Mismatch parameters
Bipolar mismatch can happen due to 3 parameters:-
Mismatch
-
Mismatch
-
Mismatch
There are 2 methods to extract the Bipolar Mismatch parameters. Let us analyze both methods to determine which method represents all 3 mismatch sources adequately.
Measurement Method 1
In this method the setup is made as shown above. Here the emitter currents are the same and the difference in the Vbe’s can be directly measured in the 2 side by side devices.
Wehave:
Mismatch only
If the BJT has
mismatch only:Beta mismatch Only
If BJT has
mismatch only:Set:
Re Mismatch only
If BJT has
mismatch only:Measurement Method 2
In this method the setup is made as shown above. Here the Vbe’s of both side by side transistors are forced equal and their collector currents can be measured.
Mismatch only
Beta mismatch Only
Since
will be same and
is the same the measured
will be the same so we do not detect any mismatch due to Beta mismatch.Re Mismatch only
Setting
Solving for
:Comparison of the Methods
Mismatch Factor | Method 1 | Method 2 | Comment |
---|---|---|---|
|
|
| Both methods represent this mismatch |
|
|
| Method 2 cannot measure the mismatch due to Beta |
|
|
| Method 1 gives a better reading |
This Method 1 should be the method used to measire the mismatch parameter for a BJT since it includes effects from all 3 mismatch parameters.
NOTE: Mismatch will increase with temperature due to
dependance