I built the following current mirror, but i didnt receive the same current at both sides:



Could anyone tell me what is the problem?

Thank you.

Attached Thumbnails

 

 

Q2 likely has a slightly higher threshold voltages (Vth) than Q1. Thus a gate voltage of 2.269V causes a current of 24.31mA in Q1 but only 18.7mA in Q2. For the two legs to have identical current, the two transistors would have to be perfectly matched.

Current mirrors work somwhat better with bipolar transistors since the unit-to-unit base voltage match is better between unmatched transistors of the same type.

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Carl
Curmudgeon Elektroniker

 

To do this properly, you need a "matched" transistor pair in a single package. These components are widely available, though I don't have a part number avilable.

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You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Best you could do without a single package pair is to match to separate devices and bond them face to face with super glue..both will then experience the same rise in temperature( well almost...)

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Eccentric millionaire financed by 'er indoors

 

Thanks guys.

I'd like to have your opinion about this too please.
I did another two experiments:

1. I Connected a 20mA IF, 2.2V(typ) VF green LED instead of R2, and indeed its voltage was 2.2V (I didnt measure the current).

2. I connected a 20mA 1.9V(typ) VF red LED instead of R2, and its voltage was indeed 1.9V (I didnt measure the current).

Does it mean that this current source works after all?
(If it wasnt a current source, i should have seen the same voltage on both LEDs).

So

 

The LED's will determine their own voltage. The fact that they didn't burn up is an indication that at least you are controlling the current, even it it's not identical in both branches of your mirror.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Screen your transistors to get two with the same beta if your're going to do it this way.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

It's working as a current source, it's just that the currents in the two halves aren't matched. The currents don't need to be matched to operate as a current mirror. It still has the high output impedance (that of the Q2's drain) and near constant current of a current source.

__________________
Carl
Curmudgeon Elektroniker

 

Now, put both your devices in thermal contact, as crutschow said above, to give you stable current regulation over temperature.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

Oh so you're saying that even if i connect a 200ohm resistor as R2, i'd get a current of ~18.7mA?

 

ummm...didnt I say that?

__________________
Eccentric millionaire financed by 'er indoors

 

Yes, Sorry

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

lmao...easy mistake...

__________________
Eccentric millionaire financed by 'er indoors

 

So,
Was i building a current source of 18.7mA, and not of 24.3mA?

 

Yes. The current is inversely proportional to the value of R1. If you want 24.3 mA then reduce the value of R1 by approximately 18.7/24.3 = 77% to 77Ω.

__________________
Carl
Curmudgeon Elektroniker