Noise Comparisons between VFB and CFB Op Amps

previous High Speed Current-to-Voltage Converters, and the Effects of Inverting Input Capacitance
In most applications of high speed op amps, it is the total output RMS noise that is generally of interest. Because of the high bandwidths involved, the chief contributor to the output RMS noise is therefore the white noise, and the 1/f noise is negligible.
Typical high speed op amps with bandwidths greater than 150MHz or so, and bipolar VFB input stages have input voltage noises ranging from about 2 to 20nV/√Hz.
For a VFB op amp, the inverting and non-inverting input current noise are typically equal, and almost always uncorrelated. Typical values for wideband VFB op amps range from 0.5pA/√Hz to 5pA/√Hz. The input current noise of a bipolar input stage is increased when input bias-current compensation generators are added, because their current noise is not correlated, and therefore adds (in an RSS manner) to the intrinsic current noise of the bipolar stage. However, bias current compensation is rarely used in high speed op amps.
The input voltage noise in CFB op amps tends to be lower than for VFB op amps having the same approximate bandwidth. This is because the input stage in a CFB op amp is usually operated at a higher current, thereby reducing the emitter resistance and hence the voltage noise. Typical values for CFB op amps range from about 1 to 5nV/√Hz.
The input current noise of CFB op amps tends to be larger than for VFB op amps because of the generally higher bias current levels. The inverting and non-inverting current noise of a CFB op amp is usually different because of the unique input architecture, and are specified separately. In most cases, the inverting input current noise is the larger of the two. Typical input current noise for CFB op amps ranges from 5 to 40pA/√Hz. This can often be dominant, except in cases of very high gain, when R1 is small.
The noise sources which dominate the output noise are highly dependent on the closed-loop gain of the op amp and the values of the feedback and feedforward resistors. For high values of closed-loop gain, the op amp voltage noise will tend be the chief contributor to the output noise. At low gains, the effects of the input current noise must also be considered, and may dominate, especially in the case of a CFB op amp.
Feedforward/feedback resistors in high speed op amp circuits may range from less than 100Ω to more than 1kΩ, so it is difficult to generalize about their contribution to the total output noise without knowing the specific values and the closed-loop gain.
The best way to make the noise calculations is to write a simple computer program that performs the calculations automatically, and include all the noise sources. The equation previously discussed can be used for this purpose (see Fig. 1-74, again). In most high speed op amp applications, the source impedance noise can often be neglected for source impedances of 100Ω or less.
High speed op amp noise summary
Figure 1-121: High speed op amp noise summary
Figure 1-121 above summarizes the noise characteristics of high speed op amps
DC Characteristics of High Speed Op Amps
High speed op amps are optimized for bandwidth and settling time, not for precision DC characteristics as found in lower frequency precision op amps. In spite of this, however, high speed op amps do have reasonably good DC performance.
Input offset voltages of high speed bipolar input op amps are rarely trimmed, since offset voltage matching of the input stage is excellent, typically ranging from 1 to 3mV, with offset temperature coefficients of 5 to 15μV/°C.
Input bias currents on VFB op amps (with no input bias current compensation circuits) are approximately equal for (+) and (–) inputs, and can range from 1 to 5μA. The output offset voltage due to the input bias currents can be nulled by making the effective source resistance, R3, equal to the parallel combination of R1 and R2.
As previously discussed, this scheme will not work with bias-current compensated VFB op amps which have additional current generators on their inputs. In this case, the net input bias currents are not necessarily equal or of the same polarity.
CFB op amps generally have unequal and uncorrelated input bias currents because the (+) and (–) inputs have completely different architectures. For this reason, external bias current cancellation schemes are also ineffective. CFB input bias currents range from 5 to 15μA, being generally higher at the inverting input.
High speed op amp offset voltage summary
Figure 1-122: High speed op amp offset voltage summary
Figure 1-122 above summarizes the offset considerations for high speed op amps.
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