8. Adaptation to Other Input Stage Topologies
So far we have considered the application of the new transimpedance stage to standard voltage feedback input stages with one differential pair only. However, it is perfectly feasible to adapt the new second stage to other input stage structures. Below I will give some examples of this.
In figure 8, an amplifier with complementary differential input pairs is sketched. The output current of each differential pair is made single-ended by an according current mirror (Q5 and Q6, or Q7 and Q8) and subsequently level-shifted to the common input node of the transimpedance stage by folded cascodes Q9 and Q10.
Figure 8: Novel transimpedance stage adapted to an input stage with complementary differential pairs.
With suitable, minor input stage modifications  such an amplifier can be designed to support very high slew rates. Note that, compared to the topology from figure 2, there is only one compensation capacitor and no need for a second stage bias control circuit. As for any of the following examples, the use of low-voltage regulated power supplies for the amplifier front-end (as detailed in section 7) is fully supported.
The simplest form of current feedback is implemented as shown in figure 9. This topology is particularly applicable to low level preamplifiers, as only one transistor, Q1, acts as primary noise source. I1 is optional to reduce the rather large bias current flowing from the inverting input terminal. Two amplifiers of this structure may be used to form the front-end of an instrumentation amplifier, such as is in frequent use in transformerless microphone preamplifiers .
Figure 9: Single-ended current feedback amplifier.
Further improvements regarding large-signal performance may be made with a complementary current feedback input stage (see figure 10).
Figure 10: Complementary current feedback input stage combined with new push-pull second stage.
Most complementary current feedback amplifiers are based on one-stage topologies; figure 10 however represents a full two-stage architecture, with the resulting advantages regarding open-loop gain (in the context of current feedback amplifiers usually referred to as transimpedance), insensitivity to loading from the power output stage and distortion. Although not discussed further, the use of two transimpedance stages and a suitable amplifier subcircuit for common-mode feedback will permit the design of fully differential amplifiers .
Coming up in Part 3: Experimental verification.
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About the author
Samuel Groner was born and currently lives in Zurich, Switzerland. He has been passionate about both art and science as long as he can remember. At present he works for Weiss Engineering Ltd. in the field of analogue hardware design and freelances as classical recording engineer/producer. Besides this, he teaches several courses at a local sound engineering school (ear training, classical music production and audio measurement) and enjoys a manifold activity as pianist, singer and choirmaster. If time permits, he is found on one of the numerous Swiss hiking trails, preferably in company with one of his cameras and a few sheets of black-and-white film. He holds a MSc degree in computer science and a MA degree as Tonmeister (recording engineer/producer).
This article originally appeared in Linear Audio Volume 2, September 2011. Linear Audio, a book-size printed tech audio resource, is published half-yearly by Jan Didden.
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