kambliarup

What is a class D amplifier?
I know about Class A, B, AB, C amplifiers but never heard of class D
Pls Help

kchriste

It is an amplifier that outputs a digital PWM signal. The amplifier is followed by a lowpass filter to filter out the highfreqency PWM but leave the signal created by varying the duty cycle of the PWM. The period of the PWM is higher than the signal you wish to amplify. These amplifiers have efficiencies of 90% or higher depending on the design.

Analog

Methinks you mean the frequency of the PWM is higher than the signal frequency being amplified. A higher (longer) period is the inverse of the frequency.

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kambliarup

Class C amplifiers have effeciency of greater than 95% so why do we need class D amplifiers????

kchriste

That's exactly what I meant. I really should proof read more!
You can get a sine wave out of a class D. Try that with a class C! Class C amps really are used in RF amplifiers in systems using FM or CW modulation. You need class A/B for AM transmissions. Class D works well in the audio range for audio power amps etc.

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Hero999

You can get a sine wave from class C, but you can't get more complex waveforms such as those found in speech and music.

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audioguru

A class-C RF circuit makes a sine-wave because it has a tuned circuit and works at only one frequency.
I guess we have all heard cheap car audio systems where the sub-woofer is tuned and makes "one-note bass" maybe with a class-C amplifier.

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Uncle $crooge

Sig239

While I do see the point you are making, class c amplifiers are used to amplify FM signals, clearly not only one frequency.

Furthermore, class c amps can and are used in high level AM systems, but they are used before the mixer/modulation step. In other words, it is more efficient to generate high power AM by amplifying the RF frequencies with a class c amplifier, then amplify the audio signal with a linear amplifier (in the right proportions of course), then mix filter etc.

Please don't get me wrong, I'm only pointing out things that I found interesting while learning about different amps. I have a lot of respect for the wisdom and knowledge expressed in this forum by the likes of audioguru, nigel goodwin, kchriste etc. etc. the list goes on. I'm very much in the learning phase, as are many on these forums. I've been a hobbyist for about 15 yrs. but have never taken on electronics as a career, and as such, that is what my knowledge reflects. Happy learning everyone!!

Nigel Goodwin

It's effectively 'one' frequency, with a little spread either side - the wider the spread required the lower the gain of the stages. Class C stages require tuned outputs in order to function.

You appear to be talking about high level AM modulation?. You simply generate a high power carrier wave, and directly AM modulate the final stage. The 'transmitter' itself is really just a CW transmitter.

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Hero999

I think he's talking about some large DSB AM transmitters which use a class C output stage; the power supply to this is fed from a large class D audio amplifier which modulates the output.

If you want to learn about class D amplifiers then Wikipedia is one of the best places to start.
http://en.wikipedia.org/wiki/Class_D_amplifier

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Sig239

I was not disagreeing with the fact that a tuned output stage is needed. However to say that a class c amplifier only works at one frequency is clearly incorrect. All tuned amplifiers have center frequency and bandwidth, and the bandwidth on a class c amp is not zero. That is all I am saying.

Yes, I clearly stated, as you quoted, that I was refering to high level AM modulation. But I was not refering to CW. DSB AM is also generated in this fashion.

audioguru

You mentioned FM.
A 100Mhz FM station has a frequency swing from 99.925MHz to 100.075MHz at max volume. I call that one frequency because the 2nd harmonic at 200MHz and the 3rd harmonic at 300MHz are filtered out by the tuned circuit.

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Uncle $crooge

Sig239

My appologies for the misunderstanding. I understood you to mean one frequency when you really meant one channel or station etc. I would not call 100 KHz one frequency. Any class of TUNED amplifier will only operate on a certain bandwidth and a class c amp requires the tuning components (at least to generate a sinewave). However if it is only pulses that you need then the class c amp. can be used at any freq. right?

On a different note, aren't the harmonics of the carrier the easiest to filter out? What about the harmonics of the baseband signal?

audioguru

If the modulator and demodulator are linear, then there won't be any added baseband harmonics, just the same harmonics as the original signal has.

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Uncle $crooge

Sig239

Your statement seems somewhat contradictory. Modulation is a multiplicative process and thus is not ever linear. When you mix two frequencies (not to be confused with adding or summing as in audio mixers) such that two new frequencies are created, you have removed all linearity.