Class A amplifier current demand - Page 1
Forum section: electronics / general

author: knudsenuk
author_rank: Contributor
timestamp: November 14, 2024, 01:41:07 pm
content: Dear allI frequently read how people differentiate between class A current demand and class B/AB. Usually the sentiment seems to be that the Class A demand is constant and the other is transient. I am confused. If you talk about the DC operating conditions I don't have a problem but when an AC stimulus is introduced, the power supply is reacting to current demand that is transient.The attachment clearly show the current demand in a common emitter circuit to be the inverse of the voltage output.There also seems to be some degree of confusion over the load. Typically, it's a resistor to set the DC operating point, but it rarely (in text books) discusses the relationship between the load and the load from the next stage.I am interested in your thoughts please....

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2024, 02:45:32 pm
content: Class A can be run "single-ended" or push-pull.Classes AB and B must be run in push-pull.Class C is only useful for RF amplifiers with a tuned load, and can be either single-ended or push-pull.

author: Tation
author_rank: Regular Contributor
timestamp: November 14, 2024, 02:52:27 pm
content: Quote from: knudsenuk on November 14, 2024, 01:41:07 pmBut, even with AC applied, the DC component of the supply current is constant in your class A circuit. The power get from the supply is constant.Class A and C have no connection to push/pull or not, both classes may be push-pull or single ended.

author: SteveThackery
author_rank: Super Contributor
timestamp: November 14, 2024, 02:56:14 pm
content: https://en.wikipedia.org/wiki/Power_amplifier_classes

author: SteveThackery
author_rank: Super Contributor
timestamp: November 14, 2024, 02:58:27 pm
content: Quote from: Tation on November 14, 2024, 02:52:27 pmSurely that's not right for a single-ended Class A, as drawn. The amplifier works by varying the current in the collector circuit, all of which comes from the power supply.EDIT: Sorry, I didn't understand your use of the term "DC component". Yes, the mean current remains unchanged, even though the instantaneous current varies in time with the input signal.

author: Tation
author_rank: Regular Contributor
timestamp: November 14, 2024, 03:01:05 pm
content: Quote from: SteveThackery on November 14, 2024, 02:58:27 pmYeah, but the mean of the current (that's the DC current) remains constant.

author: SteveThackery
author_rank: Super Contributor
timestamp: November 14, 2024, 03:03:37 pm
content: Quote from: Tation on November 14, 2024, 03:01:05 pmI agree, but the OP was querying the sinusoidal current trace in his simulation.@OP: in asingle-endedClass A amplifier like yours, the supply current does indeed vary in time with the input signal. However, the resultant dips and the peaks in the current are equal in amplitude, so the mean current stays constant. In apush-pullClass A, the supply current does actually remain constant.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 14, 2024, 03:46:15 pm
content: Class A does not mean the supply current is constant. Class A just means the current doesn't ever go to zero.For a push-pull class A amplifier you have a split rail power supply. In that case the total power is approximately constant. But the two rails indivially oscillate just as the single ended class A, but out of phase.

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2024, 04:31:53 pm
content: To distinguish classes A, B, and C:In class A, the current through the active device or devices flows (at varying level) for the entire cycle: often called 360oconduction angle. This allows single-ended operation with an approximately sinusoidal current waveform (some distortion). In push-pull class-A, with matched devices, the even-order harmonics cancel in the total output current, leaving odd-order terms. The DC (mean over time) of the supply current is high, since there is substantial current at zero input, but remains constant as the input amplitude increases, so the overall efficiency is low (ideally 50% with transformer coupling, but only at full output power). At low signal level, the efficiency is very low.In class B, the current through each active device flows for half of the entire cycle (180oconduction angle): one device pulls the output positive and the other pulls it negative. Push-pull is required to get both positive and negative output swings. At zero input signal, the current in each device is zero (ideally), or very small. Of the linear classes, class B has the highest overall efficiency (ideally 78.5%).Class AB is a compromise between A and B: conduction angle between 180 and 360 degrees, with a reasonable current draw at zero input to aid the transition between positive and negative total current.Push-pull vacuum-tube amplifiers normally use a transformer with center-tapped primary to couple the load from two identical devices. Solid-state amplifiers usually put two active devices in series across the power supply and connect the load directly or through a large capacitor from their junction.Class C uses a narrow conduction angle < 180 degrees to send pulses into a resonant circuit for narrow-band application.

