A new audio amplifier topology with push-pull transimpedance stage - Part 1: Introduction

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.

1. Introduction
Power amplifier design is a topic which finds wide interest in the audio community - not only in the sense of commercial concerns, but also as a fascinating and often demanding challenge for electrical engineers and do-it-yourself enthusiasts. The trade-offs involved are numerous; from the basic requirements for accurate audio signal reproduction (such as good frequency response, low noise and distortion, etc.) over contemplations regarding efficiency, safety and complexity to constraints as a result of current fashion,which might favour certain design techniques for no particular objective reason.

As there will never be a solution which simultaneously fulfills all demands, we can expect an ongoing development in this field and I hope to add to the state of the art with the humble contribution of this article.

Most audio power amplifiers have been built around topologies with two gain stages; the first gain stage forms a transconductance stage (i.e. a voltage-to-current converter, typically implemented by a simple differential pair), and the second a transimpedance stage (i.e. a current-to-voltage converter, usually a common-emitter transistor configuration) which also provides Miller compensation.

A full audio power amplifier capable of driving a speaker load is completed by the addition of a unity gain power output stage, e.g. a complementary Darlington emitter follower.¹ As explained by Self [1] in great detail this configuration is very suitable to build discrete power amplifiers, as it offers potentially excellent performance at low complexity.

There are amplifier topologies which use three gain stages; however proper compensation is usually difficult to achieve, and the possible performance improvement is often benign compared to the increase in complexity. On the other hand, it is also possible to build amplifiers with just one gain stage. Unfortunately they require a power output stage with very high and constant input impedance for reasonable distortion performance; furthermore typical topologies show rather high sensitivity to transistor and resistor mismatch. This makes them less suitable for discrete implementation and the required complexity for a given performance goal will often be higher than with the use of a two stage topology.

In this article I will consider - once more - the two-stage topology. Before I present the new amplifier architecture in section 3, I'll first discuss typical prior-art topologies and some of their shortcomings in section 2.

Later in this text (sections 4–7) detailed advice for the optimum practical realisation of the novel amplifier topology is given. Section 8 considers several possible adaptations and alterations of the presented circuit, while in section 9 I show experimental verifications of the new amplifier concept. This article is finished with a brief conclusion, and an appendix covering noise sources in folded cascode stages.

¹ I should add that some authors count the output buffer as explicit gain stage, so the standard amplifier topology is then referred to as three-stage amplifier. Personally I prefer the other nomenclature which is in line with the large IC operational amplifier literature, and which appreciates that for the basic conceptional functionality of amplifiers the output stage is not strictly necessary.