Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

random amplifier design questions
random amplifier design questions
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 11th August 2002, 09:50 PM   #11
peranders is offline peranders  Sweden
Electrons are yellow and more is better!
diyAudio Member
peranders's Avatar
Join Date: Apr 2002
Location: Göteborg, Sweden
random amplifier design questions
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
Group buy: DCT03 DC trap for big toroidal transformers. Sign up for interest HERE. 18 pcb's in interest.
  Reply With Quote
Old 11th August 2002, 11:22 PM   #12
P.Lacombe is offline P.Lacombe  France
diyAudio Member
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...

Regards, P.Lacombe.
  Reply With Quote
Old 30th June 2004, 10:53 PM   #13
cunningham is offline cunningham  United States
diyAudio Member
Join Date: Jun 2004
Location: Knoxville
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.
  Reply With Quote
Old 30th June 2004, 11:43 PM   #14
Nelson Pass is offline Nelson Pass  United States
The one and only
Nelson Pass's Avatar
Join Date: Mar 2001
random amplifier design questions
"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....
  Reply With Quote


random amplifier design questionsHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Amplifier design questions j.wright Car Audio 13 22nd July 2008 05:36 PM
Random electrocution? bob_v5 Everything Else 20 8th February 2008 05:06 PM
Random horn questions e-side Multi-Way 5 20th July 2006 11:48 PM
Is distortion random ? Bernhard Solid State 46 30th December 2003 02:59 AM
Questions about how to decrease the PASS A-40 amplifier into a class AB amplifier. novtango Pass Labs 2 21st October 2002 02:50 AM

New To Site? Need Help?

All times are GMT. The time now is 03:50 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio