Exercise 3.5.5. Linear Voltage Regulator
In order to get a constant (lower) voltage from a higher voltage input or a source with a broader spread of the voltage (e.g. a battery) often linear regulators are used. One example could be to get $5 ~\rm V$ from the car battery voltage (between $11 ~\rm V...14 ~\rm V$) for a microcontroller in a control unit e.g. the brake control unit. Linear regulator here means that a transistor as a variable resistor is used to drop the unwanted voltage.
Below, two types of such linear regulators are shown
- The first simulation shows a simple series regulator with a FET. „Series“ here marks the fact that the transistor is in series to the load resistor $R_\rm L$. The Zener diode $D$ has a current limiting series resistors $R_\rm D$ ahead. By the voltage divider of $R_\rm D$ and $D$, a relatively constant voltage will be created.
- The second simulation shows a more sophisticated circuit. Here, there is feedback from the output of the transistor back to the transistor controlling voltage. This feedback is given by $R1$, $R2$, and the operational amplifier.
Tasks
- In both simulations there are two sliders on the right-hand side:
- Input Voltage, which changes the ingoing voltage between $5~\rm V...20~\rm V$
- Load Resistance, which changes the load on the output between $10~\Omega...1~\rm k\Omega$
Play with these sliders and look for the differences! What are these? - The lower simulation with the operational amplifier is also called „Low DropOout“ (LDO). The dropout is the minimum voltage difference on the transistor. How can the terminology low dropoff can be explained?
- To which primitive OpAmp circuit does the LDO circuit ($R_1$, $R_2$ and OpAmp) look similar to?
How can the controlling of the transistor input voltage $U_{\rm GS}$ be explained? - Given a load resistor of $R_\rm L=1~\rm k\Omega$, an input voltage $U_\rm I=20~\rm V$, and an output voltage $U_\rm O=5~\rm V$, what is the dissipated power on the load and on the transistor?