The addition of a BJT or MOSFET increases the output current capability. Op-amps typically have an output capability of a few tens of mA max. There is a little leakage current and you can get its value from the datasheet. In practice you'll need a little "headroom" of a couple of volts at least.ġ Not quite true. The voltage at (1) will be reasonably steady for a wide range of voltages V in > V ref. All that is required is to feed it a voltage higher than it's breakdown voltage with some form of current limiting, R3 in this case. Zener diodes are designed to breakdown in a controlled manner at a specific voltage. A regular diode will not pass any current in reverse mode 1 unless you exceed the reverse breakdown voltage at which point you have probably destroyed it. Note that the Zener diode is pointing towards the positive supply so that it is reverse biased.
You've changed your schematic to a MOSFET since I started my answer but the principle is the same. My question is, how does the comparing of \$V_\text \$. Any perturbation of the input current or the load current (or even others) will affect the output voltage, but the feedback loop will quickly adjust the driving of the BJT base to stabilize the output voltage at the desired reference.īut I pretty much am lost after they talk about the output voltage being divided down and then compared to a reference voltage. Then, the op amp supplies a voltage to the gate of the MOSFET transistor such that its negative input (the output voltage) follows its positive input (the reference), due to the beautiful effect of negative feedback. A voltage divider composed by R1 and R2 provides a measure of the output voltage suitable for the opamp, i.e., if we want the output voltage to be k times higher than the voltage reference, than the voltage divider should divide the output voltage by k so that both inputs of the opamp can be equal for the desired output voltage. Note that any other kind of voltage reference, such as a bandgap voltage reference would also work (it is actually better). The zener diode provides the reference voltage to the positive input of the opamp. Here is the explanation that I have found: I am trying to understand the functioning of this circuit since I have read that it is appropriate for power regulation:
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