Bias

Current Bias

Current Bias Example

If we can work to make Op-Amp current bias’s cancel, we can use these Op-Amps with high internal current bias and make them work practically,

To understand this, we have Left R and Right R, we know that the voltage at the middle node (node -) is -$\frac{R}{2}{I}_B(+)$ so the voltage through will be Vin - Middle Node, or $V_{in}+\frac{R}{2}{I}_B(+)$ , note that no current can flow opposite direction through I_B(-) so it is ignored, and no current can flow from the Op-Amp, next, we can calculate the current flow through Right R, which is obviously going to just be the voltage as from before in addition to the $I_B(-)$ which can now have current flow, so therefore it will be $\frac{V_{in}+\frac{R}{2}{I}_B(+)}{R}-I_B(-)$ (The flow of $I_B(-)$ flows opposite). Finally that means that the voltage out will be

$$V_{out}=\frac{R}{2}{I}_B(+)-V_{in}-\frac{R}{2}{I}_B(+)+R{I}_B(-)$$ $$V_{out}=-V_{in}+R(I_B(-)-I_B(+))$$

Note that if $I_B(-)=I_B(+)$ then there is 0 error.

Ex 1:

This in general is one way to represent our error in current bias, with a I_B(-) and I_B(+) (However as this is connected to a virtual earth and nothing else, nothing happens)

Its to be noted that $I_B\ast R_2=-I_B\ast R1$ practically and the difference comes from where we represent the error on the circuit.

Another way of representing this circuit is: