how do class D amplifiers work???

[samcheetah]

ive learnt that transistors have two basic uses which are amplification and switching. for amplification transistors are operated in the active region and for switching transistors are operated at the two extremes i.e. cutoff and saturation. as we progress along the classes of amplifiers (A, B, AB, C, D) the amount of time the transistor remains in the active region reduces. in class A the transistor stays in the active region for the whole cycle of the input voltage. in class B its less than that and in class C its even lesser than class B. now in class D the transistor remains in the active region for zero time.

now my question is that isnt the behaviour of a class D amplifier like a switch. so is it a switch or is it an amplifier. i know that this switching mechanism produces amplification, but how ???

i have read that the average power delivered to a load can be varied by PWM. and PWM is the basis of class D amplifiers.

what happens next?????

the story ive heard about class D amplifiers is incomplete. would someone please tell me how switching produces amplification??

 

[Nigel Goodwin]

In a class D amplifier the input signal is converted into a PWM signal, usually by a comparator - you feed the signal into one input, and a high frequency triangle wave into the other - this produces a PWM representation of the original input signal.

This PWM signal is then amplified in the class D amplifier, which simply switches between the two supply rails - so the output is a PWM signal equal to the supply rails.

This is then fed through a high power passive low-pass filter, this removes the original high frequency switching frequency and leaves the amplified audio signal.

Obvious problems are the quality of the original analogue to PWM conversion, and the passive filtering feeding the loud speaker. But decent specifications are often claimed for them.

 

[samcheetah]

This PWM signal is then amplified in the class D amplifier, which simply switches between the two supply rails - so the output is a PWM signal equal to the supply rails

how does this happen. i know about everything before this and after this. but i dont know how this happens.

i would be glad if someone told me how this works

 

[Nigel Goodwin]

This PWM signal is then amplified in the class D amplifier, which simply switches between the two supply rails - so the output is a PWM signal equal to the supply rails

how does this happen. i know about everything before this and after this. but i dont know how this happens.

i would be glad if someone told me how this works

It's just transistor switches! - you turn the top transistor ON, the output goes HIGH, you turn the bottom transistor ON, the output goes LOW. But just like a linear amplifier you have specific drive and power requirements, so class D amplifiers are usually more complicated than class B (or A/B) ones.