FET Amplifier

[sapito]

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

 

[PMA]

Try to change the value of 15pF capacitor (in parallel with 47k) and/or place a capacitor of 33pF between the collector and base of BC639.

 

[hugobross]

maybe you must try to increase the 47µF cap there the negative and the positive part of the signal doesn't have the same amplitude.

HB

 

[UrSv]

I would think somebody here would recommend gate resistors for the output FETs at say 221 Ohms.

/UrSv

 

[hugobross]

I suggest to place two resistors of 47 ohm in series with the emitters of the differential amplifier. The 4.7K res. at the base of the BC639 is not needed when the 680 ohm res is well chosen.

HB.

 

[hugobross]

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.

 

[phase_accurate]

Hi Sapito

This circuit reminds me from an IR application note, doesn't it ?

Regards

Charles

 

[Diogo]

Why not to use souce resistors in fet output stage?

 

[mikek]

sapito

your design appears not to have any compensation
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....

 

[hugobross]

... just wondering what happened further to his amp ...

 

[halojoy]

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

 

[Zen Mod]

Re: Re: FET Amplifier

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]

Re: Re: Re: FET Amplifier

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 ...
------------------------
/halo - stays on the floor - looking up in disgust