thoughts and questions on this CCS

[keantoken]

Hello.

I have been thinking about this CCS for a while and am considering its applications and implications.

It has an interesting feature in that it can actually overcompensate for the input voltage.

If the load is made a Zener diode it will act as a precision voltage source which can be used for a power supply, although this will nullify the affect of overcompensation. If the load is a resistor, the same can be done except with the possibility of overcompensation.

What I am looking to improve here is the thermal stability; When it warms up the Vbe's of Q1 and Q3 will drop, and the output current will decrease.

So then how might I make it more thermally stable?

- keantoken

[lineup]

isnt this more a VREF, voltage reference
fed by a CCS

VREF = constant voltage, often thermally compensated

Often used in voltage reulator chips is Bandgap Reference
The bangap uses difference of VBE voltages as the konstant to create a temp insensitive reference voltage.

See this topic, with some discrete Bandgap Circuit Theory
and discrete example circuit by me:
Volt Reference - 4 Transistors with Bandgap
Volt Reference - 4 Transistors with Bandgap

[lineup]

comparison
CCS supplied voltage reference
5 different
The output voltage change, as a function of Voltage supply, 9-15 VDC.
2.50 Volt at Supply 12 VDC, is comparing point.

RED, the one to the left is like Keantoken idea. Performs well!
BLUE, in middle is the best one: JFET CSS + transistor shunt

On the right, plain JFET CSS and plain BJT CSS.
They play in different leagues.
Actually, the popular BJT CSS current source, we see in many 'hifi' amplifiers,
is terrible in performance, by comparison in this test.

This gives an indication of poor PSRR. (Power Supply Rejection Ratio)
Even if this test is not AC, it gives a good hint.

[keantoken]

Hi Lineup (:

Thanks for the info.

You can also put, say, a temperature sensor as the load (in my schematic) you will get a varying output voltage that isn't affected very much by input voltage.

Here is some more simulation and info regarding such.

The attached image is of the CCS when properly tuned. It produces second harmonics as a result of stray capacitances, unfortunately, which may make it less ideal for some applications. Tuning of Rtrim beyond or below this point will result in over- or under-compensation.

I used 7KHz as the test frequency so as to give an idea of high frequency operation. An AC analysis shows the output beginning to suffer at 2.5KHz.



More data soon...

- keantoken

[keantoken]

Here it is... "worse" values of Rtrim will overcompensate more.



- keantoken

[keantoken]

And here it is fully tuned... Notice the strange bends as a result of stray capacitance. It looks like the circuit has turned the low-value stray capacitances in to larger values to smoothen out the higher harmonics... Not sure but perhaps it is more important to use low-Cj transistors here?

Hey Lineup, try the above tests without C2, or with a much smaller value, as it degrades the performance quite a bit according to my tests. I don't know how necessary it is for noise filtering. Not to mention that if we want to use it for overcompensation we don't want to filter out the noise!



- keantoken

[keantoken]

Here is the circuit operating at 20KHz, properly tuned. If the overcompensation feature is not required simply adding a 1uF cap at the output decreases ripple significantly.

- keantoken

[keantoken]

Here it is fully tuned alongside the best one in Lineup's post.

What do you think, Lineup?



- keantoken

[sawreyrw]

keantoken,

The output voltage of your circuit will be very nearly equal to

Vbe+Vbe(1k/270) or 4.7*Vbe

where Vbe is the base emitter drop of the NPN transistor. The temperature coefficient of Vbe is about -2 mV/degree C, and Vbe for different real transistors of the same type will also be different. For these reasons this type of regulator is generally not used; the output is too dependent on transistor parameters. To see this effect run .op simulations at different temperatures. Try different transistor types as well. (The key parameters that effect Vbe are Is and temperature.)

Please note that I have given you this explanation only so that you may learn. I have no intention of finding fault with your work.

Rick

[keantoken]

Please note that I have given you this explanation only so that you may learn. I have no intention of finding fault with your work.

This was the information I was looking for. Thank you.

- keantoken