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Old 23rd April 2002, 01:59 PM   #11
jam is offline jam  United States
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Geoff,

What an excellent idea! I wonder if bonding Q2 and Q3 togther thermally would provide any thermal stability benefits?

Jam
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Old 23rd April 2002, 03:17 PM   #12
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Default peranders, could you post an ultimate current source?

As you may know these things are becoming quite popular with tube/valve people. I just installed the C4S from the bottlehead site on my tube preamp, but I was wondering if there was a way to improve on the design. Needless to say I am no EE.

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Bas
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Old 23rd April 2002, 03:46 PM   #13
Geoff is offline Geoff  United Kingdom
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Jam

Difficult to answer your question without actually building and testing the circuit. In simulation, I simply raised the global temperature. A 10degC rise in junction temperature gave a 2.5% current variation for this arrangement, compared with 3.5% for the LED schematic. Current variation with load (10ohm to 1kohm) was 0.1%, the same as the two-transistor ccs, compared with approximately 0.75% for the two LED versions. For a 50V supply rail modulated with a 2Vpeak sine wave, the current variation was 0.004% for the circuit I posted, 0.04% for the White circuit, 0.18% for a standard two-transistor ccs and 0.37% for the LED/transistor version.

Geoff
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Old 23rd April 2002, 04:48 PM   #14
Geoff is offline Geoff  United Kingdom
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I should have mentioned in my original post that, for the 4-transistor ccs, R1 = R2 = R and Iconst is 2 * Vbe / R. For initial calculations, R = 1.3 / Iconst is sufficiently accurate.
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Old 23rd April 2002, 05:17 PM   #15
sonnya is offline sonnya  Denmark
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Hi Geoff

Which BJT did you use in the simulation?

Sonny
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Old 23rd April 2002, 05:46 PM   #16
Geoff is offline Geoff  United Kingdom
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Sonny

BC550 & BC560, with R1 and R2 set at 120ohm. I adjusted all versions of the ccs to give a current of between 10 and 11mA, which was the figure the White circuit gave with the resistor value shown on Eric's schematic.

I must stress that the figures I obtained should be treated as approximate since I made no attempt to optimise each circuit. However, I believe that they are sufficiently accurate to permit an initial comparison to be made. The main disadvantage I see with the 4-transistor or 2-LED/2-transistor ccs is that a higher supply rail voltage may be needed in certain applications due to the larger PD across the ccs.

Geoff
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Old 23rd April 2002, 06:35 PM   #17
tiroth is offline tiroth  United States
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Geoff

I just the other day was drawing up a CCS, and those are exactly the transistors I was planning to use! I'll have to try out your circuit.
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Old 23rd April 2002, 07:53 PM   #18
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Default CCSWTMP

hhmmm.............
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Old 23rd April 2002, 08:09 PM   #19
sonnya is offline sonnya  Denmark
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Quote:
Originally posted by Geoff
Sonny

BC550 & BC560, with R1 and R2 set at 120ohm. I adjusted all versions of the ccs to give a current of between 10 and 11mA, which was the figure the White circuit gave with the resistor value shown on Eric's schematic.

I must stress that the figures I obtained should be treated as approximate since I made no attempt to optimise each circuit. However, I believe that they are sufficiently accurate to permit an initial comparison to be made. The main disadvantage I see with the 4-transistor or 2-LED/2-transistor ccs is that a higher supply rail voltage may be needed in certain applications due to the larger PD across the ccs.

Geoff
I can understand that, but it could be fine for something like currentsource for an input diff-pair or for driving a bunch off currentmirrors with a constant current source. It will not be affected that much by ripple on the supply rails.
I think your sims shows the fine result you can get with a 4-BJT solution compared to a "simple" current source.

Sonny
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Old 23rd April 2002, 08:12 PM   #20
Geoff is offline Geoff  United Kingdom
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Harry

Your's is longer than mine :-( Is this a competition to see who can design the most complicated current source?

Tiroth

By all means try the circuit I posted. However, if your supply rails are regulated (which they should be for low current circuits) I do not see that there will be any performance improvement over a simple 2-transistor ccs. An alternative, if the supply rail voltage does fluctuate, is a jfet (with gate and source connected to from a ccs) in place of collector resistor on the control transistor. The 4-transistor circuit only comes into its own if a two-terminal ccs is required to say replace a resistor in an existing circuit where it is difficult to provide the earth connection required by the simple 2-transistor circuit.

Don't take my reference to the BC550/BC560 as a recommendation (though I have no complaints with these transistors). I simply use the models for these devices when I want a small-signal transistor model since I have tested the models and have found that the results have good correspondence with the datasheet information.

Geoff
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