Discussing audio amplifier design -- BJT, discrete
in [Basics]
|
Prev: Watch replacemnt (button) batteries?
Next: Low Power Satellite Based Laser / Imaging System Could Easily Track Activity Disturbing Cheap Reflecting Fibers
From: David Eather on 24 Feb 2010 22:01 On 25/02/2010 4:40 AM, Jon Kirwan wrote: > On Wed, 24 Feb 2010 21:20:45 +1000, David Eather > <eather(a)tpg.com.au> wrote: > >> <snip> >> I am/was thinking of a single supply class A amp. It has a great big >> output capacitor. > > Then I think class-AB remains the mode of operation when the > 8 ohm is replaced with the 4 ohm output, per your question to > me about that. A single-sided class-A with a 1.59A sink and > an appropriately sized rails would barely work class-A with 8 > ohms. And would move into AB, driving 4. I think. > > I talked a bit about the topology I was considering, earlier, > so hopefully I didn't get that part wrong even if I did fail > to add the output cap to the description. Just to be clear, > here is what I'm imagining right now: > >> : V+ >> : | >> : | >> : |/c Q1 >> : VAS ----| TIP3055 >> : |>e >> : | C1 >> : | || BIG >> : +----||----, >> : | || | >> : | \ >> : | / R1 >> : / \ \ 8 or 4 >> : | I1 / >> : v 1.59A | >> : \ / | >> : | gnd >> : | >> : gnd > > Just an emitter follower feeding a sink and the speaker via a > cap. Maybe I'm getting that wrong, though. > > Jon Your picture is what I was thinking about as a class A amp - a single supply class A amp. I have checked through my notes and I have absolutely *no* experience with push-pull class A.
From: David Eather on 24 Feb 2010 22:13 On 25/02/2010 12:50 PM, Jon Kirwan wrote: > On Wed, 24 Feb 2010 19:50:15 -0500, "Paul E. Schoen" > <paul(a)peschoen.com> wrote: > >> "Jon Kirwan"<jonk(a)infinitefactors.org> wrote in message >> news:3rrao55usonbnvt12kv6iqg89sdeacc9no(a)4ax.com... >>> On Wed, 24 Feb 2010 21:20:45 +1000, David Eather >>> <eather(a)tpg.com.au> wrote: >>> >>>> <snip> >>>> I am/was thinking of a single supply class A amp. It has a great big >>>> output capacitor. >>> >>> Then I think class-AB remains the mode of operation when the >>> 8 ohm is replaced with the 4 ohm output, per your question to >>> me about that. A single-sided class-A with a 1.59A sink and >>> an appropriately sized rails would barely work class-A with 8 >>> ohms. And would move into AB, driving 4. I think. >>> >>> I talked a bit about the topology I was considering, earlier, >>> so hopefully I didn't get that part wrong even if I did fail >>> to add the output cap to the description. Just to be clear, >>> here is what I'm imagining right now: >>> >>>> : V+ >>>> : | >>>> : | >>>> : |/c Q1 >>>> : VAS ----| TIP3055 >>>> : |>e >>>> : | C1 >>>> : | || BIG >>>> : +----||----, >>>> : | || | >>>> : | \ >>>> : | / R1 >>>> : / \ \ 8 or 4 >>>> : | I1 / >>>> : v 1.59A | >>>> : \ / | >>>> : | gnd >>>> : | >>>> : gnd >>> >>> Just an emitter follower feeding a sink and the speaker via a >>> cap. Maybe I'm getting that wrong, though. >> >> My conception of a class A amplifier is one where instead of an active >> current sink, there is just a resistor. > > I first thought of that, as well. But for the purposes at > hand, it seemed a lot easier to plop a current sink in there. > > David had written this to me: > > "A class A amp say at 10 watts into 8 ohms will > have an output stage with a constant current > sink (or source) set at 1.59 amps. If the speaker > load changes to 4 ohms the maximum current into > and out of the speaker is still 1.59 amps. How's > the power now?" > > I didn't want to wind up "getting corrected" for failing to > read well what he wrote, introducing some pre-conception of > mine. I don't get wound up very easily by this sort of thing - I know you aren't trolling. I do hop I do not sound too harsh when I disagree - it is not my intention to bully. When I see the other person is sincere I try to put my cynical factor on hold. I think I see the problem with what I wrote. "A class A amp say at 10 watts into 8 ohms will > have an output stage with a constant current > sink (or source) set at 1.59 amps. If the speaker > load changes to 4 ohms the maximum current into > and out of the speaker is still 1.59 amps. The current sink (or source) I was referring to was a matter of design a constant current sink ... or if you like drawing everything upside down a constant current source. > > That's why you see my schematic which _uses_ a current sink. > I'm trying to read David as accurately as I can and construct > from his words what I think he may be talking about. Hope I'm not doing too badly. To do > otherwise would be to _change_ the subject on him and talk at > cross-purposes. > >> It may be in the form of an emitter >> follower as in this case with a unity gain, > > Yes, that's clear -- now that we are quickly moving to change > the subject. :) > >> or the resistor may be in the >> collector to obtain a voltage gain greater than one. > > Um. After moving the speaker/cap connection up there, too? > Right? > >> But in these cases, >> large signal linearity is not realized. > > You mean in the case where a resistor is used in the emitter > and where a collector resistor may (or may not) be used. > Right? In the case of the current sink I attempted, when > trying to follow David, it seems 'large signal linear' -ish > to me. (Speaking loosely. Except for Vbe variations on Ic > and maybe also the Early effect, anyway.) > >> So such a configuration is used for >> very small signals that are at least an order of magnitude smaller than the >> supply rails, and power levels in the milliwatt range. With a resistor >> load, the maximum power output is where the output impedance equals the >> resistor, and the maximum voltage that can be achieved is about half the >> supply rails. > > The configurations you now brought up? Or the one that I was > talking about, earlier, when trying to deal with David's > question to me? > >> When one adds an active current source or sink, it involves another >> transistor > > Yes, that's a given of sorts. And that is why I'd almost > certainly prefer to go with a push-pull style class-A of some > kind. It seems crazy to go single-ended under the > circumstances. > >> and the circuit becomes essentially a half-bridge. > > A term I need to follow a little better, I suppose. I would > use it in the case of two diodes instead of four in a > full-wave, center-tapped PS with CT to ground and only one > other rail. You are using it differently than that, here. > Which makes me feel behind the terms-curve, still. > >> For the >> circuit shown above, with I1 = 0.75V and V+ = 12 VDC, > > Um... I1=.75A? Not 'V', right? (I assume we are getting > back to my ASCII schematic, now.) > >> a sine wave of 6V >> amplitude will be reproduced across R1 = 8 ohms. > > Yes, assuming as I know you must be that the drive is nicely > centered on +6V so that it goes from 0V to +12V -- which is > what I take you to mean here. > > Actually, maybe 5.2V or so would be better, so that the > emitter can follow up and down well. > >> With R1 = 4 ohms, the >> current source must be set to 1.5A. > > Yes, this much I understand. > > The question that David was asking me, if I understood him > accurately, didn't permit me to arbitrarily change the > current sink value. As such, the example case you are > bringing up would be a more accurate analogy to his question > if you kept the current source at .75A and changed R1 to 4 > ohms. My reply, at least, was made on that basis. > >> The efficiency under these conditions >> is 25%, and 4.5 W output. But this assumes that the current source can pull >> the output below ground, > > Two issues here. One with and one without the output cap > that David wisely mentioned in his response to me. > > In the case without the output cap, the current sink needs > access to a rail _below_ that used by the speaker load's > other end. Otherwise, if they are common to each other, then > there is a DC bias current flowing through the speaker and > that's not really a good thing. > > In the case with it, the cap provides the necessary 'most- > negative' side for the speaker and allows, after a few cycles > to pump up an equilibrium voltage on