FET Amplifier

I've made this amplifier one year ago and it didn't work well.
Is has an enormous instability and, when I push it a little, a lot of oscilation occur.
The waveform at the output is not pure sinusoidal. The negative half wave is about 25% the positive.
Is the cause the output fet's I use?

Thanks for any reply.

Pedro Martins
 

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UrSv,

I'm not familiar with FETS but indeed I think there must be resistors at both gates.

I would also use a zener diode to stabilise the voltage/current of the diff. amp.

HB.

EDIT: maybe the most important thing: I think that the value (4.7k) of the res. in parallel with the inductor at the output is way too high (normal value is about 5-20 ohm); I would recommend to leave these parts away and do the tests of your amp without them (I never use them!!)

HB.
 

mikek

Disabled Account
2002-05-15 1:12 pm
sapito

your design appears not to have any compensation:eek:
the cap across the feedback resistor merely adds an inverted zero to the closed loop transfer function, but is no where near sufficient....

you need to place a 33pF miller compensation cap across the second stage base-emitter. This should give your design an approximate single pole characteristic, and at least 40 degrees phase margin......33R resistor in series with this capacitor should increase phase margin to approx. 60 degrees....
:cool:
 

halojoy

On Hiatus
2002-11-05 6:14 pm
sapito said:
I've made this amplifier one year ago and it didn't work well.
Is has an enormous instability and, when I push it a little, a lot of oscilation occur.
Is the cause the output fet's I use?
You maybe already have put some resistors GATE of MOSFETs.

Anyway we can compare your Amplifier with this:
25 Watt MosFet Audio Amplifier
It uses same IRF532 IRF9532 Output MOS

/halo - likes 25WattAmps Very Mucho ;) ;)

attachment.php
 

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Re: Re: FET Amplifier

halojoy said:

You maybe already have put some resistors GATE of MOSFETs.

Anyway we can compare your Amplifier with this:
25 Watt MosFet Audio Amplifier
It uses same IRF532 IRF9532 Output MOS

/halo - likes 25WattAmps Very Mucho ;) ;)


interesting sig;
pitty that ,like all things- otpor loose own value with time,and embrace all characteristics of one politic party-hunger for power.
 

halojoy

On Hiatus
2002-11-05 6:14 pm
Re: Re: Re: FET Amplifier

choky said:
interesting sig;
pitty that ,like all things- otpor loose own value with time,and embrace all characteristics of one politic party-hunger for power.
Yes, all movements are good when they are fresh.
Lech Walesa - Solidarity in Poland
Vaclav Havel - President of The Czech Republic
When they go into politics,
they get some of the same disease:
Hunger/fight for Power and TOP-position (and money)
-------------------

And the most bad guy - who is not afraid of stepping on others,
Becomes the Top-Dog.
(it is same in Business/Companies - Top Directors are mostly
egoistic and tough guys/girls)

The Good Ones never Reach the TOP,
they are too good and too much of team-players ... :rolleyes: :rolleyes:
------------------------
/halo - stays on the floor - looking up in :dead: disgust :dead: