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.
[Part 1 introduces an audio amplifier topology which uses a novel push-pull transimpedance stage that offers a substantial improvement in power supply rejection over standard amplifier configurations. Part 2 discusses the amplifier's biasing, stability and AC performance.]
9. Experimental Verification
To compare the audio performance of the new transimpedance stage to the standard amplifier topology from figure 1
, a model amplifier (that is, an amplifier with a small-signal output buffer only, which is powered from low voltage regulated supplies ) for each topology was built. Figure 11 depicts the implementation which was chosen for the standard topology, and in figure 12 the model amplifier for the novel transimpedance is shown.
Figure 11: Model amplifier implementation for the standard two-stage topology.
Figure 12: Implementation of the model amplifier for verification of the new transimpedance stage.
To make the results as fair as possible, the input differential pairs have the same quiescent current and emitter degeneration, and the compensation capacitors have alike values. Furthermore, the quiescent current of the emitter followers and the common emitter transistors in the transimpedance stage, as well as the emitter resistor values of the common- emitter transistors, are made equal. Obviously, also the small-signal class A output stage details are equivalent. For simplicity, the use of voltage regulators for the front-end of the new amplifier was omitted.
If measured at a noise gain of 22 (which gives a unity loop gain frequency of about 700 kHz), +20 dBu output level and within a 80 kHz bandwidth, THD+N of both amplifiers is below –112 dB across the full audio frequency range, and dominated by amplifier noise and residual contributions of the oscillator and analyzer. This indicates that both topologies have no inherent distortion mechanisms in the small-signal stages which were significant in the context of a full power amplifier design. Substantial differences however are observed if the two amplifiers are evaluated for their sensitivity to loading at the second stage output node.
In  I have introduced the use of a voltage-dependent network, consisting of two back-to-back connected 3.3 V zener diodes in series with a 10 kΩ resistor, to roughly model the loading behaviour of power output stages. There is no reason to suspect that this modeling is particularly accurate, however it enables the easy comparison of amplifier topologies regarding the sensitivity to this distortion mechanism,which is just what we need here. Figure 13 discloses the measurement results; without doubt they provide little evidence for arguing against the novel transimpedance stage.
Figure 13: THD+N measurement (+20 dBu output level, 80 kHz measurement bandwidth) of the two model amplifiers with voltage-dependent loading of the second stage output node.
At low frequencies, the standard amplifier topology shows a mixed distortion residual, while the new amplifier architecture is still limited by noise and oscillator/analyzer contributions. Above 1 kHz, both model amplifiers show increasing distortion levels, however the magnitude observed for the novel transimpedance stage remains about dB below that of the conventional arrangement. Note that the decrease in distortion above 2 kHz is due to the bandwidth limiting filter and not actual circuit behaviour.
Unfortunately detailed discussion of the reasons which lead to the superiority of the new second stage with regard to output loading are beyond the scope of this article. I can just refer to the analysis I presented in , and give some food for thought in the following listing:
- The lumped resistance at the input node of the second stage is considerably higher for the new topology; this is because of the use of folded cascodes (which have higher output resistance than a differential pair), the current mirror with high emitter resistor values (which increases its output resistance), and the high-value resistors which are used to bias the emitter followers (R9 and R10 in figure 12). The high lumped resistance leads to low second stage output impedance.
- The complementary push-pull arrangement further reduces second stage output impedance, and mitigates the dependence of output impedance on output current.
- The novel transimpedance stage shows reduced nonlinear modulation of the compensation capacitor reference voltage (i.e. the second stage input node voltage). This is because the complementary topology cancels even-order harmonics present at the transimpedance stage input node.