26-09-2013, 03:29 PM
Op Amp Regulator with Series-Pass Transistor
INTRODUCTION
What is the function of a voltage regulator circuit? It’s basically this - maintain a precise voltage
regardless of the current drawn by the load. Three basic components are needed to achieve good
voltage regulation.
1. A precision reference (zener diode) to set the output voltage.
2. A muscle component (transistor) to deliver the required current.
3. An automatic controller (opamp) to adjust the transistor drive. The “prime directive” of the op
amp is to adjust the base drive of Q1 delivering the required load current while keeping the
output voltage at a fixed value.
OUTPUT VOLTAGE
Resistors RF1 and RF2 feed a fraction of the regulator output Vo to the op amp's negative input
V-. The op amp then adjusts the drive to Q1 such that V- is equal to the zener voltage Vz. When
this occurs, the output voltage is related to the zener voltage through the RF1, RF2 divider .
LOAD REGULATION
How well does the regulator perform? One test is to apply a change in the load current and see
how well the regulator maintains its output voltage.
Current source IL generates a 1A pulse starting at 10ms and ending at 20ms. Place IL in the
circuit by removing the "*" at the beginning of the IL statement. Run a simulation and plot the
load current by adding trace I(IL) to the plot window.
The output should not change much except for the appearance of 2 small spikes. These spikes
show that the regulator takes some finite time to respond when the load current changes.