5 ms·
Yes, the definition of linearity is that f(A) + f(B) = f(A+B), which you have contradicted in your GP post. If a transistor is fully on, then doubling the input
by ballooney 8y ago
Yes, the definition of linearity is that f(A) + f(B) = f(A+B), which you have contradicted in your GP post. If a transistor is fully on, then doubling the input voltage to it will not double the output, because it's already at maximum. That is very precisely not a linear device. That's why linear amplifiers, like Class A, operate the transistor in a partially switched state, where increasing the input signal (about some bias point, and with feedback to turn a real transistor into a more ideally-behaving one) gives a corresponding increase in output signal.
Class D (switching) amplifiers are _not_ linear, they trade linearity for efficiency. The article you linked to actually says this:
"Over the years, a number of different biasing schemes have been developed for amplifiers, from extremely linear Class A operation, in which the active devices essentially remain powered on through the full sinewave cycle of an input signal, to more power-efficient schemes, such as Class D, in which multiple transistors are switched on and off to conduct different portions of an input waveform."
I think what you are doing is confusing the linearity of the transistor with the linearity of the whole audio amplifier treated as a black box, which is why I suggested you rephrase your argument rather than say you are categorically wrong, however what you said was:
"The thing is that transistors’ behavior is more linear and controlled when fully on or off rather than when used as continuously variable."
... which I'm afraid is really just wrong.
- AnthonBerg 8y agoThanks but no thanks. When a transistor is off, it’s off. Then it takes a certain input voltage to get it going. Then there’s a varying curve to full output. In contrast, fully switching on or off “gets rid of the curve” and tranforms the problem into one of timing, and one of a modulation scheme for a series of on or off states. In that respect I contend that it is an useful thought to consider transistors’ behavior (in an amp) as more linear and controlled if used in only the on or the off state. The output quality of Class-D amps in relation to price, size, complexity, and power, and cost suggests to me that the gains are not only in power efficiency. Class D amps do leave the operation of transistors in their linear domain on the table, and do gain efficiency by that. Overall linearity as a control mechanism is something I stand by saying is increased. You may present an alternate take of course.
- mark-r 8y agoIt's an interesting contradiction to build a more linear amplifier by intentionally using the transistors themselves in a non-linear fashion.
- AnthonBerg 8y agoIt is :) But it’s also only a contradiction on the surface of the words. The amplifier is quite linear even though the transistors aren’t used in what’s called their “linear region”, but I’d say they’re still used in a linear fashion.
- Junk_Collector 8y agoI think you are confused. To get a linear amplifier, you operate your output transistors in a state of Saturation. This gives a linear transfer function to the drain based on the Gate to source voltage and you use a method of choice to hold the source constant. This is what a class A amplifier does. An AB amplifier has 2 transistors to extended the range of inputs on the gate for which the output is linear at the cost some distortion when switching between the A and B (hi and low side) transistors. A class D amplifier uses a smaller amplifier internally (which can be a single transistor or more complicated) that is driven open loop so that it is either hard off or hard on but very fast and efficient. The hard on state is known as full compression because the output power will not change with input power. This is purely non-linear behavior and can be very easily verified by measuring the high IMD components it generates. Class D amplifiers employ significant filtering to suppress these non-linear terms and allow you to recover the signal. Mathematically, they violate the linearity condition of F(ax) = aF(x) Also you should reread the article you learned from. It directly contradicts you.
- AnthonBerg 8y agoIt’s not about me; The “you”-ing is incredibly offputting. We are simply and obviously applying the word “linear” a little differently. I am aware that it is a specific term for certain behavior / a certain operating mode of transistors. I’m referring to it in another wider context on purpose. As another example “linear” in “linear algebra” isn’t about saturation or gates. I’ve gone through this before in this thread and believe my perspective is now pretty complete and clear here and won’t engage further.