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Old 3rd September 2015, 06:46 AM   #1
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Unhappy Single stage class A calculations...

Hi guys, I am trying to understand how to design a single stage class A audio amp. So far I've come across the following links:

Class A Amplifier - Class-A Transistor Amplifier Tutorial

Common Emitter Amplifier and Transistor Amplifiers

The values I'm getting don't seem right at all and I feel like I am wasting my time.

The transistor I plan on experimenting with is the TIP142 and a 15V supply.
http://www.mouser.com/ds/2/389/CD00000914-249544.pdf

Will someone please point me in the right direction for information to help me find the resistor values?
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Old 3rd September 2015, 07:08 AM   #2
Mooly is offline Mooly  United Kingdom
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That is a bit of an un-answerable question without knowing what you are trying to achieve. What kind of load are you looking to drive ?
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Old 3rd September 2015, 07:10 AM   #3
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You could try starting out with a simpler single stage classA amp - a classA buffer. That is - only current amplification, unity gain voltage. This is a lot easier to grasp than an amp with voltage gain too. You could use opamps if you need additional voltage gain. You'll need two power transistors - one for the buffer and the other for a constant current source load.
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Old 3rd September 2015, 07:48 AM   #4
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I am trying to drive a 4 or 8 ohm speaker load. The power to the speaker load I thought would depend on the current available from the 15V supply but if that isn't how things work then I would choose anything under 5W.

I have an adjustable voltage op amp stage that I can drive the input of the class A if that will be useful.
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Old 3rd September 2015, 08:00 AM   #5
Mooly is offline Mooly  United Kingdom
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Well it should depend on the current available and also on the current that the collector load resistor allows to flow.

Maybe if you just draw out what you have and then we can take a look. You will never get a single ended stage to give 5 watts (rms ?) from a 15 volt rail and for that to be developed across 4 or 8 ohms.

Draw out what you have and then we can test it and see.
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Old 3rd September 2015, 06:46 PM   #6
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Click the image to open in full size.
I'm trying to make this circuit produce as much power as possible with a TIP142 and a 15V supply.

It doesn't matter to me how much power it is able to make, I'm only trying to learn how to design for this circuit.

I am attaching the above file for LTSpice in case that is helpful to anyone. The datasheet link and other guides for this circuit are in post#1.
Attached Files
File Type: asc Class A design.asc (1.3 KB, 3 views)
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Old 3rd September 2015, 06:50 PM   #7
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Quote:
Originally Posted by abraxalito View Post
You could try starting out with a simpler single stage classA amp - a classA buffer. That is - only current amplification, unity gain voltage. This is a lot easier to grasp than an amp with voltage gain too. You could use opamps if you need additional voltage gain. You'll need two power transistors - one for the buffer and the other for a constant current source load.
That sounds fine. Can you show me where to look to see how to find what resistor values I need to use?

Is it really that complicated to add voltage gain to it too, even in a simple circuit like this?
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Old 3rd September 2015, 07:18 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

The first thing to realise is a single device amplifier is a
very poor option and there is no good way to design it.

Death of Zen - A new Class-A power amp

rgds, sreten.
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Old 3rd September 2015, 07:27 PM   #9
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That much I understand. You have to start somewhere though right? And it doesn't get any simpler than this circuit so it's a start.
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Old 3rd September 2015, 07:33 PM   #10
Bigun is offline Bigun  Canada
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I recommend this, from what I have read (I have not built it myself - it is on my 'list'...)

SEWA - Seven Watt Amplifier
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