LED CCS questions...

ash_dac · 23rd September 2006, 10:59 AM

Hi,

http://www.ing.iac.es/Astronomy/obse...otes/tn107.pdf

Use od LED for CCS and cascode stages

For audio is it worth the effort of compensating a particular LED for temperature ?

http://www.rapidonline.com/netalogue/specs/56-0300e.pdf

Should I look for the steepest curve IV LED ?

AndrewT · 23rd September 2006, 11:20 AM

Hi,
by steep do you mean less voltage change for a wide range of operating current?

If you are looking for a voltage reference then you want little change in voltage, provided the temperature coefficient does not get significantly worse.

mikeks · 23rd September 2006, 11:21 AM

Quote:

Originally posted by ash_dac
Should I look for the steepest curve IV LED ?

Use Red LED for temp. compensation; mind the voltage drop at choosen standing current; 10mA (at least) may be required; ''steepest slope'' curve is a ''good thing''.

mikeks · 23rd September 2006, 11:23 AM

Quote:

Originally posted by ash_dac

For audio is it worth the effort of compensating a particular LED for temperature ?

Only in input diff. stage in power amps.

johnnyx · 23rd September 2006, 11:37 AM

In Doug Self's discrete preamp article, I read that the tempco of a LED is very close to that of a BJT. To check this, I rigged up a test circuit comprising a transistor ccs, biased with a red LED. I monitored the current with my meter whilst heating the assembly with a hairdryer. If both the transistor and LED were heated equally, there was little change in current. If either was heated more than the other, the change was significant. This experiment was crude, but it showed that an LED biased ccs is good enough for me.

ash_dac · 1st October 2006, 05:13 PM

Hi,

An old amplifier I have uses the 'ANF current source' in the differential and VAS. Would a LED current source be better in this situation ?

mikeks · 2nd October 2006, 12:40 AM

Ultimately not worth the hassle.

sam9 · 2nd October 2006, 01:09 AM

Isn't the function of the diode (Red or Green LED or just 1N4148) in the CCS to provide a voltage drop with which to bias a transistor? If that's the case, isn't the issure the stability of the voltage drop rather than the current?

I seem to recall from an experience trying to get a CCS / LTP to do what I wanted, where I was scrathing my head and making the repeated measurement to try to fathom things, hat the only thing measurement that always came out the same was the thedrop across a red LED. Hot, cold or luke warm. However, if you swapped out for a different PN from as different mfg -- then you would get a change.

johndiy · 2nd October 2006, 01:21 AM

yeah we want a steady vdrop to have stable i in ccs

thank you guys for your valueable inputs

john

23rd September 2006, 10:59 AM	#1
ash_dac diyAudio Member Join Date: Jan 2005 Location: UK	LED CCS questions... Hi, http://www.ing.iac.es/Astronomy/obse...otes/tn107.pdf Use od LED for CCS and cascode stages For audio is it worth the effort of compensating a particular LED for temperature ? http://www.rapidonline.com/netalogue/specs/56-0300e.pdf Should I look for the steepest curve IV LED ?

23rd September 2006, 11:20 AM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, by steep do you mean less voltage change for a wide range of operating current? If you are looking for a voltage reference then you want little change in voltage, provided the temperature coefficient does not get significantly worse. __________________ regards Andrew T.

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23rd September 2006, 11:37 AM	#5
johnnyx diyAudio Member Join Date: Jan 2004 Location: manchester	In Doug Self's discrete preamp article, I read that the tempco of a LED is very close to that of a BJT. To check this, I rigged up a test circuit comprising a transistor ccs, biased with a red LED. I monitored the current with my meter whilst heating the assembly with a hairdryer. If both the transistor and LED were heated equally, there was little change in current. If either was heated more than the other, the change was significant. This experiment was crude, but it showed that an LED biased ccs is good enough for me.

1st October 2006, 05:13 PM	#6
ash_dac diyAudio Member Join Date: Jan 2005 Location: UK	Hi, An old amplifier I have uses the 'ANF current source' in the differential and VAS. Would a LED current source be better in this situation ?

2nd October 2006, 12:40 AM	#7
mikeks Account Disabled Join Date: Jun 2004 Location: Animal farm	Ultimately not worth the hassle.

2nd October 2006, 01:09 AM	#8
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	Isn't the function of the diode (Red or Green LED or just 1N4148) in the CCS to provide a voltage drop with which to bias a transistor? If that's the case, isn't the issure the stability of the voltage drop rather than the current? I seem to recall from an experience trying to get a CCS / LTP to do what I wanted, where I was scrathing my head and making the repeated measurement to try to fathom things, hat the only thing measurement that always came out the same was the thedrop across a red LED. Hot, cold or luke warm. However, if you swapped out for a different PN from as different mfg -- then you would get a change.

2nd October 2006, 01:21 AM	#9
johndiy Banned Join Date: Aug 2006	yeah we want a steady vdrop to have stable i in ccs thank you guys for your valueable inputs john

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