Voltage Regulator

Use an error amplifier and a pass transistor to hold an output voltage under changing load.

VR supplies a 2.5 V reference. RF = RG = 10 kΩ return half the output voltage to A1. The amplifier drives Q1’s base until the divided output matches the reference. Q1 supplies current from the 12 V source to RL = 1 kΩ.

$$V_{\mathrm{out}}\approx V_{\mathrm{ref}}\left(1+\frac{R_F}{R_G}\right)=5\,\mathrm{V}$$ $$P_Q\approx(V_{\mathrm{supply}}-V_{\mathrm{out}})I_{\mathrm{load}}$$

Vref is green, Vout red and load current blue. The table should show about 5 V and 5 mA. The emitter-follower drop is inside the feedback loop and is largely compensated. This simplified DC example omits current limiting, thermal protection, dropout details and loop-compensation design.

Read the results. The saved operating-point table shows the probe values. The first voltage probe is green and the second red. The first current probe is blue; a second current probe is purple.

Model limits. The three-terminal op-amp model has implicit ±12 V output limits, 200,000 low-frequency gain and 10 MHz gain-bandwidth product. Supply pins are intentionally absent in this functional model. In Schmitt-trigger and relaxation/integrator oscillator examples, the switching amplifier uses a 100 kHz gain-bandwidth setting to resolve transitions cleanly at the saved time step. It omits detailed slew-rate, input common-mode, output-current and thermal limits.

Try it. Set RL to 500 Ω; the output remains near 5 V while load current doubles. Restore the original values after comparing the results.