Question E7D06:
What is the purpose of Q1 in the circuit shown in Figure E7-2?  

Choices:  
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 is  C: It controls the current to keep the output voltage constant  

Correct answer: C — It controls the current to keep the output voltage constant.

Why C is correct:
- In a linear series regulator, Q1 is the series-pass transistor. The error amplifier compares a reference to a sample of the output and drives Q1’s base so Q1 conducts just enough current to the load to hold the output voltage constant, despite changes in input voltage or load.
- By adjusting Q1’s conduction, the circuit drops a variable voltage across Q1 and maintains the desired output.

Useful ways to view it:
- Emitter follower style regulator: $$V_{out} \approx V_Z - V_{BE}$$ where a Zener sets the base and Q1 follows at one diode drop lower.
- Op-amp controlled regulator: $$V_{out} \approx V_{ref}\left(1 + \frac{R_2}{R_1}\right)$$ The op-amp modulates Q1 so the sensed output equals $V_{ref}$.
- Pass device dissipation: $$P_Q = (V_{in} - V_{out})\, I_{load}$$

Why the other choices are incorrect:
- A: “It provides negative feedback to improve regulation” — Negative feedback is provided by the sensing network and error amplifier. Q1 is the controlled element inside the loop, not the feedback path itself.
- B: “It provides a constant load for the voltage source” — That describes a shunt regulator or dummy load. In this circuit Q1 is in series with the load, not across the supply.
- D: “It provides regulation by switching or ‘chopping’ the input DC voltage” — That’s a switching regulator function. Figure E7-2 is a linear regulator using a continuously variable pass device, not a switch.

Rules of thumb:
- Series-pass regulator: Q1 in series controls current to regulate voltage.
- Shunt regulator: device in parallel sinks current to regulate voltage.
- Dropout considerations: for an emitter follower pass device, $$V_{drop} \approx V_{BE} \,(\text{or } \approx 2V_{BE} \text{ for Darlington})$$ so $V_{in}$ must exceed $V_{out}$ by at least this amount plus headroom for the driver.