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Old 11th April 2012, 10:10 PM   #1
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Default Op. Amp + MOSFET Follower

Apmlifier with discrete MOSFET follower

Output stage bias - 14mA. Works in AB class.
Supply - +/-6V -+/-8.4V
Gain - x2,6

What do you think about this amp?

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Old 12th April 2012, 10:51 AM   #2
rjm is online now rjm  Japan
Richard Murdey
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What are the output devices that you need 3 per side to drive headphones?
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Old 12th April 2012, 12:39 PM   #3
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It's for lower power consumation.
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Old 12th April 2012, 02:17 PM   #4
godfrey is offline godfrey  South Africa
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Interesting circuit! What type of opamp and MOSFETs are you using?
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Old 12th April 2012, 07:23 PM   #5
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I used irf7317 comp pair - same as irf550/9540 but low power. - HEXFET MOSFETS(can be used in audio)
I am used ad op27(ti opa2227 a little better)
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Old 13th April 2012, 12:18 AM   #6
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Richard Murdey
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Surface mount.

More pertinent: the input capacitance is about 850 pF per device, the op amp has to drive 5 nF all told. The op amp may or may not be happy driving this capacitance at full swing.
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Old 13th April 2012, 04:23 PM   #7
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Quote:
Originally Posted by rjm View Post
Surface mount.

More pertinent: the input capacitance is about 850 pF per device, the op amp has to drive 5 nF all told. The op amp may or may not be happy driving this capacitance at full swing.
I'm not sure I understand this circuit, but it looks to me that the opamp is only driving the first pair of mosfets, which are arranged to drive the other mosfets in a way that I do not understand.
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Old 14th April 2012, 12:12 AM   #8
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Richard Murdey
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You're right. The op amp drives the first two MOSFETs only, they appear to be connected in common-gate configuration. The gate is tied to the drain so the stage becomes 100% degenerate and the gain is unity. [My interpretation, may be incorrect!]
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Old 14th April 2012, 12:19 AM   #9
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The input mosfets are transdiodes that do level shifting for the output transistors. This is the most simple form of a mosfet diamond buffer. It is a circuit that allows DC coupling without conventional bias circuits.
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Old 15th April 2012, 09:28 PM   #10
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Joachim Gerhard,
that's right
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