Question
What is the purpose of Q1 in the circuit shown in Figure E7-2?
Answer Options
- A) It provides negative feedback to improve regulation
- B) It provides a constant load for the voltage source
- C) It controls the current to keep the output voltage constant
- D) It provides regulation by switching or “chopping” the input DC voltage
Correct Answer: C
Explanation
Figure E7-2 shows a basic linear voltage regulator circuit. Q_1 is a pass transistor wired in series with the input and output. It serves as the primary control element responsible for dropping the excess voltage from the +25 \text{ V} input down to the regulated +12 \text{ V} output.
The purpose of Q_1 is to actively control the current to keep the output voltage constant. A feedback network (not fully shown, but implied by D_1) senses the output voltage and adjusts the base drive to Q_1. This causes Q_1 to vary its conduction (acting as a variable resistor), which precisely drops the necessary amount of excess voltage across its collector-emitter junction.
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