In Electronics A2 (GCE), we are currently studying voltage regulators.
An example voltage regulator circuit is provided using an op-amp and a power MOSFET.
I would judge this to be a voltage follower.
The workbook however claims it uses on-off control. In other words, if the output voltage is below the reference, the MOSFET is turned hard-on; otherwise, it is turned off. Since the cap takes time to charge, this causes the circuit to regulate at a voltage.
However, I would have thought that due to the lack of inertia in the circuit (at most the on-resistance of the power MOSFET, around 0.02 ohms) would cause it to constantly overshoot if it did this; instead, it simply regulated normally.
When the gate waveform was checked, it was steady, which further convinces me that it is essentially a power follower for the op-amp.
The teacher claimed that it was probably switching on and off too fast to see at the gate using "only" a 50MHz scope. The time constant to charge the 4.7uF cap (changed from diagram) through the 0.02 ohm RdsON would be on the order of nanoseconds, so this could be true, but I can't see how a general purpose TL071 op amp could possibly do this, especially without a gate driver and high speed PCB layout.
Interestingly, the original circuit used 470nF which worked up to around 40mA load; the cap had to be increased to 4.7uF to maintain a stable output up to 250mA (otherwise, it did appear to exhibit a form of oscillation, around 500mVp-p noise at a few kHz.)