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Electronics 102 - Lesson 4 - Answers

Question 1

I changed the stop value of the DC sweep to 1 A and ran the simulation. I obtained this plot:

You can see that the output voltage starts dropping much faster around 0.52 A. This is due to the transistor not having enough base drive. As an exercise, you can try reducing resistor R1 to see if you can get more current. However, be aware that the 2N2222 is only rated 0.5 A maximum collector current, so if we had ran the test on an actual part, the transistor would be dead by now :-)

Please note that the part of the curve where the output voltage becomes negative is an effect of the ideal current source model in SPICE. In an actual circuit, the output of the supply would not be driving an ideal current source, it would probably be driving a number of other components which could not drive the output negative, and the maximum current would be limited to around 0.7 A.

Question 2

Here is the plot with both the output voltage and the base voltage:

You can see that while the output voltage drops somewhat as the load current is increased, the base voltage remains constant. This is due to the transistor needing more base current as the load is increased, and as a result the Vbe increases.

If we had a way to compensate for the increase in Vbe of the transistor, we would have even better regulation.

We will study an improved circuit in the next lesson.

Question 3

The ratio of output voltage ripple versus input voltage ripple is called Ripple Rejection Ratio and is usually expressed in dB:

Ripple Rejection Ratio = - 20 * log(Output Vr / Input Vr) dB

In this case, the ripple rejection ratio is 36 dB.

Question 4

The variation between 25 and 75 degree C is about 80 mV.

Done!!!

May 11 2008 16:44:13