DC Imperfections

Wednesday, 10 July 2019

7:29 PM

Consider an inverting amplifier where there is no input into either terminals

 

Ideally,  would be zero, but this is not the case in real life.

There are DC imperfections within the op-amp which cause  to be non-zero

 

The transistors within the op-amp draw some current - known as the input bias current ( and )

The mismatch of the transistors for the non-inverting and inverting terminals cause

 

The input offset current

 

There is also an input offset voltage, where the non-inverting and inverting terminals have different potentials

         BJT | Base-Emitter Voltage

MOSFET | Gate-Source voltage

 

Modelling the DC Imperfections

We need to consider the three causes of DC imperfection: input bias current, input offset current, and input offset voltage

 

We can calculate the maximum (worst case) output voltage offset with

 

So, then

 

 

 

Apply superposition

 

To calculate , disable  and  and apply golden rules

To calculate , disable  and  and apply golden rules

To calculate , disable  and  and apply golden rules