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early effect voltage in current mirror

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lucky8

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Hi,

I'm aware that current mirror circuit is effected by early effect voltage, Va. I only know one method to reduce it, which is applying emitter degeneration into the circuit.

Is or are there any other way(s) to reduce this effect?
 

Try to use cascode structure. Since the effect of early voltage is reflect by the finite output resistance of current mirror, using cascode can boost your output resistance and thus equivalently reduce the effect of early voltage.
 

    L

    lucky8

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thanx for the reply.
is it necessary to use both cascode circuit and emitter degeneration or any one of them will do?
 

Emitter degeneration is wasteful as it involves a resistor. To increase the output impedance of a current mirror, cascoding is almost always good enough, unless your voltage range is very wide. Very often, it's not the early effect but the inherent mismatch that kills, hence, the key is to use units and large devices to improve matching.
 

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    lucky8

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lastdance,

what do you mean by inherit mismatch? do u mean the current mismatch?
 

lastdance said:
Emitter degeneration is wasteful as it involves a resistor. To increase the output impedance of a current mirror, cascoding is almost always good enough, unless your voltage range is very wide. Very often, it's not the early effect but the inherent mismatch that kills, hence, the key is to use units and large devices to improve matching.
Click to expand...

Why do we use large device to improve matching. For capacitor normally we break it into a few unit size of capacitor which is smaller. That is mean smaller device is better for matching. Correct me if I wrong
 

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    lucky8

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Inherent mismatch are mismatch due to the process, you dont see it on sim.
In current mirror, you can have geometrical mismatch, for eg., you want W to be 5u, but due to the process tolerance, it could end up to be 4.8u. To improve matching, you draw larger devices. You also have beta and Vth mismatch which are often the dominant mismatch parameters. Therefore, you need to design for larger overdrive to reduce effect of these mismatches. If you could, do common centroid. I normally do not use dummies, they dont seem to help.
What I meant was to draw larger devices in units. It's true that if you break a large device into many small unit pieces, you get the averaging effect and hence better matching. But I dont think you should use minimum size devices when it comes to accuracy.
 

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    lucky8

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lastdance said:
Inherent mismatch are mismatch due to the process, you dont see it on sim.
In current mirror, you can have geometrical mismatch, for eg., you want W to be 5u, but due to the process tolerance, it could end up to be 4.8u. To improve matching, you draw larger devices. You also have beta and Vth mismatch which are often the dominant mismatch parameters. Therefore, you need to design for larger overdrive to reduce effect of these mismatches. If you could, do common centroid. I normally do not use dummies, they dont seem to help.
What I meant was to draw larger devices in units. It's true that if you break a large device into many small unit pieces, you get the averaging effect and hence better matching. But I dont think you should use minimum size devices when it comes to accuracy.
Click to expand...

So, thats mean we should break our large devices into unit device rather than make it one big single device. Is this one apply for transistor also?
 

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    lucky8

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I think dummy devices is very important in matching, since your external devices will see different boundary condition thus different etching rate compared to your internal devices.

By the ways, how should we divided a devices to have the greatest matching performance? For example I want to match two 1pF capacitor, should we divided it into units of 500fF, 200fF, 100fF, 50fF or even smaller? Do it really the smaller the better??
 

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    lucky8

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terryssw said:
I think dummy devices is very important in matching, since your external devices will see different boundary condition thus different etching rate compared to your internal devices.

By the ways, how should we divided a devices to have the greatest matching performance? For example I want to match two 1pF capacitor, should we divided it into units of 500fF, 200fF, 100fF, 50fF or even smaller? Do it really the smaller the better??
Click to expand...

I don't think smaller capacitance has better match, the reason we break the capacitance :
1. decrease the well or diffussion resistance.
2. layout can mixed these two capacitances for better match. if you break the capacitance but not mix it. not good.
3. always add dummy patern in the layout boundary.
4. process variation makes small percentage in the big componet.
 

