Op. Amp + MOSFET Follower

[stridervvv]

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?

An externally hosted image should be here but it was not working when we last tested it.

 

[rjm]

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

 

[stridervvv]

It's for lower power consumation.

 

[godfrey]

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

 

[stridervvv]

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]

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]

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]

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]

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]

Joachim Gerhard,
that's right

 

[rjm]

Doh! I see it now. MOSFET diamond buffer. How does that work in practice? Does it matter if the devices are not especially well matched?