The jury is still out on TIM. Some people believe this distortion to be caused by a combination of other well known distortion types.
One of the misconceptions is the square wave output testing into a capacitive load, measuring the overshoot-ringing to evaluate transient response/distortion of the amplifier. It was proved to show nothing about the performance of the amplifier, but only the resonating of the Q-damped output inductor with the load.
One of the misconceptions is the square wave output testing into a capacitive load, measuring the overshoot-ringing to evaluate transient response/distortion of the amplifier. It was proved to show nothing about the performance of the amplifier, but only the resonating of the Q-damped output inductor with the load.
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audioguru2
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Hi AN920,
I agree that an amplifier should be designed for low distortion before a modest amount of negative feedback is applied.
The OPA134 opamp has a very low distortion of 0.00008% at 1kHz with a gain of 1, and 10 times more with a gain of 10. Its open-loop gain at 1kHz is 10,000 times so its open-loop distortion might be about 0.8% which is pretty good.
I agree that an amplifier should be designed for low distortion before a modest amount of negative feedback is applied.
The OPA134 opamp has a very low distortion of 0.00008% at 1kHz with a gain of 1, and 10 times more with a gain of 10. Its open-loop gain at 1kHz is 10,000 times so its open-loop distortion might be about 0.8% which is pretty good.
Another problem with feedback is that you need to increase the slew rate BW of the front end gain stages including the feedback network by the factor of feedback (40 to 50 times) to prevent/minimize dynamic intermod distortion.
This was the reason so many transistor amplifiers sounded so bad in the early 70's. This situation improved after it was discovered and pointed out by Dr. Otala in his published papers. He suggested that minimum slew rates for power amps with a 30kHz BW should be 100V/us !
Also adding negative feedback to single-ended, differential pair or push-pull output stages using FET's can be very surprizing, as it actually may boosts higher order intermod distortion over the full spectrum.
This was the reason so many transistor amplifiers sounded so bad in the early 70's. This situation improved after it was discovered and pointed out by Dr. Otala in his published papers. He suggested that minimum slew rates for power amps with a 30kHz BW should be 100V/us !
Also adding negative feedback to single-ended, differential pair or push-pull output stages using FET's can be very surprizing, as it actually may boosts higher order intermod distortion over the full spectrum.
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Here is a good explanation about the subtle differences between biased Class-B and AB
From the works of "Wang & Chien"
It is important to realize this. Many people and even most engineering text books get this part wrong or don't understand the difference.
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audioguru2
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Hi AN920,
I have seen very simple class-B amplifiers (like the LM324 opamp) that do not have their bias "ridgidly controlled" (most do not have any bias) and they produce terrible crossover distortion.
All the class-AB amplifiers that I have seen have a fairly small idle current of about 50mA which produces no crossover distortion, a very low level of TIM distortion and little heating.
I have seen very simple class-B amplifiers (like the LM324 opamp) that do not have their bias "ridgidly controlled" (most do not have any bias) and they produce terrible crossover distortion.
All the class-AB amplifiers that I have seen have a fairly small idle current of about 50mA which produces no crossover distortion, a very low level of TIM distortion and little heating.
audioguru2
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Hi AN920,
I mentioned the LM324 opamp because it and its sister the dual opamp LM358 are the first low-power opamps and they use insufficient bias current in their output transistors to reduce power consumption. They are the only opamps that have horrible crossover distortion.
Class-AB power amplifiers use an idle current of about 50ma to avoid crossover distortion.
I mentioned the LM324 opamp because it and its sister the dual opamp LM358 are the first low-power opamps and they use insufficient bias current in their output transistors to reduce power consumption. They are the only opamps that have horrible crossover distortion.
Class-AB power amplifiers use an idle current of about 50ma to avoid crossover distortion.
From another author: Randy Slone's book
Enthusiasm for class A today is often supported by
a variety of misunderstandings and myths. The following list provides
some major examples.
1. "Crossover distortion in class B is a constant. At low listening
levels, it becomes very prominent."
This is an understandable assumption, but fortunately it is not true. The absolute level of crossover distortion decreases as the output signal level is decreased,
but not in a proportional manner. The principle behind this phenomenon will be discussed later. However, the end result is only a slight increase in THD as the output level is drastically reduced.
