eapbg #51 Amplifier Classes A, B, AB, C, and D

[@xtalplanet]

so many dislikes? why. Your lecture is very informative and straightforward

[@eapbg]

That's a good question.

[@felixcat4346]

Excellent. Please do a complete series on RF. You brought a historical perspective to amplifies. So class A from the the turn of the last century, class B came next, class AB is from the 1970's. Class D is the newest tek. Almost all the stuff you buy today are single chip class d amplifiers. To save the planet.

[@eapbg]

The only real drawback of a class D is that they have the highest distortion of all the different amps. RF is not my strong suit by any means. I've been tinkering with a software-defined radio and may do some videos on that. I was planning on doing some frequency domain stuff too which will include some am and fm stuff.

[@arzoch]

Thank you for a nice clear explanation and introduction to these circuits.

[@scottdurrwachter7127]

wow...I cant speek for anyone else...I think ya did a great job...I am presently trying to fix a big PA amp 1000watt...it is a class AB...instead of diodes setting up the bias it is transistors...and this amp has a variable resistor.....do I put my dc voltmeter at the middle and adj to zero volts???I guess the bias adj for component values changing....and....your dc supplies drifting over time c...correct?

[@eapbg]

Without a schematic, it is difficult to say what is going on. There are hundreds of different ways to do this including but not limited to preamplifiers, negative feedback, and overcurrent protection. What I would suggest though is to feed a small test waveform into the input and then look at the result of the output with an oscilloscope. This will show you whether the waveform is centered and if there is any crossover distortion.

[@hybridskeeter]

I was wondering, could you feed some of the output from a class C amp back into the input, so that you could start it with a single pulse and it just keeps on going?

[@eapbg]

The short answer is: no. The long answer is: tying the output to the input would generate a feedback loop. I'm sure you've heard what it sounds like when a microphone is held to a speaker. This type of connection is known as positive feedback. In an amplifier, the input is amplified by some amount. Also, amplifiers have a small amount of phase delay meaning what happens at the input does not instantly happen at the output, it happens some very tiny amount of time later. Armed with this knowledge it can be seen that the output would make the input a bit greater. The input would make the output a little greater and then the output would make the input a little greater. This reaction would run away until the amplifier saturates. If lucky it won't explode. If unlucky the drive element will burn out. Regardless, no matter how much the output is reduced there will always be a positive relationship which would cause a runaway condition. There is also a negative feedback loop. This type of loop can keep an amplifier from running away.

[@hybridskeeter]

@@eapbg That makes sense. Many thanks and happy new year!

[@maxxsmaxx1901]

Very clear and precise lecture . Thank you !

[@billwilliams6338]

What amplifier class biases the FET from the input drive level?

[@eapbg]

I'm not sure what you're asking.

[@billwilliams6338]

When you increase the input drive level amplitude into the FET amplifier, the input drive level with Bias the FET. This happens in Class AB, but what kind of biasing is this called? its not a DC bias its based on the input drive level to the gate of the FET.

[@eapbg]

Still not sure what you are asking. In an AB the bias has to be there all the time or you will get cross over distortion.

[@boybravo689]

Sir may I ask if a class b amplifier be configured two same type of transistor and different type of transistor tnx Sir

[@eapbg]

They have to be a matched set of pnp and npn transistors.

[@boybravo689]

@@eapbg Tnx for the response Sir

[@siavoshkasravi398]

I think the trick in D class is that the filter after MOSFETs actually creates sinusoidal waves. I hate resistors. They are always there and nobody knows what they are doing.

[@eapbg]

Correct, the filter strips out the higher frequencies and all that is left behind is the sinusoidal audio signal.

[@iamwangine8752]

Class A loss 50% without any signal at start , is les than 30% eficiency.

[@eapbg]

Without any signal, the loss is 100%. At best 50% can be delivered to load with a signal.

[@iamwangine8752]

eapbg I never seen more than 30% eficiency on classA . If you speak only math calculation on paper. Yea is 50% only teoreticaly , Sorry for my english.

[@BobLeeSwagger8264]

I am Wangine According to the math equations, 50% is theoretically possible. In practice, it is far less than that. Probably at best 30% for a class A amplifier and wouldn't be surprised with numbers down to 5%.

[@felixcat4346]

Who care what the efficiency is? It all comes down to how does it sound. Are we worried that the efficiency of a dopey amplified in your home will deplete the ozone layer and increase global warming? Is that what the concern is?

[@siavoshkasravi398]

@@iamwangine8752 when you calculate the efficiency, it will be an average somewhere between 0% and 50%. As I understood 50% is ideal. so you should take 40% instead.