In this article I have presented a new transimpedance topology, suitable for combination with a wide range of input stages to form amplifiers with two gain stages. Compared to prior amplifier designs, the novel architecture shows distinct advantages. First of all, it features inherently good power supply rejection, which is easily improved further by the use of voltage regulators for the amplifier front end. These voltage regulators are easily implemented at low cost and complexity.
The transient current output capability of the new transimpedance stage is considerably enhanced as well, and will easily support the design of amplifiers with very high slew rate. This comes along with reduced sensitivity to loading at the second stage output node - this is a very welcome property, as it simplifies the design of the power output stage. Also the clipping behaviour was shown to be superior to the standard amplifier topology.
The novel second stage topology requires level shifting at the input stage via folded cascodes. I have outlined how their impact on offset and voltage noise can be minimised, and shown that the noise performance of an amplifier using the new transimpedance stage may even be superior to prior implementations; that is because of reduced current mirror noise contribution. There is a complexity penalty associated with the new circuits, compared to the standard amplifier implementation.
However, there is no significant cost increase to be expected, as just small-signal transistors and standard passive components are needed. Also the increased quiescent current is usually of no relevance within the context of a full power amplifier design.
I must leave the design of a fully worked out power amplifier design, using the new concepts presented here, to future research. However, the interested reader will find it easy to derive his own implementation from the presented model amplifier. If, as suggested in section 7, voltage regulators for the amplifier front-end are used, the design from figure 12 will only need minor modifications in the folded cascodes of the transimpedance stage to accept high voltage power supply rails. With the addition of a suitable power output buffer, and the necessary control and protection circuitry, a complete power amplifier is realised.
To the best of my knowledge this new amplifier topology is first officially published in this article. However it is not possible to investigate all prior art, particularly all the information buried within the extensive patent repository. I shall be happy to hear about any findings with this respect from my valued readers.
In Part 4: Appendix: Noise in folded cascode stages.
I'm very grateful to Bob Cordell for reviewing this text and for providing valuable thoughts on it. I'd like to express my thanks to Jan Didden for his efforts in publishing this article and for providing us with an excellent resource on audio electronics.
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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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