2. "Even though class A amplifiers are inefficient, they are easy to
design, and the sonic quality will be exceptional even in poorly
designed units."
In reality, a well-designed class A amplifier is about
equal in complexity to a good class B design, and the class A amplifier
will virtually always be more expensive.
3. "In comparison to class B designs, class A amplifiers sound better."
A well-designed class A amplifier operating under favorable conditions
compared to a well-designed class B amplifier operating under
the same conditions will not reveal any perceivable differences. Otherwise,
the human ear would have to be capable of hearing distortion
levels into the hundredths of a percent at 20 kHz. But we must be careful
to compare apples with apples. There are many poorly designed
class B amplifiers around, and their inferior performance characteristics
are readily detectable.
CLASS-AB
Class AB is not really a class but rather a poor marriage of both class A
and class B characteristics. It has been erroneously taught that a class
AB amplifier becomes a class B amplifier when all bias is removed from
the OPS. This is incorrect. Technically speaking, class B pertains to an
OPS wherein the output devices are conducting for one-half (that is, 180
degrees) of the signal cycle. In order to accomplish this action, a small
forward bias must be applied to the output devices to overcome their
inherent Vbe drop. If this forward bias is removed from a class BOPS,
the output devices begin to conduct for less than 180 degrees of the signal
cycle, placing the OPS closer to the category of class C than class B.
Class AB pertains to a class BOPS that has been overbiased so that
each output device conducts for more than 180 degrees of the signal
cycle. This was thought to improve crossover distortion through the
mental imagery of the output devices sliding through the crossover
region in linear class A operation. However, what actually results is a
form of crossover distortion referred to as gm doubling. That is, in the
crossover region while all of the output devices are conducting simultaneously,
their current gain factors are doubling (i.e., summing), creating
a severe wobble in the linearity. Both Fourier analysis and distortion analysis
prove that this doubling effect causes distortion
harmonics as bad as if the output devices were severely underbiased.
Class AB operation also causes increased power dissipation in the
OPS, decreasing efficiency and reliability. Since class AB operation
provides no advantages whatsoever and only serves to degrade linearity
and create additional heat problems, it should be entirely dismissed
as a "good idea that just didn't work out."
CLASS B
As defined previously, class B pertains to an OPS wherein the output
devices are biased to conduct for 180 degrees of the signal cycle. In
times past, class B operation was referred to as push-pull operation
(analogous to sourcing-sinking action), but this is a misnomer. Output
devices in a class B OPS will source current to a load for a half-cycle,
but they do not sink current during the opposite half-cycle: they are
cut off. The term push-pull should be confined to class A type stages.
At least 99 percent of all audio power amplifiers utilize a class B
OPS. I make no apology for the fact that this book is devoted to the
methodology and development of class B audio amplifiers since that
appears to be the only practical and viable choice at our current technological
level.
Enthusiasm for class A today is often supported by
a variety of misunderstandings and myths. The following list provides
some major examples.
1. "Crossover distortion in class B is a constant. At low listening
levels, it becomes very prominent."
This is an understandable assumption, but fortunately it is not true. The absolute level of crossover distortion decreases as the output signal level is decreased,
but not in a proportional manner. The principle behind this phenomenon will be discussed later. However, the end result is only a slight increase in THD as the output level is drastically reduced.
2. "Even though class A amplifiers are inefficient, they are easy to
design, and the sonic quality will be exceptional even in poorly
designed units."
In reality, a well-designed class A amplifier is about
equal in complexity to a good class B design, and the class A amplifier
will virtually always be more expensive.
3. "In comparison to class B designs, class A amplifiers sound better."
A well-designed class A amplifier operating under favorable conditions
compared to a well-designed class B amplifier operating under
the same conditions will not reveal any perceivable differences. Otherwise,
the human ear would have to be capable of hearing distortion
levels into the hundredths of a percent at 20 kHz. But we must be careful
to compare apples with apples. There are many poorly designed
class B amplifiers around, and their inferior performance characteristics
are readily detectable.
