MOSFET amplifier

[ash_dac]

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.

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

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

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]

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]

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]

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]

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]

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

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

 

[Elvee]

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