bias spreader bypass cap - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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
Reply
 
Thread Tools Search this Thread
Old 22nd April 2013, 01:41 AM   #1
diyAudio Member
 
Dan Moos's Avatar
 
Join Date: Mar 2013
Location: Lynden, WA USA
Default bias spreader bypass cap

considering a simple Vbe multiplier type bias spreader, how does one determine the size of the bypass cap. Cordell's book shows many examples with 10uf, but I can't find how that number was reached. I assume it needs to be a pretty good sized cap, but I want to know how to calculate it. Surely it can't be as simple as 2pi*the divider resistances*the freq I want to pass. Intuitively I feel the transistor matters in the calculation too, but I can't see how to do this.
  Reply With Quote
Old 22nd April 2013, 01:43 AM   #2
diyAudio Member
 
Dan Moos's Avatar
 
Join Date: Mar 2013
Location: Lynden, WA USA
Oh, obviously in my example of what I'm pretty sure is NOT the relevant formula, I would divide one by my answer.
  Reply With Quote
Old 22nd April 2013, 02:00 AM   #3
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Sure does.
And the value is much lower than the resistive divider suggests, because the transistor both has current gain and passes much higher current.
On first approximation, consider an "equivalent resistance" of: Req=V/I and define a capacitor which has that impedance at, say, 16 Hz or something, so the transistor is bypassed in all of the Audio range.
  Reply With Quote
Old 22nd April 2013, 02:17 AM   #4
diyAudio Member
 
Dan Moos's Avatar
 
Join Date: Mar 2013
Location: Lynden, WA USA
so I would imagine a resistor who's value is the collector voltage/collector current of the transistor, put it in parallel with the divider resistors and use the result as the "R" variable in the standard equation I mentioned earlier?

Since the base current portion of the total current is probably on the order of 1/100th of the total, it can be ignored I assume?

Wouldn't 16 hertz be kinda high? I presume what we don't want is the signal modulating the bias voltages. Wouldn't using 16 hertz as my -3dB point cause the cap to attenuate noticeably still at 20 hertz where it is audible?

I'm not disagreeing. I am very much a novice here. Just trying to understand is all.
  Reply With Quote
Old 22nd April 2013, 02:23 AM   #5
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Yes that's the idea.
This is just an estimation, to have some idea about what we are talking.
The 16 Hz cutoff was just a personal estimation, personally I don't lose sleep about what happens at 20 Hz, given that most speakers don't reproduce them, there's no music program material that low and I doubt I can hear it anyway, but that's a personal/practical choice.
  Reply With Quote
Old 22nd April 2013, 02:46 AM   #6
diyAudio Member
 
Dan Moos's Avatar
 
Join Date: Mar 2013
Location: Lynden, WA USA
I realize this is just an estimation, but can I assume such an estimation is sufficient in this case?

I guess my thinking 16htz was high stems from the fact that I have been doing guitar amps. With tube guitar amps, many bad things come from attempting to make the lower bandwidth limit of a stage too low. Blocking distortion and such.

Now that I'm learning hi-fi, I may be chomping at the bit to have, at least on paper, the crazy bandwidths that were impossible with previous projects. Don't look to hard for any real logical rational for this line of reasoning .
  Reply With Quote
Old 22nd April 2013, 03:52 AM   #7
diyAudio Member
 
Join Date: Nov 2010
Location: The City, SanFrancisco
As JMFahey pointed out the multiplier is typically a single stage feedback regulator. The designs can vary but even the minimilast will have loop gain above 40, or a low output impedance from dc up to near a MHz. This impedance should be less than 20 ohms, thus there is really no benefit adding a large capacitor (it would take hundreds of uf's). Once the multiplier nears its crossover frequency its impedance will rise so here bypassing is critical (predrivers shouldnt be fed from an inductive source). But values near .01u are sufficient to keep the impedance near its dc value.
The charge necessary to remove from the predriver bases during crossover can be minimized to the point where these 0.01u or less are sufficient.

Hope this helps.
-Antonio

Last edited by magnoman; 22nd April 2013 at 03:53 AM. Reason: typo
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help understanding bias spreader Dan Moos Solid State 7 5th March 2013 01:52 PM
Mundorf M-Cap Supreme as bypass cap? MGH Power Supplies 38 11th September 2012 12:09 AM
Cap to bypass bias battery dsavitsk Tubes / Valves 4 24th February 2011 11:00 AM
How to calculate bypass cap across bias resistor? Henry8 Solid State 27 14th November 2010 02:20 PM
Polar electro with bypass cap to replace nonpolar as coupling cap? at77 Parts 0 12th January 2010 02:05 AM


New To Site? Need Help?

All times are GMT. The time now is 11:30 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2