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Bias Current Compensated Bipolar Input Stage

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A simple bipolar input stage such as used in Fig. 1-22 exhibits high bias current because the currents seen externally are in fact the base currents of the two input transistors.
By providing this necessary bias currents via an internal current source, as in Figure 1-23 below, the only external current then flowing in the input terminals is the difference current between the base current and the current source, which can be quite small.
Most modern precision op amps use some means of internal bias current compensation, examples would be the familiar OP07 and OP27 series.
A bias current compensated bipolar input stage
Figure 1-23: A bias current compensated bipolar input stage
The well-known OP27 op amp family is good example of bias compensated op amps (References 2 and 3: George Erdi, "Amplifier Techniques for Combining Low Noise, Precision, and High-Speed Performance," IEEE Journal of Solid-State Circuits, Vol. SC-16, December, 1981 pp. 653-661 and George Erdi, Tom Schwartz, Scott Bernardi, and Walt Jung, "Op Amps Tackle Noise-and for Once, Noise Loses," Electronic Design, December 12, 1980). The simplified schematic of the OP27, shown in Figure 1-24 (opposite), shows that the multiple-collector transistor Q6 provides the bias current compensation for the input transistors Q1 and Q2. The "G" grade of the OP27 has a maximum input bias current specification of ±80nA at 25°C. Input voltage noise is 3nV/√Hz, and input current noise is 0.4pA/√Hz. Offset voltage trimming by "Zener-zapping" reduces the input offset voltage of the OP27 to 50μV maximum at +25°C for the "E" grade device (see Reference 4 for details of this trim method: George Erdi, "A Precision Trim Technique for Monolithic Analog Circuits," IEEE Journal of Solid-State Circuits, Vol. SC-10, December, 1975 pp. 412-416).
Bias current compensated input stages have many of the good features of the simple bipolar input stage, namely: low voltage noise, low offset, and low drift. Additionally, they have low bias current which is fairly stable with temperature. However, their current noise is not very good, and their bias current matching is poor.
These latter two undesired side effects result from the external bias current being the difference between the compensating current source and the input transistor base current. Both of these currents inevitably have noise. Since they are uncorrelated, the two noises add in a root-sum-of-squares fashion (even though the DC currents subtract).
Since the resulting external bias current is the difference between two nearly equal currents, there is no reason why the net current should have a defined polarity. As a result, the bias currents of a bias-compensated op amp may not only be mismatched, they can actually flow in opposite directions! In most applications this isn't important, but in some it can have unexpected effects (for example the droop of a sample-and-hold (SHA) built with a bias-compensated op amp may have either polarity).
In many cases, the bias current compensation feature is not mentioned on an op amp data sheet, and a simplified schematic isn't supplied. It is easy to determine if bias current compensation is used by examining the bias current specification. If the bias current is specified as a "±" value, the op amp is most likely compensated for bias current.
OP27 op amp uses bias current compensated input stage
Figure 1-24: OP27 op amp uses bias current compensated input stage
Note that this can easily be verified, by examining the offset current specification (the difference in the bias currents). If internal bias current compensation exists, the offset current will be of the same magnitude as the bias current. Without bias current compensation, the offset current will generally be at least a factor of 10 smaller than the bias current. Note that these relationships generally hold, regardless of the exact magnitude of the bias currents.
It is also a well-known fact that, within an op amp application circuit, the effects of bias current on the output offset voltage of an op amp can often be cancelled by making the source resistances at the two inputs equal. But, there is an important caveat here. The validity of this practice only holds true for bipolar input op amps without bias current compensation, that is, where the input currents are well matched. In a case of an op amp using internal bias current compensation, adding an extra resistance to either input will usually make the output offset worse!
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