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Old 22nd September 2011, 02:07 PM   #1
Elvee is offline Elvee  Belgium
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Default Improved 2W current sources (II)

Some improvements to the basic CCS's have been described here:
Improved current source/sink

Here are some further circuits, a bit more complicated but having a very wide compliance.

On the left is the basic circuit.
The performances are already quite good, with a variation of 0.25mA (for 10mA nominal) for a voltage swing of 2.5V to 62.5V.


The middle circuit has two improvements: the 22Meg, to avoid relying on leakage currents for start-up, and a 300K compensating resistor decreasing the delta current to ~25µA, 1/10th of the initial value.

If the tempco of -0.3%/°C is problematic, each pair of silicon diodes can be replaced by an LED, providing a first order compensation.
The circuit on the right uses the infrared LEDs of an optocoupler to minimize the working voltage.
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Last edited by Elvee; 22nd September 2011 at 02:10 PM. Reason: Pic
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Old 22nd September 2011, 03:09 PM   #2
RJM1 is online now RJM1  United States
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62.5V to2.5V 10ma Nom. Deviation 8 µA.
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Old 23rd September 2011, 08:53 AM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by RJM1 View Post
62.5V to2.5V 10ma Nom. Deviation 8 µA.
That is an impressive result, but of course, this is not what these threads are about.

I have described a number of real-world solutions to implement 2W. current sources (that is, fully floating).

It is obviously possible to do much better if you drop the 2W. requirement, and to do even better if one is allowed to use "sim components", like perfect floating sources.
If that route is allowed, there is an obvious shortcut, giving even better results, see below: voltage range=anything, Inom=10mA, deviation=0.

The circuits I propose are shown in a simulated form, but they can readily be built using normal, physical components, and I am confident the discrepancies between the simulated and actual versions will not exceed several percents.
Simulators are primarily intended as a means to an end, but they can also be used for phantasy and intellectual masturbation.
This is not what I am doing here.

That said, the circuit you propose is not without merit, and for that matter can be found in textbooks.
But it would need some adaptations to qualify.

First, the way it is drawn is misleading: in fact the 5V source is in series with the stimulus, which means that in fact the actual voltage swing is from 7.5 to 67.5V.
The elements could be rearranged in a more normal manner, to avoid having the main current passing through the 5V bias.
This would limit the minimum operating voltage to ~5V, and then would remain the problem of creating the 5V bias from the voltage available, without disturbing the controlled current.
Certainly achievable, but not simple or straightforward.

But anyway, I rule out the use of FETs. They may look great in sims, but they suffer from big dispersions in reality.

The Idss of the 2N3819 can range from 2 to 20mA. This means that you have to discard first hand half of your samples, and then adjust individually each circuit for the rest.
Even if you resort to sorted devices, like the BF245A, B or C, there is still a dispersion range of more than 1 to 2 in the Idss.

BJT's may give somewhat lower performances, but the current and tempco are predictable, and in most instances they won't require any adjustment or sorting.
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Last edited by Elvee; 23rd September 2011 at 08:56 AM.
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