CCS question

[Bazukaz]

Hi,
I need a constant current source from positive supply , but using NPN transistor. Is it possible to achieve ?
Regards ,
Lukas

[janneman]

Like this?

Jan Didden

[AKSA]

I don't believe it is. You must use a PNP.

Hugh

[Bazukaz]

Thanks ,
I have found similar already ,
But i need the current source feed from +Vs signal. Actually , i am trying to design an A class amplifier with an IC , one NPN transistor , and a current source - but i am restricted to use NPN devices for a current source too , because it is a high - voltage amp.
What should i use instead of I1 ? Restor seems to add too much distortion.


Regards ,
Lukas

[HFGuy]

Why do you want to use a CSS as a load ?? a CSS is used as a load when you want very high gain, but you dont need more open loop gain as the opamp has more then enough. you can also use a LM317 voltage regulator as a current source.

[jcx]

probably related to:
ESL Headphone direct drive

where a high V floating ccs is wanted

jan's battery bias would work as a proof of concept test to see if the ccs helps much but a more permanent solution requires a way to generate bias for the ccs pass device, with a bipolar Q this is hard, a Supertex depletion mode fet LD150 used as a ~1ma ccs could bias a npn ccs

a enhancement mode VN0545 mosfet (DigiKey) could be biased with a opto driver designed for solid state relay control http://www.irf.com/product-info/data...a/pvi5013r.pdf (DigiKey)
with the opto gate driver looking like a battery and a souce series R setting the current

Digikey also has VP0545 p channel mosfet which could make a single ended ccs hanging from the V++ supply


another possible improvement is to linearize the pass transitor - the Baxendal "super-pair" may be useful here: (mje340 not recommended at this V, its just that I think the model comes with LtSpice)



(the dc bias is hiding in the input Vsource)

[Bazukaz]

Well ,
I have built this circuit with a resistor and measured distortion levels of around 0.2%.I have read that nonlinearities may arise if a current source is a resistor.
I cannot use LM317 because the supply is 350 - 700 Volts.

[Bazukaz]

Sorry , didnt read last reply .
Yes , it is related to another thread
So , as i understand , i would need a separate low voltage source to set bias for Q3 ?

Lukas

[jcx]

a 9V battery could work - for a while, long enough to see if it helps

[HFGuy]

The LM317 can be used with an external BJT or MOSFET so your higher operating voltage is just about choosing the right external BJT. A resistor is a distortionless component, all things being equal a CCS will result in higher distortion. Have you considered using a source follower output stage instead of your common emitter ???