it, a DC center of 0A > for the speaker. > >> which is not possible with any practical >> component. > > Without the cap. With the cap, you are still right in that a > 0V on the base of Q1 does not mean that the emitter can sink > to -0.8V or whatever, since there is no rail there for it. > (Unless some extra windings are added to the transformer to > get it, of course.) > >> And I did not factor in the power provided by the current >> source, so actual efficiency will be lower. > > Yup. Understood. I think Self says 12.5% is the best to be > hoped. I've not done my own double-check. But with your > estimate and adding in an equal amount for the sink, that > seems to get to about there. > >> If I use a 4 ohm resistor as the emitter load, and bias Q1 so that there is >> equal clipping at the output, I can get an output of about 7.6 volts P-P, >> or 1.8 W. The efficiency is about 8%. If I bias the resistor for 1/2 the >> supply rail (6 V), I can get at most about 2.2 VRMS into 4 ohms, or 1.2 W. >> Efficiency is 6.6%. >> >> With a realizable current source made from a 2N3055 and a 0.2 ohm emitter >> resistor, set at 1.55A, I can get about 3.9 watts into 4 ohms, and an >> efficiency of 20%. >> >> Now, I decided to see if I could get better efficiency by adding a variable >> current sink. Essentially I am now making a push-pull circuit where the >> lower half is not pulling so much when the upper half is pushing, and then >> it pulls harder when it needs to do so during the negative excursions of >> the signal. It simply required two additional components. My LTSpice >> simulation shows an output of about 3.9 W into 4 ohms, with an efficiency >> of about 27%. > > Thanks for the circuit. I also ran it under LTspice. > Selecting from 100ms to 500ms as a range by which things have > settled out well, the resistor shows about 3.9 watts and 14.1 > watts from the rail supply. Which gets to your number. As > you hoped, most of the 14 watts is in Q1, at about 6W. Q2 > shows about 2.9W. > >> This design is similar to class A in that it burns up about 15 watts with >> low level signals. As such, maybe it is not so much and amplifier as an >> "Apple-fryer" :) > > :) You've got to love class A. The only thing better is to do class A with valves! > >> And under those conditions the output stage is running 1.2A. But that is >> better than running 1.5 A as was the case with the original design. There >> does not appear to be any crossover distortion, and at high signal levels >> you just get clipping, and that occurs within 1 volt of the supply rails. >> As Scotty might say, "Cap'n, she just caint give ye no more!" >> >> OK, I've played around enough. I've attached the ASC file if anyone wants >> to play with it or criticize it. I just used a "shotgun" approach with >> simplicity in mind. Maybe it's worth building, but I'm happy enough with >> the usual Class B or AB amplifiers that don't function as space heaters >> when they're just sitting there. And a class A power amplifier will never >> win an Energy Star! Go green! Use PWM! > > Thanks, Paul. All discussion is most welcome to me. I > appreciate it very much. > > Jon Lead on Jon!
From: Jon Kirwan on 24 Feb 2010 22:19 On Thu, 25 Feb 2010 13:01:22 +1000, David Eather <eather(a)tpg.com.au> wrote: >On 25/02/2010 4:40 AM, Jon Kirwan wrote: >> On Wed, 24 Feb 2010 21:20:45 +1000, David Eather >> <eather(a)tpg.com.au> wrote: >> >>> <snip> >>> I am/was thinking of a single supply class A amp. It has a great big >>> output capacitor. >> >> Then I think class-AB remains the mode of operation when the >> 8 ohm is replaced with the 4 ohm output, per your question to >> me about that. A single-sided class-A with a 1.59A sink and >> an appropriately sized rails would barely work class-A with 8 >> ohms. And would move into AB, driving 4. I think. >> >> I talked a bit about the topology I was considering, earlier, >> so hopefully I didn't get that part wrong even if I did fail >> to add the output cap to the description. Just to be clear, >> here is what I'm imagining right now: >> >>> : V+ >>> : | >>> : | >>> : |/c Q1 >>> : VAS ----| TIP3055 >>> : |>e >>> : | C1 >>> : | || BIG >>> : +----||----, >>> : | || | >>> : | \ >>> : | / R1 >>> : / \ \ 8 or 4 >>> : | I1 / >>> : v 1.59A | >>> : \ / | >>> : | gnd >>> : | >>> : gnd >> >> Just an emitter follower feeding a sink and the speaker via a >> cap. Maybe I'm getting that wrong, though. >> >> Jon > >Your picture is what I was thinking about as a class A amp - a single >supply class A amp. I have checked through my notes and I have >absolutely *no* experience with push-pull class A. Okay. At first, I wasn't understanding what you meant well. I guess this says we are on the same page, now. The push-pull version I was thinking about at first looks exactly like (and is, if the bias is set that way) what I've see as a class-B push-pull stage. The only difference is that the Vbias value is set "high" enough to cause both BJTs to always conduct. This is what I'm thinking about, regarding class-A push-pull: >: V+ >: | >: | >: |/c Q1 >: ,-----| TIP3055 >: | |>e >: + | | >: --- | >: - Vbias +---- to speaker >: --- | >: - | >: | |<e Q2 >: +-----| TIP2955 >: | |\c >: | | >: | | >: VAS V- In the above case, Vbias can be set about right for near class-B operation (is anything truly exactly class-B?) from each BJT, with some cross-over distortion. If you increase Vbias above that, you start getting class-AB operation from each BJT and eventually with high enough Vbias both BJTs always have some non-zero collector currents and are each operating class-A. Given my vague undertanding of things, anyway. There are a few other push-pull class-A output stage topologies, too. For example: >: V+ >: | >: | >: / \ >: | I1 >: v V+ >: \ / | >: | | >: | |/c Q1 >: +-----| >: | |>e >: | | >: |/c Q3 | >: VAS ----| +----- to speaker >: |>e | >: | | >: | |/c Q2 >: +-----| >: | |>e >: | | >: \ | >: / R1 V- >: \ >: / >: | >: | >: V- And at least one other one, as well. Thanks, Jon
From: Jon Kirwan on 24 Feb 2010 22:20 On Wed, 24 Feb 2010 21:55:18 -0500, "Paul E. Schoen" <paul(a)peschoen.com> wrote: >"Paul E. Schoen" <paul(a)peschoen.com> wrote in message >news:pQjhn.13494$Ab2.6638(a)newsfe23.iad... >> >> When one adds an active current source or sink, it involves another >> transistor and the circuit becomes essentially a half-bridge. For the >> circuit shown above, with I1 = 0.75V and V+ = 12 VDC, a sine wave of 6V >> amplitude will be reproduced across R1 = 8 ohms. With R1 = 4 ohms, the >> current source must be set to 1.5A. The efficiency under these conditions >> is 25%, and 4.5 W output. But this assumes that the current source can >> pull the output below ground, which is not possible with any practical >> component. And I did not factor in the power provided by the current >> source, so actual efficiency will be lower. > >Like 13%. The magic active current source was contributing about half the >power. Self says 12.5% is the best to hope for. Jon
From: Jon Kirwan on 24 Feb 2010 22:38
On Thu, 25 Feb 2010 13:13:12 +1000, David Eather <eather(a)tpg.com.au> wrote: ><snip> >I don't get wound up very easily by this sort of thing - I know you >aren't trolling. I do hop I do not sound too harsh when I disagree - it >is not my intention to bully. When I see the other person is sincere I >try to put my cynical factor on hold. ><snip> Sometimes, the fastest way from A to B is through some 'oscillation' in discussion. I think of this as critically damped in the best of cases and maybe a little underdamped more than would be liked, most of the time. I don't mind that if you don't. It would be a slower discussion if we spoke way overdamped and never oscillated. Yes, I am seriously studying and seriously trying to engage myself to every sentence I'm gifted with by you or Paul or pimpom (and others, too.) I'm pouring through Self's latest book every night, working some equations on my own. So this is a serious attempt on my part. Whether or not it is serious to anyone else, of course, is a different matter. I'm pretty hard to teach, at times, and won't pretend otherwise. I do appreciate everything you add. I just hope that my responses aren't off-putting. Jon |