CCS explained

[catalin gramada]

Hello. Complete newbie regard CCS. Found this schematic on internet .May someone explain how it works (for ignorants) ? and how can be adjusted in respect with negative supply voltage and constant current please ? I.m tempted to adapt this to a -100v negative source to assure 2 mA constant current to 12ax7 cathodes. Thanks.

[Dave H]

The R8,R10 potential divider sets 23V across R10, subtracting 1V for the Vbe of Q1 puts 22V across R12 (2k2) which defines a current of 10mA for Q1 emitter and collector (ignoring the small base current). 10mA is 5mA for each half of V1. 5ma across R9 is a voltage drop of 5 x 39 = 195V. B+ is 335V, 335V - 195V gives a plate voltage of 140V

[catalin gramada]

The R8,R10 potential divider sets 23V across R10, subtracting 1V for the Vbe of Q1 puts 22V across R12 (2k2) which defines a current of 10mA for Q1 emitter and collector (ignoring the small base current)

Ok. I got it. Thank you. How the values of R8, R10 divider was choosed, arbitrary ? Do we need a certain current to pass through negative supply - R10 - R8 string please ?

[Dave H]

Ok. I got it. Thank you. How the values of R8, R10 was choosed, arbitrary ?

They must have run out of 10k

I'd chose the value so that the potential divider current is large compared to Q1 base current.

[catalin gramada]

Q1 base current meant the current settled by R 12 right ? Well, large meant at what order at magnitude minimal , optimal ? 10x maybe ...?

[catalin gramada]

So, with same -45v supply, for instance if choose R10- 1.5k R8-15k and set same current by R12- 4k I will get same result, the ccs it will work as same please ? Why -23v into the base was set and not -10v or -40v please ?

[catalin gramada]

Does it was settled voltage into the middle (at-23v) with a reason ? It.s like a bias set adjustment please ?

[catalin gramada]

There is an other picture with similar ccs but with R12 tied to the ground instead minus supply rail, like in previous sample . I.m completly confused. May someone enlighten me please ? Thanks.

[uneumann]

There is an other picture with similar ccs but with R12 tied to the ground instead minus supply rail, like in previous sample . I.m completly confused. May someone enlighten me please ? Thanks.

This one is a drawing error. The bottom of R12 is shown connected to ground. That can't be right. It should be connected to the bottom of C3.

[loudthud]

The schematic in post #8 doesn't look like it would work to me. I suspect a mistake in the drawing.

Here is a slightly different current source that uses two transistors from a Fender Super Sonic.

[Axtman]

Pardon my ignorance but what is "CCS"?

[Helmholtz]

CCS means "constant current source".
The term is used for a circuit that provides a very large AC impedance while dropping only moderate DCV.

In our example here it is used to improve the balance/symmetry of the differential amp (long tailed pair phase inverter).

[Dave H]

Q1 base current meant the current settled by R 12 right ? Well, large meant at what order at magnitude minimal , optimal ? 10x maybe ...?

Yes, an order of magnitude (or more). The current set by R12 is the emitter current. The base current is emitter current divided by transistor current gain (100 say) i.e 10mA / 100 = 0.1mA. Potential divider current is 45V / 16.4k = 2.7mA which is 27 x the base current.

[Helmholtz]

Wost case assumption would be the transistor having the minimum Hfe of 30, resulting in a current ratio of 9.

https://www.onsemi.com/pdf/datasheet/mje340-d.pdf

[Dave H]

So, with same -45v supply, for instance if choose R10- 1.5k R8-15k and set same current by R12- 4k I will get same result, the ccs it will work as same please ? Why -23v into the base was set and not -10v or -40v please ?

I think you have the resistor values reversed. Make R10 15k and R8 1k5 then it should work. Why not 23V ?