# Electronic – How is a diode used to regulate voltage

diodesoperational-amplifiervoltage-regulator

Hi all, I was wondering how a diode is used to control the voltage here? Assuming the voltage coming in from the left is greater than the reference voltage provided by the potentiometer, the diode is somehow acting to ensure the voltage going out is equal to the reference voltage. How does it do this, and if I tried to create this in real life what should I watch out for when choosing a diode part?
Thanks

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) The basic voltage follower. (b) Still a voltage follower. (c) The modified follower.

• (a) Vout = VR1. It's just a voltage follower.

• (b) Vout = VR3. It's still just a voltage follower but with the added work of fighting whatever VIN and R4 are providing.

• (c) If VIN ≤ VR5 then the op-amp output will swing high and D1 will be reverse biased. The op-amp will have no affect on VOUT.

If VIN > VR5 then the op-amp output will start to swing low until the inverting input is equal to the non-inverting input. The effect is that OA3's output will settle at 0.7 V or so below VR5 and VOUT = VR5.

Andy has made some good suggestions on diode selection.