CLASS-AB
Class AB is not really a class but rather a poor marriage of both class A
and class B characteristics. It has been erroneously taught that a class
AB amplifier becomes a class B amplifier when all bias is removed from
the OPS. This is incorrect. Technically speaking, class B pertains to an
OPS wherein the output devices are conducting for one-half (that is, 180
degrees) of the signal cycle. In order to accomplish this action, a small
forward bias must be applied to the output devices to overcome their
inherent Vbe drop. If this forward bias is removed from a class BOPS,
the output devices begin to conduct for less than 180 degrees of the signal
cycle, placing the OPS closer to the category of class C than class B.
Class AB pertains to a class BOPS that has been overbiased so that
each output device conducts for more than 180 degrees of the signal
cycle. This was thought to improve crossover distortion through the
mental imagery of the output devices sliding through the crossover
region in linear class A operation. However, what actually results is a
form of crossover distortion referred to as gm doubling. That is, in the
crossover region while all of the output devices are conducting simultaneously,
their current gain factors are doubling (i.e., summing), creating
a severe wobble in the linearity. Both Fourier analysis and distortion analysis
prove that this doubling effect causes distortion
harmonics as bad as if the output devices were severely underbiased.
Class AB operation also causes increased power dissipation in the
OPS, decreasing efficiency and reliability. Since class AB operation
provides no advantages whatsoever and only serves to degrade linearity
and create additional heat problems, it should be entirely dismissed
as a "good idea that just didn't work out."
CLASS B
As defined previously, class B pertains to an OPS wherein the output
devices are biased to conduct for 180 degrees of the signal cycle. In
times past, class B operation was referred to as push-pull operation
(analogous to sourcing-sinking action), but this is a misnomer. Output
devices in a class B OPS will source current to a load for a half-cycle,
but they do not sink current during the opposite half-cycle: they are
cut off. The term push-pull should be confined to class A type stages.
At least 99 percent of all audio power amplifiers utilize a class B
OPS. I make no apology for the fact that this book is devoted to the
methodology and development of class B audio amplifiers since that
appears to be the only practical and viable choice at our current technological
level.
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audioguru2
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I disagree. 99% of amplifiers are certainly not class-B, they are class-AB. Many of these class-AB amplifiers have extremely low distortion. If they have a very small amount of gm-doubling distortion then it is buried way down in the noise level.
It is anybody's right to disagree 
Just remember that these statements are made by persons that are considered world renowned authorities on the subject and have presented numerous studies and papers to bodies like the IEEE and other professional publications. They have backed up their statements with simulation, experiments and measurements in a lab.
Just remember that these statements are made by persons that are considered world renowned authorities on the subject and have presented numerous studies and papers to bodies like the IEEE and other professional publications. They have backed up their statements with simulation, experiments and measurements in a lab.
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audioguru2
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The experts say that an amplifier with a peak output current of 5A (100W into 8 ohms) operates in class-B when one transistor conducts 180 degrees and the other transistor conducts the other 180 degrees.
But most of the amplifiers made have both transistors conducting 50mA (class-AB) at idle, which is only 1% of the peak current. Then one transistor conducts for 180.5 degrees and the other transistor conducts another 180.5 degrees with only 1% of overlap. Do the experts call it class-B by mistake?
Douglas Self talked about his class-B amplifier with insufficient bias, optimal bias and excessive bias. Class-B? No, the transistors conduct for more than 180 degrees each so it must be class-AB.
The crossover distortion or gm-doubling distortion on these excellent amplifiers is 100dB down, much lower than other distortions and noise. It is completely inaudible.
But most of the amplifiers made have both transistors conducting 50mA (class-AB) at idle, which is only 1% of the peak current. Then one transistor conducts for 180.5 degrees and the other transistor conducts another 180.5 degrees with only 1% of overlap. Do the experts call it class-B by mistake?
Douglas Self talked about his class-B amplifier with insufficient bias, optimal bias and excessive bias. Class-B? No, the transistors conduct for more than 180 degrees each so it must be class-AB.
The crossover distortion or gm-doubling distortion on these excellent amplifiers is 100dB down, much lower than other distortions and noise. It is completely inaudible.
audioguru2
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I wish I have all the documents written by Self. His article in Wireless World magazine probably has more details.
