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Old 20th March 2011, 11:54 PM   #1
ea6b607 is offline ea6b607  United States
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Default Class AB Amplifier - New to the analog world

First, been browsing for a bit and this is a great site. Now onto the issue at hand.

First I'm an EE major and we are required for a semester project to build an audio device that consists of a lot of requirements which I won't go over here.

The power amplifier stage is required to be a Class AB amplifier with between 3W and 25W of output power and a THD < 1%. While breadboarding I had issues with thermal runaway and asked the professor for suggestions. Where upon he gave some horrible advice to remove the rubber diode I was using to bias the power transistors and simply feed the output of the emitters from the power transistors through the feedback loop of the opamp (essentially having the opamp handle the biasing of the transistors). Now the design works fine in simulation of course, but I have to wonder if it is even possible to meet the THD requirement without the rubber diode (or regular diodes) if quality opamps are used?

I'm not looking for high quality audio just to meet the requirements for right now. If funds and time allow I may come back and make a better design but if what I have will work then I need to move on to other parts.

I appreciate any insights, insults and inferences.
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Old 21st March 2011, 12:38 AM   #2
djoffe is offline djoffe  United States
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please post your schematic as it is...give a few comments about the parts of concern to you.
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Old 21st March 2011, 12:48 AM   #3
ea6b607 is offline ea6b607  United States
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Sure,

Here's the schematic (after the professors suggestion):

Click the image to open in full size.

My previous design was the same but with a 3904 used to create a bias voltage on the power transistors. Here's the simulation (the input is not a sin wave). It works fine but this is with an ideal op amp so the slew rate is infinite:

Click the image to open in full size.

The opamp I'm planning on using is a OPA134.
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Old 21st March 2011, 12:49 AM   #4
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Quote:
Originally Posted by ea6b607 View Post
The power amplifier stage is required to be a Class AB amplifier with between 3W and 25W of output power and a THD < 1%.

I'm not looking for high quality audio just to meet the requirements for right now. If funds and time allow I may come back and make a better design but if what I have will work then I need to move on to other parts.

I appreciate any insights, insults and inferences.
You should be able to meet these criteria quite easily with a fairly simple design and have it sound decent without breaking the bank. For the more expensive items like power transformer, heatsink and filter capacitors you can reclaim used parts from other dead equipment, the rest should be readily available and inexpensive.

Post your current schematic and perhaps get some ideas off the board. Have you consulted any published resources from Bob Cordell, Dougls Self or Randy Slone? Any of these would be a good starting point for an introduction to class AB amplifier design.
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Old 21st March 2011, 02:14 AM   #5
ea6b607 is offline ea6b607  United States
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Ok, I started reading up more and this appears to be a class B push-pull amplifier if I am not mistaken? What exactly is the difference between a class AB and a class AB push-pull? If I am reading it correctly then the dc biasing of the transistors that place them into forward active region is what makes them the class AB.

I apologize for my ignorance in this subject, I'm much better with a MCU.
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Old 21st March 2011, 02:43 AM   #6
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Rane uses this same topology in their 6 channel rack mount headphone amplifier.
I used to have one and it sounded very good. jer
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Old 21st March 2011, 02:51 AM   #7
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OK, so the opamp is basically required to slew through the output buffer transistors crossover region where they aren't conducting quickly enough to avoid serious crossover artifacts. This actually works with a sufficiently fast opamp. This would be is the strictest sense actually closer to class C as the OPS has a relatively broad area of non-conduction.

There ia no real true class B as device physics do not allow exactly 180 degrees of conduction per output device. An 'optimally biased' arrangement is often called a class B design and has a relatively modest bias current. Increasing the bias to produce a broader region of mutual conduction results in a class AB design where a small amout of the amps output is class A and beyond that the outputs cut off alternately with the signal, a-la class B. Really jacking up the bias results in a class A amp where the output devices are always in the linear region and never cut off or saturated. Class AB or B will always be 'push-pull', class A can be 'single ended' or 'push pull'.

Is the opamp required in the design? Could you handle something fairly simple that is a discreet design? If the opamp is a required part of the design I'd suggest using it as the amps IPS and using a conventional discreet VAS and OPS.

For a simple discreet, look at something like this: It can give you power at the upper range you are looking for with signifigantly less distortion than you are allowed. The diodes would be mounted to the heatsink to stabalize bias. You could sub those with a Vbe multiplier, but the goal is as dead simple as possible with reasonable utility and performance.
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Old 21st March 2011, 03:38 AM   #8
ea6b607 is offline ea6b607  United States
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This is starting to make a lot more sense now. It sounds like the op amp feedback design will work, the opamp I choose has a very good slew rate so that should help.

In regards to the schematic jkuetemann posted, what is the advantage of using a diferential amplifier for the gain and feedback stage as oposed to an opamp? I've seen a lot of designs that used that but always thought that opamps provided a more linear gain and they have a differential amplifier as the output stage anyways.
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Old 21st March 2011, 05:09 AM   #9
CBS240 is offline CBS240  United States
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If the design criteria is to be class AB, then the circuit in post 3 will not cut it. For class AB the output devices must have some bias. This complicates things because you now run into thermal vs transconductance issues, and in particularly with BJTs the reduction of Vbe vs temperature, which can also be expressed in conductance.
BTW, at what impeadance does this 3-25W amp need to drive? Is there any specific criteria for the devices you can use? If there are no restrictions, I would suggest you look into the 'Thermaltrack' BJT output devices from On-semi. A simple bias arrangment for the outputs following an op-amp would be my suggestion. Granted they are capable of more power than your goal, but it would simplify things quite a bit. Mr Cordell has plenty of information about these devices in his book....not hard to reference here at this site.
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Old 21st March 2011, 05:24 AM   #10
ea6b607 is offline ea6b607  United States
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Ya I realized it's not a class AB, my professor is however has no knowledge of what he is teaching, most of the TA's I've had are much more knowledgeable. The load is 8ohms. Those thermaltrak transistor/diode pairs are pretty cool, the entire project has to have a BOM of less then $150 however and $20 for power transistors alone plus a bit more for heatsinks is a little too much, added it to my bookmarks though. My opamps are coming in today and I'll let everyone know how the initial design works.
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