Sorry for the short title but I having that blank page bug and reducing the title was the only way to get past it.


So my question is:

When designing a current mirror, I know that a Widlar current source adds a Resistor in the emmiter of the transistitor Q2.

But why would you want to make both current be different? Wouldn't it defeat the purpose of a current mirror which is to replicate Iref in Iout?

Here's an image of the Widlar Current source:

Thanks in advance.

 

Perhaps this will help.

 

Thanks for replying... I have been searching for a while in many sites and I just can't find the answer, for small signal currents with that configuration it doesn't really replicate the current. It comes out differently than the input.

 

The purpose of a current mirror can be whatever is needed. If you need an output current smaller than the reference current, then you can use the Widlar Current Source.

 

It's what I generically call an unbalanced current mirror since the currents through each branch are intentionally different. It has a fairly wide range of uses. Inside linear IC's, the common design configuration uses a reference current source with a number of other current sources feeding circuit branches.

If you also add a resistor in the emitter leg of Q1, it lets you vary the current through Q2 to just about any ratio you want. It also provides degenerative feedback to compensate for VBE variations between the two transistors.

 

But why would you want to make both current be different? Wouldn't it defeat the purpose of a current mirror which is to replicate Iref in Iout?

That's a common point of misunderstanding, and "current mirror" is probably an unfortunate term but it's the one commonly used. I never define a current mirror as something that must have identical currents. rather, the changes in the ref current are mirrored over to the other. In other words, the ref current controls the other current. In reality, they are rarely the same but used to have a small current from a precision circuit control a large current. But when the currents are different, I make it clear by calling it an unbalanced current mirror since the currents are not equal.

In IC's, you have one "control current" in a bias chain setting the larger currents in many other circuit branches.

 

I see..I understand it now.

I appreciate it guys, Thanks.

 

That's a common point of misunderstanding, and "current mirror" is probably an unfortunate term but it's the one commonly used. I never define a current mirror as something that must have identical currents. rather, the changes in the ref current are mirrored over to the other. In other words, the ref current controls the other current. In reality, they are rarely the same but used to have a small current from a precision circuit control a large current. But when the currents are different, I make it clear by calling it an unbalanced current mirror since the currents are not equal.

In IC's, you have one "control current" in a bias chain setting the larger currents in many other circuit branches.

Yeah my professor just introduced the concept of current mirrors as being a replica of each other and didn't really explain too much as we went along, so this was a little confusing. Thanks again for clearing it up.