Op. Amp + MOSFET Follower

stridervvv · 11th April 2012, 10:10 PM

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?

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

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

11th April 2012, 10:10 PM	#1
stridervvv diyAudio Member Join Date: Feb 2012	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?

12th April 2012, 10:51 AM	#2
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	What are the output devices that you need 3 per side to drive headphones? __________________ RJM Audio (phonoclone.com / G+)

13th April 2012, 12:18 AM	#6
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	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. __________________ RJM Audio (phonoclone.com / G+)

14th April 2012, 12:12 AM	#8
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	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!] __________________ RJM Audio (phonoclone.com / G+)

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12th April 2012, 12:39 PM	#3
stridervvv diyAudio Member Join Date: Feb 2012	It's for lower power consumation.

12th April 2012, 02:17 PM	#4
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	Interesting circuit! What type of opamp and MOSFETs are you using?

12th April 2012, 07:23 PM	#5
stridervvv diyAudio Member Join Date: Feb 2012	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)

14th April 2012, 12:19 AM	#9
Joachim Gerhard diyAudio Member Join Date: Mar 2009	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.

15th April 2012, 09:28 PM	#10
stridervvv diyAudio Member Join Date: Feb 2012	Joachim Gerhard, that's right

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