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Old 11th August 2002, 08:50 PM   #11
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Default Re: random amplifier design questions

Originally posted by mirlo

1) What is the mechanism of the instability that gate resistors are supposed to ameliorate? If you have a compact enough layout and drive the gate from a high impedance (as high as the get resistor would have been) does a gate resistor do any good?
The reason why MOSFET's are instable is the large capacitances which in combination with very small inductances (the connections to the transistor) forms a very good oscillator. I have seen the equations for that in some application note somewhere (Hitachi?) but I can derive them because I'm too tired.

When you insert a ferrite bead or a resistor in series with the gate you create losses, enough to stop the oscillation.


Small (50-100 ohm) or no resistance = risk for oscillation but a fast transistor

Medium value (100-470 ohm) = no oscillation (if you are lucky) and a little bit slower transistor

Too high (470-2 kohm) value = Way too slow transistor!
Originally posted by mirlo

3) What sorts of time constants / and ESPECIALLY what capacitor types (low leakage electrolytics??) are good to use for the DC rejection cap that makes the amp have unity gain at DC? Or is it better to use a servo op-amp...
Check my monster phono amp for the design of the DC-servos. I use "plastic" caps in the DC-servo and acheive 0,4 Hz. I use try to use max 1 µF plastic for DC-servos.

Check also my input bias servo (at the input of the preamp), this servo is extremely slow with MC-pickup, minutes! With MM pick under 1 minute (don't remember the exact value)

If you use plastic caps there is no problem with leakage and if you don't have so high resistor values you won't get trouble with bias currents. Don't use more the 100 kohm to 1 Mohm (depending on opamp).

To answer your question: If you don't use DC-servo (and use a ordinary DC-blocking cap) you don't need to think about leakage. An electrolythic cap has around 1 Mohm parallel resistance as worst case and this compared to 100-2200 ohms is nothing. The gain will be slightly higher than 1.

What is the best?

DC blocking cap
Pros: Cheap, small space
Cons: Don't last forever, limited lifetime when it's warm.

Pros: Easy to get desired speed, long lifetime
Cons: Complicated (more than one cap), takes up space, can be more expensive than a single cap.

Distortion? Not really a big difference.

Audiophileness: Some like DC-servos, some don't
/Per-Anders (my first name) or P-A as my friends call me
PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid.
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Old 11th August 2002, 10:22 PM   #12
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I don't know the exact mechanism which causes distortion when driving impedance is high, but I have experienced this, and this phenomenon has been reported by Jean Hiraga in l'Audiophile, 15 years ago.

Very high transconductance jfets have a large gate, which can exhibit significant non-linear leakage current. Such current, for instance, 1 nA, in 1 Mohm impedance, can cause 1 mV voltage drop. If the input signal is 100 mV, this can cause 1% distortion... depending on the exact nature of the driving impedance.

And, as you have written, non linear capacitances can cause further distortion, by non linear Miller effect, but especially at high frequencies, and depending on the effective drain load.

Another possible cause is reverse transconductance, but I have no informations about this...

Regards, P.Lacombe.
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Old 30th June 2004, 09:53 PM   #13
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could you use a FET to act as a bias servo for a MosFET output stage, as it is used in a BJT circuit, to keep from crossover distortion as it is heated. (BJT's have + coefficient & FET's have
- coeficient.)

to maintain a steady DC bias current regardless of temp.
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Old 30th June 2004, 10:43 PM   #14
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"Check out Bob Pease's book......Troubleshooting Analog Circuits. Good reference on measuring PSRR and CMRR the right way. Lots of other very useful stuff. Just don't tell Bob you work on exotic audio stuff."

And don't mention cables under any conditions....
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