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Old 5th September 2005, 12:19 AM   #31
andy_c is offline andy_c  United States
Join Date: Apr 2003
Here is my version of the two-transistor current source.

As you can see, the two transistors aren't visible.

Always glad to be of help!
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Old 5th September 2005, 12:39 AM   #32
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Hi Andy.

Very funny

Out of context mate. The post was about the PSRR of various diff'l tails. everyone knows what a 2BJT CCS is, but dimitri had to highlight a partly shown piece of the schematic. that's all. It was simply an experimental lash up and had nothing meritorious in itself to display so there was considered no need to show it.

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Old 5th September 2005, 12:43 AM   #33
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with diode is not as precise as with transinstor base connected to collector
high precision is mastertech guys

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Old 5th September 2005, 12:58 AM   #34
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Hi Mastertech,

The diode in question was added to the 2BJT CCs for a reason - I needed a 1.2V active component across the 1.2k so I built up the BJT Vbe junction voltage with a diode. Why? because the CCS is temperature comp as well and either the diode or BJT were to be relocated to the heatsink. As the thermal comp required was half that of a std BJT I added an equal but non thermally connected component with a much smaller dynamic impedance than a resistor which would dramatically impair the CCS performance.

So I got my 2mV/degree in 1.2V, temp comp CCS.

Clear as mud.

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Old 5th September 2005, 01:02 AM   #35
dimitri is offline dimitri  United States
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so the CCS current can be calculated as Ube plus Diode drop divide on 1200 Ohms

What about thermal stablity?

oops, you are fast - but why 2 mV/deg, 2 pn junction - 4.4mV/deg
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Old 5th September 2005, 01:20 AM   #36
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Hi Dimitri,

I wanted a CCS that had a -Ve tempco of half that of a BJT so put the diode in the sensing leg and EITHER the sensing BJT or diode can go to the heatsink while the other monitors ambient. Simple low cost solution! As you're aware the simple BJT CCS has a normal 2mv/deg tempco of output current , this was simply my way of halving it.

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