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Old 5th July 2011, 09:33 AM   #2081
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Twin LEDs CCS

I've met oscillations with a cascoded CCS using two LEDs in series, the first one defining the reference voltage for the current, and the second one defining the reference voltage for the output common base transistor. Cure was done with a base stopper for this transistor. With a 22 Ohm stopper, this scheme was recommended by Roender on this forum. However, replacing the second LED with a resistor of adequate value gave a similar result to me. This scheme has been used by Morgan Jones.
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Old 5th July 2011, 12:17 PM   #2082
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forr,
its mainly a matter of source impedance.

In the article, Hiraga is talking about reduced leakage current in the J-FET by the cascode, right?
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Old 6th July 2011, 01:29 AM   #2083
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Quote:
Originally Posted by forr View Post
Twin LEDs CCS

I've met oscillations with a cascoded CCS using two LEDs in series, the first one defining the reference voltage for the current, and the second one defining the reference voltage for the output common base transistor. Cure was done with a base stopper for this transistor. With a 22 Ohm stopper, this scheme was recommended by Roender on this forum. However, replacing the second LED with a resistor of adequate value gave a similar result to me. This scheme has been used by Morgan Jones.
Hi forr,

I'm curious, how much current were you running through the LEDs when you had the oscillations, and at what frequency were the oscillations? I must admit, I've never tried this particular arrangement.

Cheers,
Bob
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Old 6th July 2011, 10:49 AM   #2084
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Originally Posted by Bob Cordell View Post
Hi forr,

I'm curious, how much current were you running through the LEDs when you had the oscillations, and at what frequency were the oscillations? I must admit, I've never tried this particular arrangement.

Cheers,
Bob
Hi Bob,
I was experiencing CCSs about 25 years ago, the currents in the biasing LEDs and the CCS output were both about 3 mA. I just observed the results on a little scope and can't say what were the oscillation frequencies encountered.
However, Morgan and Roender who are certainly more skilled than me arrived at schemes similar to mine.

Here some links to a discussion about CCS in 2006 :

Roender's circuit

Searching the "best" CCS

Click the image to open in full size.

As we know, bootstrapping the resistor biasing the LEDs to the negative power supply rail la Self or la Jung much enhances PSRR.

Darkfenriz used a Zener diode
Searching the "best" CCS

Click the image to open in full size.

My comments

Searching the "best" CCS

In Roender's circuit, replacing R12 by a short and D6 with a 1 kOhm resistor (about 1.6 mA bias current) gave almost the same performance. I was pleased to discover that Morgan Jones used the same circuit in his valve amps.

Regards.
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Old 7th July 2011, 02:03 PM   #2085
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Quote:
Originally Posted by forr View Post
Hi Bob,
I was experiencing CCSs about 25 years ago, the currents in the biasing LEDs and the CCS output were both about 3 mA. I just observed the results on a little scope and can't say what were the oscillation frequencies encountered.
However, Morgan and Roender who are certainly more skilled than me arrived at schemes similar to mine.

Here some links to a discussion about CCS in 2006 :

Roender's circuit

Searching the "best" CCS

Click the image to open in full size.

As we know, bootstrapping the resistor biasing the LEDs to the negative power supply rail la Self or la Jung much enhances PSRR.

Darkfenriz used a Zener diode
Searching the "best" CCS

Click the image to open in full size.

My comments

Searching the "best" CCS

In Roender's circuit, replacing R12 by a short and D6 with a 1 kOhm resistor (about 1.6 mA bias current) gave almost the same performance. I was pleased to discover that Morgan Jones used the same circuit in his valve amps.

Regards.
Hi forr,

Thanks for this info. Good stuff. I cover current sources in my book in figures 2.10a-f, but those current sources are not as sohpisticated.

Do you know if anyone has simulated the current source you had oscillation issues with, and whether the simulation pointed to instability?

Cheers,
Bob
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Old 7th July 2011, 02:29 PM   #2086
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Quote:
Originally Posted by WuYit View Post
Howdy,

Without a base stopper the LED CCS is not stable either.
Data?
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Old 7th July 2011, 03:22 PM   #2087
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Quote:
Originally Posted by Bob Cordell View Post
Hi forr,

Thanks for this info. Good stuff. I cover current sources in my book in figures 2.10a-f, but those current sources are not as sohpisticated.

Do you know if anyone has simulated the current source you had oscillation issues with, and whether the simulation pointed to instability?

Cheers,
Bob
Hi Bob,

There is some goof stuff in this thread :

Improved current source/sink

My ancient experiences probably suffered from lack of local decoupling and inadequate breadboard implementation. Instabilities and unexepcted oscillations may be due to a reactive input impedance on the base or gate of a transistor. This is usually cured by base stoppers.
Erik Margan wrote a paper on the subject in Electronics World, April 1998, page 311-312. I'll scan it and send it to interested persons (leave me an MP).

I recently made up my mind that CCSs more sophisticated than the ones you present in your book (if your R2 is bootstrapped to the negative rail power supply rail) are generally unnecessary.

Regards.
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Old 7th July 2011, 05:21 PM   #2088
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Hi SY,
instability is a function of Iout, you may get away with no base stopper at low Iout levels, but knowing the mechanism, I always use a base stopper.
In a wider perspective, I never apply a capacitor directly to the base.
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Old 7th July 2011, 05:25 PM   #2089
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Perhaps I wasn't clear for which my apologies. I was asking for data supporting your declaration that an LED-based CCS is not stable without a base stopper. I've built a few dozen CCS with that topology and never saw an oscillation. So if you have contrary data, I'd like to see it.

The capacitor in your diagram is curious, given the low dynamic impedance of LEDs.
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Old 7th July 2011, 05:25 PM   #2090
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This arrangement has pretty high Zout. T2`s Vbe provides a more stable and independent reference voltage than T1`s Vgs.
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