go_away

Author Topic: Help understanding AB amplifier (not exactly robot related...)  (Read 1151 times)

0 Members and 1 Guest are viewing this topic.

Offline z.s.tar.gzTopic starter

  • Supreme Robot
  • *****
  • Posts: 540
  • Helpful? 5
  • Linux Guru
Help understanding AB amplifier (not exactly robot related...)
« on: September 29, 2011, 08:14:39 AM »
I'm trying to wrap my head around making a class AB amplifier but I'm having a very hard time understanding more than the basic principal.
For example: http://electronics-diy.com/electronic_schematic.php?id=714 is a promising-looking schematic for a ~20W amplifier.
I get that the first NPN acts as sort of a "class A preamp" in the circuit and that the other transistors are darlington pairs.
What I don't understand is why there are so many resistors. Why does this design have so many and others so few? What are their functions?

If anybody can help me with a link or a quick explanation I would greatly appreciate it as I've been mulling this over for over a week now.
Save yourself the typing. Just call me Zach.

Offline ambcorp2000

  • Jr. Member
  • **
  • Posts: 21
  • Helpful? 0
Re: Help understanding AB amplifier (not exactly robot related...)
« Reply #1 on: September 29, 2011, 06:37:54 PM »
I do not know your background so here is a bit of theory: AB power amplifiers are supposed to give the amplification efficiency of a class B amplifier and also the reduced distortion of a class A amplifier.

Other aspect of an amplifier design - mainly to reduce distortion is to provide a slight negative feedback. - there is a need for biasing resisor for the transistor base and collector. Analog theory is unfortunately very complex involving mathematical modeling and I experienced that very few of my friends during the 4 year BS course actually understood it (not trying to discourage you - its a fact).

The reality is that even after I understood this complex theory, I was not able to successfully design a single commercial grade amplifier. Not just because of my laziness, there are factors like variation from piece to piece of a BJT (yes! that what these little transistors are called - Bi Junction Transistor)

Fortunately, unless you are planning to use valves (which per circuit you are not), thanks to the more recent techniques developed, Class-T can give much better efficiency and surprissingly great sound.

Check this link out for this very inexpensive class-T amplifier that blew me away when I heard it for the first time. Unfortunately the company closed down and copycats are the only ones available but if the underlying principle has not changed, I wouldn't think that the sound quality would change much:

http://www.ebay.com/itm/NEW-Tripath-TA2020-20W-Class-T-Digital-Audio-Amp-Chips-/120751783917?pt=LH_DefaultDomain_0&hash=item1c1d5dffed

You should be able to get the data sheet on the internet that will guide you on how to use the chip. If you do not want to assemble it, just buy a readymade amplifer for around $20...now THAT is cheap!!

Offline z.s.tar.gzTopic starter

  • Supreme Robot
  • *****
  • Posts: 540
  • Helpful? 5
  • Linux Guru
Re: Help understanding AB amplifier (not exactly robot related...)
« Reply #2 on: September 29, 2011, 07:31:30 PM »
Here's what I know: Class A amplifiers use a single transistor that is biased enough to let it handle the whole input waveform. These are extremely inefficient in any sort of power amplifier. Class B amplifiers aren't biased very much (note: I'm guessing what "bias" actually means) and use two transistors in a "push and pull" set up. The problem with this is that there is crossover distortion below 0.7V. Class AB takes the two transistors from B and the biasing from A and creates a balance between efficiency and audio quality.

This is what I've learned from reading tutorials on the subject. I think my real problem though is that I don't have a full understanding of semiconductors.

Also, I'm not going to buy something I can make. End of story.
Save yourself the typing. Just call me Zach.

Offline billhowl

  • Supreme Robot
  • *****
  • Posts: 375
  • Helpful? 32
  • Your success is in your hands.
Re: Help understanding AB amplifier (not exactly robot related...)
« Reply #3 on: September 30, 2011, 04:30:52 AM »
Read the designer blog site for more info.
http://mrcshobbies.blogspot.com/
class AB amplifier tutorial
http://www.electronics-tutorials.ws/amplifier/amp_1.html
http://en.wikipedia.org/wiki/Electronic_amplifier
Quote
Class 'AB' amplifiers use two groups of transistors like class 'B' amplifiers. In most respects, class 'AB' and class 'B' amplifiers are very similar. A class 'A' amplifier is very inefficient. This is not good for a audio amplifier. Some people believe that class 'B' amplifier can never produce clean audio because their output transistors aren't biased 'on'. A class 'AB' amplifier is generally considered to be the best compromise. A class 'AB' amplifier is a class 'B' amplifier which has a small amount of 'bias' current flowing through the output transistors at all times. This eliminates virtually all of the crossover distortion that's possible with class 'B' amplifiers. The bias current is flowing because the output transistors are always conducting current (even without an audio signal). This differs from a pure class 'A' amplifier in the amount of current flow. A pure class 'A' amplifier typically has an enormous amount of current flowing through its output transistors with NO audio signal. A pure class 'B' amplifier has NO current flowing through its outputs with no audio signal. A class 'AB' amplifier is much more efficient than the class 'A' but without the possible distortion of the class 'B'.


This is an OCL amplifier (output capacitor-less amplifier) with direct coupled capacitorless output. It bias at quiescent current of about 30mA. Most of the design will be using differential amplifier (the long-tailed pair) for the negative feedback to allows the amplifier to compare the input to the actual output. But this circuit design had no differential amplifier therefor many of the resistors are used for forming the negative feedback. If your OCL amplifier has no protection circuit, you could potentially fry your speaker, unless you use something like this speaker protection circuit



Offline Soeren

  • Supreme Robot
  • *****
  • Posts: 4,672
  • Helpful? 227
  • Mind Reading: 0.0
Re: Help understanding AB amplifier (not exactly robot related...)
« Reply #4 on: October 11, 2011, 07:22:04 PM »
Hi,

I'm trying to wrap my head around making a class AB amplifier but I'm having a very hard time understanding more than the basic principal.
For example: http://electronics-diy.com/electronic_schematic.php?id=714 is a promising-looking schematic for a ~20W amplifier.
I get that the first NPN acts as sort of a "class A preamp" in the circuit and that the other transistors are darlington pairs.
What I don't understand is why there are so many resistors. Why does this design have so many and others so few? What are their functions?

This is a prime example of someone pushing a ghastly and (by him) ill understood circuit around in a simulator until the results are bearable.

To name just a single (but very distinct) thing about it... The function of the two diodes is negated in this circuit. The diodes are there to counter for two b-e drops (which you have in the conventional AB-circuit), but since he Darlington'd the output, two diodes won't match this, so he compensated with an X-mas three of bias resistors.
In use, there will be cross over distortion as the components heat up and by this change the bias need.


If you just need a 20W amp that works, there's plenty of power op-amps and integrated audio amps that will do, or you can use a regular op-amp to drive a couple of power transistors.
If, OTOH, you're trying to learn, I'd recommend some of the audio sites like Elliots, DIY-audio or similar.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


Get Your Ad Here