author: Tation
author_rank: Regular Contributor
timestamp: November 14, 2024, 05:20:42 pm
content: Quote from: ejeffrey on November 14, 2024, 03:46:15 pmYeah. But in the (single ended) class A amplifier of the OP, the DC supply current is (OK, approximately) constant, no matter the AC amplitude (without clipping).EDIT: and the maximum peak supply current is twice the DC supply current.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 14, 2024, 08:07:44 pm
content: Quote from: TimFox on November 14, 2024, 04:31:53 pmYes, to reduce "crossover distortion".

author: Tation
author_rank: Regular Contributor
timestamp: November 14, 2024, 10:21:43 pm
content: Quote from: Analog Kid on November 14, 2024, 08:07:44 pmAbout this B vs. AB, on his book "Audio Power Amplifier Design" Douglas Self states that:Class-AB is less linear than either A or B, and in my view its only legitimate use is as a fallback mode to allow Class-A ampliﬁers to continue working reasonably when faced with a low-load impedance.In the same book he performs a detailed analysis of B & AB showing that, in fact, AB gives more distortion than B.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 14, 2024, 10:35:56 pm
content: Interesting. I never heard that.Here's his page on that subject online. I'm reading it now.So class AB reduces crossover distortion but introduces its own set of distortions.Which is why they call it "the art of electronics".

author: SteveThackery
author_rank: Super Contributor
timestamp: November 14, 2024, 10:40:57 pm
content: Quote from: Tation on November 14, 2024, 05:20:42 pmI think this terminology is incredibly misleading for a beginner.You are using the term "DC supply current" to refer to themeansupply current. But "DC" doesnotequal "mean"! "DC" means the polarity of the current never changes. That's all. It saysnothingabout whether the current is steady or varying, instantaneous or mean.* Go on, look it up.If you want to refer to the mean supply current, rather than the instantaneous current, then for goodness' sake say "mean supply current"! Don't say "DC current" because that is ambiguous.I cannot believe I'm having to make this argument.EDIT* The output from a full wave rectifier is direct current by definition: it's polarity never changes. However, there at least four legitimate values of this DC current: the instantaneous value, the minimum value, the peak value, and the mean value. It is misleading to pick one of those values and call it "the DC current". They areallDC current. The better approach is to be specific about which value you are referring to.

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2024, 10:55:12 pm
content: Quote from: SteveThackery on November 14, 2024, 10:40:57 pmNo."DC supply current" refers to the reading on a DC ammeter connected between the supply and the load, which is the mean value (over time) of the current drawn by the load.This is often called "DC component" of a waveform, meaning the Fourier component at zero frequency.

author: SteveThackery
author_rank: Super Contributor
timestamp: November 14, 2024, 11:07:40 pm
content: And how much damping does the needle have? It is a crucial parameter! And over how much time? Another crucial parameter. What if the needle moves about? Is the "DC supply current" varying? Or are we just not averaging it over a long enough period?I can totally live with "the DC component" because it has a solid mathematical derivation. But "DC supply current" is ambiguous and lazy. The concept of "mean supply current" is only meaningful when you have specified the measuring interval, which it hasn't been here.We should be specific in our terminology. Please see the edit I made to my previous post.

author: TimFox
author_rank: Super Contributor
timestamp: November 14, 2024, 11:27:59 pm
content: Normally, a power amplifier has a serious bypass (or reservoir) capacitor on each power rail (only one for a grounded power supply).For a practical amplifier, a DC ammeter connected from the power supply to that rail will show a different value as a function of output power, and the ammeter does not require any particular damping value for amplifiers operating in the audio range.A class-A amplifier, single-ended or push-pull, shows a small change in that DC current value (value shown on DC ammeter) as the power increases from zero to maximum output.A class-B amplifier, on the other hand, has very small DC current with zero excitation, increasing to a large value at maximum output power.This is seen, for example, on the data sheets for tubes intended for power amplifier application.Yes, theterm"direct current", abbreviated "DC", means a current that does not reverse.However, when avaluefor direct current is quoted quantitatively, it means the mean current averaged over time.

