noise is to write a simple spreadsheet calculation program to automatically calculate, including all noise sources. The equations discussed in guide mt-049 can be used for this purpose.
Noise considerations for VFB and CFB operational amplifiers
VFB operational amplifier
Provide low voltage noise (< 1 NV / √ Hz)
Provide low current noise (JFET and CMOS inputs)
Inverting and in-phase input current noise are equal and uncorrelated
The feedback network and external resistance must be considered when calculating the total noise
CFB operational amplifier
Low voltage noise (1 to 5 NV / √ Hz)
High current noise (5 to 40 PA / √ Hz) is usually the main factor
The feedback network and external resistance must be considered when calculating the total noise
summary
For most general-purpose or high-precision low-frequency and low-noise applications, VFB operational amplifier is usually the best choice. VFB operational amplifiers are also ideal for single power applications, as many of these amplifiers provide rail to rail inputs and outputs.
VFB operational amplifier has a very flexible feedback network, so it is suitable for the design of active power filter.
CFB operational amplifier has the best bandwidth, slew rate and distortion performance, but at the expense of DC performance, noise and the requirement of using fixed value feedback resistance. The application of CFB operational amplifier in active power filter is
limited to Sallen key equivalent phase configuration.
limited to Sallen key equivalent phase configuration.
The following advantages can be obtained by selecting VFB operational amplifier
High precision, low noise and low bandwidth
Rail to rail inputs and outputs
Feedback network flexibility
Active filter
The following advantages can be obtained by selecting CFB operational amplifier
Ultra high bandwidth, slew rate and very low distortion
The bandwidth under different gain is relatively constant
Sallen key active power filter
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