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Class A with AB Output-stage
Class A with AB Output-stage
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Old 1st December 2003, 12:03 AM   #11
millwood is offline millwood  United States
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Originally posted by Steven

Have a look at http://www.diyaudio.com/forums/showt...900#post247900


thanks, Steven for the reference.

I do have a quick question. The article seems to be concerned about open-loop operation of a class b amp. Will the same hold true when feedback is present?

I did a quick simulation on a modified citation 12 (with a t driver section that works in class a). lowest thd is obtained at about 29mv (130ma iq over 0.22 ohm emitter resistor). so I suppose that it holds true in closed loop as well.
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Old 1st December 2003, 07:24 AM   #12
Steven is offline Steven  Netherlands
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Hi millwood,

Yes, the same holds true for amps with feedback. Maybe even more. Discontinuities in the crossover region of pushpull output stages manifest themselves in higher order harmonic distortion. The sharper the discontinuity, the higher the order of the harmonics in the distortion spectrum. Feedback is a circular mechanism, the output signal is fed back to the input and has to travel along the forward path of the amplifier again, and again, and again... The amplitude of the lower order harmonics will decrease by feedback, but the amplitude of the higher harmonics may even increase, because harmonics of the harmonics are created every time the signal is subjected again to the forward path non-linearity. In general this is the big problem of using feedback in audio amplifiers. While low order harmonic distortions are very well acceptable, since our own ears suffer from low order harmonic distortion too, higher order harmonic distrortions tend to be very annoying, especially the odd harmonics from the fifth upwards.
So, also in amplifiers that use feedback, it is very desirable to keep the open loop distortion as low as possible to avoid the creation of many high order distortion products by the feedback mechanism itself.

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Old 1st December 2003, 09:31 AM   #13
Cobra2 is offline Cobra2  Norway
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Class A with AB Output-stage Class A with AB Output-stage
Default Tweaking...?

Interesting, but one Q (If I were to do some tweaking):
should I adjust the bias-current, or keep the same total bias-current, and change resistor size?

Arne K
Don't believe everything you think...
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Old 1st December 2003, 02:21 PM   #14
sreten is offline sreten  United Kingdom
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If your going to tweak then the bias voltage should remain
fairly constant. Optimium Vbias changes slightly with different
Re's and consequently Iq's, but not by much.

So varying the emitter resistors will also vary Iq.

If you keep Iq constant and vary the resistors you will be
varying Vbias.

Also true for keeping the resistor the same and varying Iq.

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Old 1st December 2003, 06:49 PM   #15
PRR is offline PRR  United States
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Curl is right. There is a theoretical optimum. And there is an "artful optimum" that is higher.

For voltage-drive, in silicon, theory says around 25mV-30mV, depending on the exact transistor.

Lower leads to distortion and, in practice, thermal instability.

Higher sounds better to the ear.

1V is way too high. (John's not the only one to go too far.) 1V can make a sub-optimum Class A stage, but sucks for any Class "B" operation.

50mV is a nice semi-safe trial value for experimenters. If the bias junction(s) is well coupled to the output devices, then bias stability is fine, idle current is on the safe/better side of the theoretical optimum, and the ear likes this side of the "optimum" better than the theoretical optimum.

> should I adjust the bias-current, or... change resistor size?

Figure the idle current you want. Higher is almost always better. Limit is how hot you want the beast to idle. You usually do not want it either hot nor cold, just warm. (You certainly do not want to run the devices down where Beta or Ft is dropping rapidly.) Calculate the idle current from the idle-power. The emitter resistors are then 0.05V/Ibias. In smoke-test, start at much lower bias and let it cook. You probably have humm-busting and other chores to do first. When it basically passes OK audio, sneak up to half the calculated current, 0.025V per resistor. Try that a while, then try 0.05V per resistor. The difference should be subtle.

Changing resistor sizes is really a re-design, and it won't be easy to make meaningful comparisons with such major change.
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