Audio amplifier output stage comparison

Triple emitter follower vs. Diamond buffer

Abstract

The triple emitter follower configuration is widely used as output stage for audio amplifiers. Diamond buffer output stages are used in operational amplifiers and headphone amplifiers a lot, but I haven't seen this circuit used for power amplifiers yet and wonder why. The only company using Diamond output stages for power amplifiers seems to be Ayre. I don't know how their output stage looks like, but assume it is some variant of the Diamond configuration since it carries that name. I tend to swim against the (main-) stream and this is why not following the trend of using a triple emitter follower output stage is appealing to me. In this article, I will compare a variant of a triple emitter follower output stage with a variant of a Diamond output stage based on circuit simulation to find out whether a Diamond output stage could match the performance of a triple emitter follower output stage. Calling the circuit I made up a Diamond buffer is a stretch, but there is basic similarity so I still consider this to be a variant of the Diamond buffer. Circuit simulation is only an approximation and results depend on model accuracy a lot, but may indicate real life performance to some extend.

Schematics

Below schematics show the circuits I set up for investigation. Note that this variant of the Diamond Buffer requires elevated supply rails for the drivers. The triple emitter follower does not, but the emitter follower output stage has elevated supplies nonetheless in order to have mostly equal conditions. The current sources powering the Diamond Buffer stem from an earlier investigation of high performance current sources. The current sources used in this example require slightly over 4V to work, so the supplies are elevated by 5V for some margin. Both output stages are set up with 100mA standing current in the driver stage and 1A in each power transistor. Given a +/- 30V supply, this results in a powerful class A output stage for 8Ω (purely resistive) loads. Power transistor base resistor is 2.2Ω and emitter resistor is 0.22Ω.

Triple emitter follower

Diamond buffer

DC bias

Stable DC bias is less important for class A output stages, like for the setup in this example. It matters most for output stages biased into class AB. I probably will investigate this case as well, but in another article. The main benefit of class AB bias is much higher efficiency and higher output power, which comes at the expense of higher distortion and, more important, this type of distortion is crossover distortion, which is entirely unrelated to the audio signal and therefore most objectionable.

The bias spreader of the triple emitter follower looks unusual, but is actually nothing special - just a complimentary bias spreader with extra diodes held at constant temperature to lower the compensation factor while increasing the DC bias current. The typical bias spreader (like shown for example in figure 14.10 of Bob Cordell's book) that compensates six VBE, totally overcompensates in simulation. Although that does not mean that this will happen in real life as well, some options to tweak compensation are nice as this may help to get the bias stable both in simulation and reality. There are literally countless possibilities to get the triple emitter follower DC bias stable. The decision is not only which bias spreader arrangement to chose, but also, which transistors to put on common or separate heat sinks and whether or not to sense the heat sink temperature and so on. This cannot entirely be assessed in simulation, but needs a lot of fine-tuning on the bench. Setting DC bias of the triple emitter follower is easily done by varying R52 in this example. This resistor also sets the compensation factor, which complicates setting the bias. This is why I introduced D5 and D6: To elevate the bias into class A without affecting the compensation factor.

The DC bias of the Diamond buffer is set by the driver transistors, which need to be on the same heat sink as the power transistors in order to sense temperature. Using cascoded current sources, the cascode transistors can be put on the same heat sink. Compensation factor is much lower and it is yet to see whether this is sufficient in real life. Power transistor DC bias depends on resistors R11 and R12, as well as the standing current of the driver stage. For setting the typical class AB bias current, probably compromises in driver stage standing current needs to be made.

Stability

Resistance of the input voltage source for the stability analysis was stepped with 10Ω, 100Ω, 1kΩ and 10kΩ.

Triple emitter follower

This is the triple emitter follower stability uncompensated, i.e. with R1 and C1 shunt compensation removed:

The plot shows that the uncompensated triple emitter follower is outright unstable with any impedance of the signal voltage source.

This is the effect of the shunt compensation network on stability:

Note that the compensation is far from perfect, but illustrates the effect. In order to optimize compensation, the input voltage source impedance needs to be considered.

Diamond buffer

This is the diamond buffer stability uncompensated, i.e. with R2 and C2 shunt compensation removed:

The Diamond buffer shows only slight instability when left uncompensated.

This is the effect of the shunt compensation network on stability:

Note that the compensation is far from perfect, but illustrates the effect. In order to optimize compensation, the input voltage source impedance needs to be considered.

Distortion

Assessing distortion in simulation is difficult, but the result is good enough for a rough estimate. Below table shows distortion at 1kHz near clipping at 30V supply rails with 8Ω load.

Input resistance Triple emitter follower Diamond buffer
10Ω 0.002% 0.007%
100Ω 0.002% 0.009%
1kΩ 0.003% 0.030%
10kΩ 0.009% 0.176%

It is obvious that the triple emitter follower performance is mostly independent from input resistance while the Diamond buffer shows a strong dependence. At 10kΩ input resistance this goes so far that visible attenuation of the Diamond buffer output can be observed. This indicates that the input impedance of the triple emitter follower is much higher than the input impedance of the Diamond buffer.

Honestly, the numbers from simulation do not make any sense to me because I expected distortion being higher some orders of magnitude. I assume that the models and setup used for simulation are somewhat unrealistic. With distortion numbers that low, I wonder whether it could be a good idea to exclude the output stage from the feedback loop and just run it open loop instead.

Conclusion

Driven from a low impedance source, the Diamond buffers shows nearly the distortion performance of the triple emitter follower. In any case the triple emitter follower outperforms the Dialog buffer in terms of input impedance and therefore also distortion. On the other hand side, the triple emitter follower is less stable in the frequency domain. Without bench experiments it is difficult to tell which one has more stable DC operating point.