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Old 26th August 2002, 05:05 PM   #1
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Question 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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Old 26th August 2002, 06:21 PM   #2
PMA is offline PMA  Europe
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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.
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Old 27th August 2002, 03:59 PM   #3
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maybe you must try to increase the 47F cap there the negative and the positive part of the signal doesn't have the same amplitude.

HB
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Old 27th August 2002, 04:01 PM   #4
UrSv is offline UrSv  Sweden
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I would think somebody here would recommend gate resistors for the output FETs at say 221 Ohms.

/UrSv
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Old 27th August 2002, 04:16 PM   #5
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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.
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Old 27th August 2002, 04:20 PM   #6
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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.
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Old 27th August 2002, 04:27 PM   #7
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Hi Sapito

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

Regards

Charles
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Old 30th August 2002, 04:19 PM   #8
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Why not to use souce resistors in fet output stage?
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Old 31st August 2002, 02:22 AM   #9
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Smile 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....
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Old 2nd February 2003, 04:26 PM   #10
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... just wondering what happened further to his amp ...
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