Constant current generator

[MacBam]

Hi

I have designed a simpel constant current generator to deliever 5mA. The problem is that it start to oscillating.

I have attached the circuit diagram, but i am using different transistors. Q2 BCX56 and Q1 is BC847B.

Any ideas, why it is ocillating?

Thanks....

 

[crutschow]

Don't see why it should oscillate.
Are there any long leads in your layout?
What is the amplitude and frequency of the oscillation at what point in the circuit?

 

[Audioguru]

Your schematic shows no supply voltage. If it is too high then the transistors have avalanche breakdown and oscillate.
Your schematic also shows no supply bypass capacitor. Maybe the supply voltage is jumping up and down caused by the current.
The circuit will oscillate if one transistor is upside down then it becomes a zener diode.

 

[MacBam]

Hi

it is a 17Mhz oscillation. It happens when i a use multimeter to measure... No long leads are used.

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The supply voltage range is between 6V to 40 V. Transistor should be able to handle that.

"The circuit will oscillate if one transistor is upside down then it becomes a zener diode."

How, could you be more specific?