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Old 4th September 2009, 03:51 AM   #1
Borat is offline Borat  United States
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Default dumb question about class AB

what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power ?
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Old 4th September 2009, 05:43 AM   #2
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Nothing , no difference. A class B can be a class A, just grossly overbias it.
This would be unrealistic for even a NJW21193/4 (SOA considerations), so IRFP240/9240's would be essential.
(attachment is a happily overbiased class B amp)
OS
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File Type: gif T3T_A.gif (14.3 KB, 461 views)
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Old 4th September 2009, 12:51 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
there is a design situation where the ClassA amplifier is exactly the same as the ClassAB amplifier.
Take a 25W into 8r0 ClassA amplifier as the reference.
The peak output voltage for a continuous sinewave output signal is 20Vpk.
The maximum output current when driving a resistive test load is 2.5A.

The bias of the ClassA output stage is just >1.25A, say ~1.3A.

Now look at a ClassAB amplifier with a 4pair output stage using 130W To247 devices and Re = 0r068
The optimum bias for ClassAB is Vre <26mV. Take Vre=22mV as the optimum determined by distortion/spectra analysis.
Ib~324mA/device.
The total bias current is 1.294A, it is biased identically to the reference amplifier.
Both amplifiers are 25W into 8r0 and both deliver 25W of ClassA into 8ohm.
Both will deliver substantially more ClassAB power into lower impedance loads.
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Old 4th September 2009, 02:09 PM   #4
stoc005 is offline stoc005  United States
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what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power

They will sound different. The sonic signature or summation of all the distortions and noise of each is different. Other than that, they are very similar.
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Old 4th September 2009, 02:21 PM   #5
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Quote:
Originally Posted by ostripper View Post
(attachment is a happily overbiased class B amp)
OS
With very bad 2nd harmonic distortion.
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Old 4th September 2009, 04:52 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
The optimum bias for ClassAB is Vre <26mV. Take Vre=22mV as the optimum determined by distortion/spectra analysis.
Ib~324mA/device.
The total bias current is 1.294A, it is biased identically to the reference amplifier.
Both amplifiers are 25W into 8r0 and both deliver 25W of ClassA into 8ohm.
Both will deliver substantially more ClassAB power into lower impedance loads.
Quote:
Originally Posted by stoc005 View Post
They will sound different. The sonic signature or summation of all the distortions and noise of each is different.
why will they sound different?
Please explain your thoughts.
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Old 4th September 2009, 06:26 PM   #7
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Quote:
Originally Posted by Borat View Post
what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power ?
It is difficult to offer a general answer, as much depends on
the devices and the conditions.

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Old 4th September 2009, 07:03 PM   #8
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Quote:
Originally Posted by stoc005 View Post
what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power [snip].
My 2 eurocents worth: Assuming that the AB bias is the same as the class-A bias, this implies that the class AB has higher supply/higher max output.
They will both work in class A up to that bias point. After that, the class A will start to clip, while the AB goes into, well, AB. As long as they stay below the class A clip point, difference depends on topology and devices.

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Old 4th September 2009, 11:16 PM   #9
Borat is offline Borat  United States
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if the two amps are mostly the same except one clips transients and another doesn't - then why not build all class A amps as heavily overbiased class AB amps ?

have transition from A to AB occur lets say 10db below clipping. if used to actively drive a tweeter such an amp would spend 99% of the time in class A.

?

Last edited by Borat; 4th September 2009 at 11:19 PM.
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Old 5th September 2009, 02:10 AM   #10
eyoung is offline eyoung  United States
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Hola Compadres,

Is not the point of class a bias to get the transistor to a more linear part of the curve and to avoid crossover distortion. Taking into consideration my simple mind and how things work once you get past the working area of class a most designs go to ab as a sort of soft clipping. Just a disclaimer I am not an engineer and I defer to those that I revere.

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