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Old 25th January 2011, 11:49 AM   #1
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Default Small low power beginner amp

Heres a true beginner circuit:

http://i.imgur.com/yLaRn.jpg

And no its not two transistors, its darlingtons like MJE703/803 or TIP142/147.

The two resistors sets the bias current, around 10-20mA is a good starting point, above that you need a bias compensation circuit.
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Old 25th January 2011, 06:56 PM   #2
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So where is the image that we are looking for.
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Old 25th January 2011, 07:02 PM   #3
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Here:

http://i.imgur.com/aI0Gx.jpg

But it needs a cap in series with input and feedback.
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Old 25th January 2011, 07:56 PM   #4
jaycee is offline jaycee  United Kingdom
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Yes the important part is you must make the opamp drive a constant load... then the output transistors follow the current being drawn by the opamp. Of course this only works with single opamps.

You might be surprised to find that this idea is actually used a fair bit, more refined than what you show here, but the same principle never the less. Usually it involves current mirrors which then drive the later stages, but the principle is the same.
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Old 25th January 2011, 08:41 PM   #5
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Like this:

http://i.imgur.com/ZFtQA.jpg
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Old 25th January 2011, 09:01 PM   #6
jaycee is offline jaycee  United Kingdom
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If you want to explore this further, look at the Alexander amp, its an Analog Devices app note AN-211.
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