author: Tation
author_rank: Regular Contributor
timestamp: November 15, 2024, 07:11:04 am
content: Just checked, none of my multimeters have buttons or selectors labeled "average/mean" or "RMS value of the current/voltage after previous subtraction of the average", they simply say DC or AC.

author: knudsenuk
author_rank: Contributor
timestamp: November 15, 2024, 09:07:55 am
content: I thanks everyone for their contributions. For me, it seems to be clear that a PSU in any amplifier needs to be capable of handling transients and also there is a relationship between frequency applied and the ability to handle transients, in much the same way mains rectification and switch mode psu's require different approaches.It seems a misnomer, to just write off the dynamic requirements of a PSU supplying a class A system. In fact the only real difference is that a class A system sits at 50% of maximum current in quiescent conditions and a class AB design has just enough bias to turn on the output devices (broadly).

author: SteveThackery
author_rank: Super Contributor
timestamp: November 15, 2024, 10:01:27 am
content: Quote from: TimFox on November 14, 2024, 11:27:59 pmI'm arguing that the common usage of "DC supply current" to mean "the mean current over an unspecified interval" is ambiguous and potentially misleading. I think we should always qualify which current we are referring to: the instantaneous current; the minimum current; the peak current; the mean current over <interval>; etc.Anyway, I realise I'm on a losing wicket here. 😄EDIT: Sorry, OP, drifting off topic. 😀

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 02:46:39 pm
content: Quote from: knudsenuk on November 15, 2024, 09:07:55 amClass AB and B amplifiers have substantial variation in DC current draw with power output, while the DC for class A (especially push-pull) varies only slightly with power output.There should be reasonable capacitance on the rails to help reduce the voltage ripple caused by the periodic variation of instantaneous current.Transients are short bursts of power, and will have corresponding changes in instantaneous current draw for the various classes.Note that if you want to add voltage regulation to the rail or rails to cope with this, the regulators should be as good as the amplifiers themselves, since bad effects on the voltage rails will propagate to the output.

author: macboy
author_rank: Super Contributor
timestamp: November 15, 2024, 02:53:06 pm
content: Quote from: Analog Kid on November 14, 2024, 10:35:56 pmTo find the text and graphs, search for "Fig 14a,b,c showing THD residuals for under- biasing, optimal, and over-biasing".While the data is probably valid, I think that it is more than a little disingenuous.He presents an under-biased case (clearly Class B), and "optimally biased Class B" (apparently tweaked to just a hair below conduction? He doesn't explicitly say), and a grossly over-biased Class AB (operating in Class A fully one quarter of the time!). Oddly, he does not present an "optimally biased Class AB" case to go along with it. Why? I believe it is because that might have the lowest distortion of all the cases, and that would weaken the (valid) point he's trying to make about a unique form of distortion introduced by the Class AB output stage. And look at the channel 2 (the distortion trace) vertical sensitivity for all three graphs. What do you notice? The over-biased Class AB case has 2.5x higher sensitivity, making its distortion trace visually appear to be much higher by contrast. Why?He does make a potentially valid point however, that that a very highly biased Class AB might not be the perfect combination/compromise between Class A performance and Class B efficiency, that some people think it is. However, in my opinion, by choosing to not present all the facts and by choosing to present the facts he does do in the way he does, he taints the data's validity and credibility, which is a shame. What data he presents does showsomething, but I would never conclude that "Class B is better than AB" based on it. But who am I to argue: he literally wrote the book on audio amplifier design, and I am just another EE with a keyboard and an opinion. Look at the data; draw your own conclusions.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 02:58:20 pm
content: One problem with the usual analysis of Class B amplifiers is that it does not include any time-dependent effects in the active devices, such as time required to switch off.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 15, 2024, 09:56:38 pm
content: Yes, the analysis linked to above, about class B and AB biasing, is clearly, uh, biased.