What are the inherent differences in asymetric/smetric audio amp output stages? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 16, 2023, 05:37:57 pm
content: The drive circuitry aside, what inherent differences do different output transistor arrangements make in a push pull audio amp?Like Symmetric NPN+PNP where the emitters are towards the speaker outputOr Symmetric NPN+PNP where the emitters are towards the power rails (I presume this allows for higher voltage swing right?)Or Asymmetric NPN+NPN or PNP+PNP with emitters all pointing down, or all pointing up.Or Asymmetric? NPN+NPN or PNP+PNP where the emitters are both towards the speaker output.Or the same but the emitters point towards the power rails (haven't seen a design like that tho)If those designs which have the emitters towards the power rails have higher voltage swing capability with the same supply voltages. Do they have different distortion characteristics or something else different too?And also wouldnt the base current from arrangements where the emitters are towards the speaker output cause minor distortion? If so, why even do this instead of connecting the emitters to the power rails?

author: wasedadoc
author_rank: Super Contributor
timestamp: August 16, 2023, 06:14:05 pm
content: I don't think I've ever seen a transformerless push-pull power output with emitters towards the power rails. That would mean the speaker at the collectors and collectors are high impedance points, ie current sources not voltage sources. Not what you want for directly driving apeakers. With a centre tapped transformer primary stepping down to drive a low impedance speaker was a very common circuit in transistor radios a long time ago.Actually both emitters pointed to the same power rail and both collectors to the other power rail on the transformer pimary's centre tap. Just the equivalent of the conventional valve/tube push pull output arrangement which could never have the luxury of complementary devices.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2023, 07:40:26 pm
content: Quote from: wasedadoc on August 16, 2023, 06:14:05 pmEvery rail to rail output opamp works this way, and some are specified for audio use under substantial loading, like driving headphones. If there are any rail to rail power amp chips (I dunno) then they also need to work this way, obviously.I too used to think that output impedance would be horrible, but I don't think so anymore. Oftentimes Miller compensation is included around the output stage, which AC couples the base to the collector, so that any collector voltage swing results in similar base voltage swing (mind base-emitter capacitance, though). Such stage ought to have only slightly worse output impedance than an emitter follower at high frequencies, and at lower frequencies the global loop takes over controlling the base to regulate appropriate output voltage.Case in point, TL431 has a Miller-compensated common emitter Darlington output stage which is current-driven by the earlier stage. The whole thing operates at a few mA (the earlier stages at sub-1mA combined) and specified output impedance is fairly low and flat to 100kHz. It is also stable with capacitive loads up to a few nF. Unfortunately, more interesting ICs like opamps don't have their schematics disclosed in nearly the same detail.I think common emitter outputs aren't popular in discrete amplifiers mainly because there is no pressure to maximize output swing on very modest supply rails and because they are harder to bias into push-pull class B than complementary emitter followers. The circuits used in opamps seem to vary from "meaningfully more complex than a Vbe multiplier" to "way too complex and finicky for discrete builds". I think the most promising approach could be the one devised by Monticelli at National Semiconductor long ago, which can be seen in contemporary OPA211 datasheet. It is analogous to a traditional bias spreader for Darlington EF output stages split in two halves and connected to supply rails, which means it should have all the same thermal coupling problems as the traditional solution and hopefully no other thermal problems.NE5534/5532 famously has an NPN/NPN output stage. I think this could be reproduces in a discrete amp, if the diode which permits the VAS to sink load current is thermally coupled with the bias spreader (or part thereof). Another clever solution for NPN/NPN was given by John Linsley-Hood in his class A amp, but I think it's only good for class A.Then there is the whole world of amplifiers with Sziklai pair output stages. But despite the use of common emitter power transistors, they are fundamentally voltage follower output stages and not much different from Darlington pairs. Some controversies seem to exist over which is better, both in terms of THD and bias stability.

author: mikerj
author_rank: Super Contributor
timestamp: August 16, 2023, 08:02:39 pm
content: Quote from: ELS122 on August 16, 2023, 06:23:38 pmI've never seen a transformer-less audio power amp with common emitter output stages either, are you maybe thinking of Sziklai pair output stages? They behave as emitter followers (i.e. voltage gain ~1, high current gain, low output impedance) with similar biasing requirements.

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2023, 02:00:20 am
content: Quote from: wasedadoc on August 16, 2023, 06:14:05 pmI have designed some which worked that way. Current outputs make current sharing easy so multiple output stages can be easily parallelled for higher power.In my design, slow 2 MHz output transistors yielded 600 kHz full power bandwidth so performance was very good. I used a separate control loop to manage output stage idle current for class-AB operation. The design failed at higher frequencies because the output transistors were too slow to turn off, so simultaneous conduction would overheat them. The solution to that is simply to bandwidth limit the input signal, which should be done anyway.The open loop output resistance is higher, however negative feedback lowers it to acceptable levels. Emitter follower outputs should have inherently lower distortion for the reason you give, but I am not sure that there is a practical difference.Quote from: ELS122 on August 16, 2023, 05:37:57 pmCommon collector output stages have just as high a swing if the base drive is bootstrapped or a slightly higher voltage supply is available for the driver circuits.Each side has a variation in output resistance which depends on the voltage across the transistor and the current through the transistor, which causes a variation in open loop gain as the output voltage changes. With symmetric stages, this can somewhat cancel out however with asymmetrical stages, the additional variation in gain will cause more distortion. This is what the extra diode in compensated quasi-complimentary stages helps with.QuoteThe base currents do not matter. The variation in open loop output resistance is what matters.