Provide low bias current (JFET, CMOS or bias current compensation) (< 200 FA)
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
Provide rail to rail inputs and outputs
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.
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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