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    lucky8

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alchen77 said:
terryssw said:
I think dummy devices is very important in matching, since your external devices will see different boundary condition thus different etching rate compared to your internal devices.

By the ways, how should we divided a devices to have the greatest matching performance? For example I want to match two 1pF capacitor, should we divided it into units of 500fF, 200fF, 100fF, 50fF or even smaller? Do it really the smaller the better??
Click to expand...

I don't think smaller capacitance has better match, the reason we break the capacitance :
1. decrease the well or diffussion resistance.
2. layout can mixed these two capacitances for better match. if you break the capacitance but not mix it. not good.
3. always add dummy patern in the layout boundary.
4. process variation makes small percentage in the big componet.
Click to expand...

What I means is break it into units and then mixed it (i.e. using common centroid). Then how will you choose your unit size? For matching two 1pF capacitor, should we divided it into units of 500fF, 200fF, 100fF, 50fF or even smaller?
 

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    lucky8

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i think you do not gain much breaking into too fine a unit. Albeit saying so, of course, u can divide into finer units, only have to make sure the device geometry is not so small as to upset the absolute value by a large margin. But i guess u burn more area by doing finer units.
But do things like common centroid, dummies(not so sure of its relevance these days, some have said it is not as useful due to better processing techniques), contacts matching, etc.
 

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    lucky8

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Definately, breaking a larger device into smaller pieces in layout will surely help. Only constraint is the area. In design, it is always advised to have a wider device because it helps the device to hold good even in corner conditions
 

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    lucky8

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Vamsi Mocherla said:
Definately, breaking a larger device into smaller pieces in layout will surely help. Only constraint is the area. In design, it is always advised to have a wider device because it helps the device to hold good even in corner conditions
Click to expand...

How small is small anyway?
 

    L

    lucky8

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Well smaller size is the size where the unit transistor(or any other cell) can give reliable results .

Consider if we need a device of W1/L1 for a current of I1. You should be able to set a device of size W1/L1*1/K which will reliably give a current of I1/K. Hence K would be the number of fingers
 

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    lucky8

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Vamsi Mocherla said:
Well smaller size is the size where the unit transistor(or any other cell) can give reliable results .

Consider if we need a device of W1/L1 for a current of I1. You should be able to set a device of size W1/L1*1/K which will reliably give a current of I1/K. Hence K would be the number of fingers
Click to expand...

Can you discuss more on how reliable should be, or how you can measure if the device give reliable current? Since larger unit devices will always produce more reliable current, but we always preferred smaller unit devices in terms of matching? Is there some explicit limit on the choice of unit devices?
 

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    lucky8

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The simplest solution would be cascoded current mirrror.
 

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    lucky8

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terryssw said:
Vamsi Mocherla said:
Well smaller size is the size where the unit transistor(or any other cell) can give reliable results .

Consider if we need a device of W1/L1 for a current of I1. You should be able to set a device of size W1/L1*1/K which will reliably give a current of I1/K. Hence K would be the number of fingers
Click to expand...

Can you discuss more on how reliable should be, or how you can measure if the device give reliable current? Since larger unit devices will always produce more reliable current, but we always preferred smaller unit devices in terms of matching? Is there some explicit limit on the choice of unit devices?
Click to expand...

If we break big resistor into smaller, we will have more contact right. ANd this contact has resistance, therefore will incerase the overall resistance value.... Seems like problem.... any comment
 

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    lucky8

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pbs681 said:
If we break big resistor into smaller, we will have more contact right. ANd this contact has resistance, therefore will incerase the overall resistance value.... Seems like problem.... any comment
Click to expand...

I think it would be not much problems, since although you have more contact, you must have identical contact structure over every small resistors, and they could be well match (the overall resistance has increase, but people usually don't care absolute resistance. Your resistor ratio still not changed)

The main problems is, can I get better matching using small units ?
 

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    lucky8

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