MOSFET amplifier

ash_dac · 3rd December 2006, 10:38 AM

Hi,

I found this amplifier design in a text book so I had a go at simulating it in LTSpice. In the original design the transitors were 2N929 and 2N657 but i've swapped these for BC547 for the simulation.

On the FFT analysis (not shown) there was a very large second harmonic (input stage used ?) .

Looks like a bit of a unstable design.

Has anyone ever built an amplifier like this ?

Elvee · 4th December 2006, 08:44 AM

Hi,

I think you should check your schematic: there seems to be a number of errors: i.e. coupling capacitors should be included in the gates of the power MOSFETs.
LV

ash_dac · 4th December 2006, 01:17 PM

Quote:

Originally posted by Elvee
Hi,

I think you should check your schematic: there seems to be a number of errors: i.e. coupling capacitors should be included in the gates of the power MOSFETs.
LV

Sorry missed the coupling caps.

Elvee · 4th December 2006, 02:26 PM

Quote:

Originally posted by ash_dac

Sorry missed the coupling caps.

It isn't the only mistake: the arrangement of C3 and C4 makes no sense. To get meaningful comments (and simulation results), you need to check thoroughly the whole schematic.
LV

ash_dac · 4th December 2006, 06:26 PM

Quote:

Originally posted by Elvee

It isn't the only mistake: the arrangement of C3 and C4 makes no sense. To get meaningful comments (and simulation results), you need to check thoroughly the whole schematic.
LV

Thanks for the pointer

ash_dac · 4th December 2006, 07:09 PM

Redrawn somewhat. C4 and C3 appear as on the page and may be a printing error.

Elvee · 5th December 2006, 09:55 AM

There are lots of oddities: both FETs have the same average gate potential, the lower boostrap/feedback circuit seems completely awkward, etc.
In addition, even if the problems were to be fixed, it wouldn't be a very good circuit, with all the RC couplings, the need for a tightly regulated supply, the ground referenced to the negative rail, etc etc. It is a lousy text-book design and you' d better design something decent from scratch rather than lose time with such a lame duck.
LV

3rd December 2006, 10:38 AM	#1
ash_dac diyAudio Member Join Date: Jan 2005 Location: UK	MOSFET amplifier Hi, I found this amplifier design in a text book so I had a go at simulating it in LTSpice. In the original design the transitors were 2N929 and 2N657 but i've swapped these for BC547 for the simulation. On the FFT analysis (not shown) there was a very large second harmonic (input stage used ?) . Looks like a bit of a unstable design. Has anyone ever built an amplifier like this ? __________________ Kind regards, Ashley.

4th December 2006, 07:09 PM	#6
ash_dac diyAudio Member Join Date: Jan 2005 Location: UK	Redrawn somewhat. C4 and C3 appear as on the page and may be a printing error. __________________ Kind regards, Ashley.

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4th December 2006, 08:44 AM	#2
Elvee diyAudio Member Join Date: Sep 2006	Hi, I think you should check your schematic: there seems to be a number of errors: i.e. coupling capacitors should be included in the gates of the power MOSFETs. LV

5th December 2006, 09:55 AM	#7
Elvee diyAudio Member Join Date: Sep 2006	There are lots of oddities: both FETs have the same average gate potential, the lower boostrap/feedback circuit seems completely awkward, etc. In addition, even if the problems were to be fixed, it wouldn't be a very good circuit, with all the RC couplings, the need for a tightly regulated supply, the ground referenced to the negative rail, etc etc. It is a lousy text-book design and you' d better design something decent from scratch rather than lose time with such a lame duck. LV

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