|11th August 2002, 08:50 PM||#11|
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Re: random amplifier design questions
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!
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
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|11th August 2002, 10:22 PM||#12|
Join Date: Jul 2001
Location: Orleans, France
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...
|30th June 2004, 09:53 PM||#13|
Join Date: Jun 2004
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
to maintain a steady DC bias current regardless of temp.
|30th June 2004, 10:43 PM||#14|
The one and only
"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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