Input Protection for an Electronic Load
For reverse-voltage protection, you could consider (conceptually) something like this. Diodes are the inherent body diodes of the N-channel MOSFETs, not discrete components. 

Each input could go negative as much as one (body) diode drop so some biasing  may be necessary depending on the amplifier/comparator, perhaps a resistor to ground on each input of the comparator and a resistor to +5 to bias the input above ground. Some amplifier/comparators may require only a divider network. 

Anyway, if Vx is negative, then the output of U1 is ~0V so Q2 stays off, and Vx can be as high as the breakdown voltage of Q2 without causing damage (assuming R4, R5 are high enough to avoid damage to U1). If Vx is positive more than a few mV, then Q2 turns on fully, adding only milliohms to the circuit. 







simulate this circuit &ndash; Schematic created using CircuitLab