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How to tell if this amp is in class A or AB ???
How to tell if this amp is in class A or AB ???
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Old 9th November 2017, 08:51 PM   #1
piz1 is offline piz1  Italy
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Default How to tell if this amp is in class A or AB ???

First of all, I'm sorry for my poor english.
In the last month I've learned a lot about power amplifiers by reading books like "High-Power Audio Amplifer Construction Manual" by Slone, and many others by Douglas Self etc..
I graduated from a technical school so I know a little about designing an amp and how the several classes work, but i have a big doubt: I designed a circuit like this and I'm pretty sure that it isnt operating in class-b. Let me explain: I first started designing a Single-end amp (that operate in class-a) then in order to increase the efficency I added a second transistor which changed the output stage in a push-pull one. The highly biased transistor makes "cross-over distortion" appear (and consequently switch to class-b) only when you quadruplicate the value of R1 and R2. So by now (with this value) i'm pretty sure that the output stage operate in class A or AB but i don't know how tell if it is still in A and when it switch to AB.

[R3 and R4<1 Ohm, R5 is the speaker and the Op amp are buffer linked to the Volt amplification stage]
[Note that the arrow in Q2 is reversed by mistake]
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Last edited by piz1; 9th November 2017 at 10:16 PM. Reason: Error in schematics
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Old 10th November 2017, 12:14 AM   #2
Lazy Cat is offline Lazy Cat  Slovenia
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What you show is nonsense, better make Class A like this
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Old 10th November 2017, 12:18 AM   #3
piz1 is offline piz1  Italy
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It's just a topology, it isnt a finish circuit however it works, run so hot but works and the THD is <0.05%
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Old 10th November 2017, 12:24 AM   #4
Lazy Cat is offline Lazy Cat  Slovenia
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Again, your sch is nonsense, what you do in real could be anything else. Sch from post #2 has three mistakes, since you are an expert, I’m sure you have knowledge to locate and correct them.
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Old 10th November 2017, 12:32 AM   #5
piz1 is offline piz1  Italy
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I'm not an expert, is the first time that i design an amp from 0. However this is the full circuit, i'm testing it on a breadboard and works. At the moment i'm using really cheap transistor
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Old 10th November 2017, 05:24 AM   #6
Lazy Cat is offline Lazy Cat  Slovenia
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Your engine is red hot and choking, even SanKens wouldn’t help. Read some online cook-amp-book before your lab burn in flames. Use something like LME49810 instead, have a cold beer and enjoy.
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Old 10th November 2017, 06:48 AM   #7
jerryo is offline jerryo  Isle of Man
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piz1......

I think Lazy Cat is being a bit harsh on you. If you have a circuit that works it means that you are learning with a degree of success, so good luck to you. Onwards and upwards. Hopefully others will offer more encouraging comments.
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Old 10th November 2017, 07:04 AM   #8
Mooly is offline Mooly  United Kingdom
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How to tell if this amp is in class A or AB ???
Try altering your circuit like this.

1/ Link the base of the output transistors.
2/ Do not connect the lower opamp output.
3/ Move the feedback point of the top opamp to the output node.
4/ Change the 10uF to something larger as shown.
5/ Increasing R3 and R7 (keep them equal) will make it an easier load for the opamp. Try 2k2 or 3k3.

Doing this will give you an idea of what class B operation looks like and from here it can be altered to class AB.
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Old 10th November 2017, 10:28 AM   #9
piz1 is offline piz1  Italy
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I think there's been some misunderstanding, excuse. I know that it isn't a good schematic, i try to understand the role of every single component before adding a new one. My question was: "how do i realize if the amp is class A instead of AB?"

I can take hundreds schematics better than this but i want to understand the project choices that lead the designer.

I took a look at this Push-Pull vs. Single-Ended in Class A Operation and realize that i have to watch the current (across the emitter resistor) instead of voltage cause there is no difference between the classes if i analize only the voltage with the scope.

Thank you all.
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Old 10th November 2017, 10:53 AM   #10
Mooly is offline Mooly  United Kingdom
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How to tell if this amp is in class A or AB ???
Your output stage most closely resembles a push pull type (but with very ill defined operating points for the output transistors, particularly in a real build with real parts).

During the time the transistor current swings from the bias current value up to but not exceeding twice that level of bias current then the stage is in class A. Once that limit is exceeded then the stage has slipped into AB operation.

Your circuit will have problems in translating from simulation to a real build due to wide transistor parameter variation.
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