Quick Fundas: Op-amp#2

Another basic question that is sometimes asked to the fresh engineers during interviews:

When a positive feedback is added to an op-amp configured as comparably as shown in the picture below, how does the circuit behave? What is this circuit popularly known as? Could you please draw the transfer characteristics for the same?

In the picture below, VREF is a reference voltage, VIN is the voltage input and VOUT is the output of the Op-Amp.

Ans:

Without the positive feedback added using R2 above, the circuit behaves as a comparator. In that condition, considering the Op-Amp an ideal Op-Amp, when VIN rises just greater than VREF, the output of Op-Amp is driven to positive saturation voltage level VH. Again when VIN falls just lower than VREF, then VOUT goes to negative saturation voltage level VL. With R2 added as positive feedback, the circuit behaves as a Schmitt triggered comparator /buffer. With R2 in the circuit, when VIN rises more than VTH (see equation below) then VOUT is driven to positive saturation voltage level VH. Again when VIN falls lower than VTL (see equation below) then VOUT goes to negative saturation voltage level VL. Please note that VTH > VTL and the voltage difference (VTH - VTL) is the hysteresis band. The transfer characteristics is shown below:

VTH = VS - (R1/R2)*VL; 

VTL = VS - (R1/R2)*VH;

Where, VS = (1 + R1/R2)*VREF.