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-   -   Op. Amp + MOSFET Follower (http://www.diyaudio.com/forums/headphone-systems/210584-op-amp-mosfet-follower.html)

stridervvv 11th April 2012 10:10 PM

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?

http://img-fotki.yandex.ru/get/6207/...c148105_XL.jpg

rjm 12th April 2012 10:51 AM

What are the output devices that you need 3 per side to drive headphones?

stridervvv 12th April 2012 12:39 PM

It's for lower power consumation.

godfrey 12th April 2012 02:17 PM

Interesting circuit! What type of opamp and MOSFETs are you using?

stridervvv 12th April 2012 07:23 PM

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)

rjm 13th April 2012 12:18 AM

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.

dirkwright 13th April 2012 04:23 PM

Quote:

Originally Posted by rjm (Post 2983168)
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.

rjm 14th April 2012 12:12 AM

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!]

Joachim Gerhard 14th April 2012 12:19 AM

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.

stridervvv 15th April 2012 09:28 PM

Joachim Gerhard,
that\'s right


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