15.6: Procedure

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1. In this exercise, a DC source is being used to simulate the output of a rectified and filtered power source in the interest of simplicity.

2. Note that the op amp is acting as a series-parallel feedback amplifier where the load voltage is equal to \(V_{Zener}\)(\(1+R_{scale}\)/47k \(\Omega\)). Changes to the load resistor should cause no change in the load voltage. Also, note that \(I_{out}\) is equal to the transistor’s \(\beta\) times the output current of the op amp. The transistor is therefore being used as a current booster, and because it is located inside of the feedback loop, it should not affect the load voltage.

3. Connect the circuit of Figure 15.5.1 using an \(R_{scale}\) of 47k \(\Omega\), and an \(R_{load}\) of 100k \(\Omega\).

4. Calculate and record the values for \(V_{load}\), \(I_{load}\), \(I_{out}\), and \(I_{out-op-amp}\) in Table 15.7.1. A typical \(\beta\) would be in the range of 50 to 100, depending on the pass transistor used.

5. Measure and record the values for \(V_{load}\), \(I_{load}\), \(I_{out}\), and \(I_{out-op-amp}\) (i.e., at point A) in Table 15.7.2.

6. Change \(R_{scale}\) to 100k \(\Omega\) and repeat steps 4 and 5.

7. Change \(R_{scale}\) back to 47k \(\Omega\), change \(R_{load}\) to 100 \(\Omega\) and repeat steps 4, 5, and 6.

15.6.1: Computer Simulation

8. One limitation of the preceding exercise is that it does not show how well the regulator suppresses AC ripple that might exist on the input power source. This can be accomplished easily in a simulator. Build the circuit in a simulator using \(R_{load}\) equal to 100 \(\Omega\) and \(R_{scale}\) equal to 100k \(\Omega\). To create the ripple, simply insert an AC power source in series with the 20 volt DC source (i.e., inserted between ground and the negative terminal of the 20 volt source). Set the AC source to 120 Hz and 2 volts peak. This will mimic filtered full-wave rectified 60 Hz power ripple. Run a Transient Analysis to determine the load voltage. In the technical report, include a plot of the simulated load voltage along with the voltage applied to the collector of the pass transistor.