Electronic – Help with understanding op amp circuit using diodes at the output

circuit analysisdiodesoperational-amplifier

I'm trying to understand this circuit and having trouble figuring out how the diodes will affect it:

enter image description here

I'm trying analyze this and understand how \$V_{\text{out}+}\$ and \$V_{\text{out}-}\$ will be affected by different values of \$V_i\$ but I am unsure. I believe \$V_{\text{out}+}\$ will see a higher voltage with lower \$V_i\$ values and once \$V_{\text{out}+}\$ is past 0.6V the current will go through both of the diodes and \$V_{\text{out}-}\$ will begin to see an increase. I'm unsure of what to think about the op amp in the center though.

Best Answer

The OpAmp is used in an inverting configuration, i.e. when you apply a positive Vi, the output Vo will be negative. So, the current flows through R3 and the upper path (R1, D1), while current through the lower path is blocked by D2.
So, for a positive input, Vo+ will be negative and Vo- will be zero.
The exact value on Vo+ is determined by the ratio of R3 and R1, as the output terminal Vo will be a value which cancels out the voltage at the negative input terminal.

To examine this a bit more, you can do a simulation, even here. I've drawn your circuit using the circuit designer of EE. Click on simulate this circuit and do a DC sweep for Vi.


simulate this circuit – Schematic created using CircuitLab

The following picture shows the result, blue is Vo+, orange Vo-. I've also added Vo (brown) just to see the effect of the diodes. (Click the image to enlarge) enter image description here