Voltage Regulators Questions and Answers

Answer: C

The current sense resistor is placed in series with the emitter of the pass transistor.

In a series regulator, the whole load current flows through the pass transistor and out of its emitter to the load. A small resistor in that path develops a voltage proportional to the load current. When this voltage reaches about 0.6 to 0.7 V, it turns on a current-limit transistor that diverts base drive from the pass transistor, limiting the output current and protecting the circuit from overload or short circuit.

A resistor across the pass transistor or in parallel with the load would not carry the load current, and the collector side is the unregulated input, so the standard current-limiting arrangement senses at the emitter.

Answer: D

The LM723 is suited to regulation from 2 to 37 V.

The LM723 is a general-purpose adjustable regulator. Its data sheet specifies an output voltage adjustable from 2 V to 37 V, with input voltages from 9.5 V up to 40 V, and an output current of up to 150 mA without an external pass transistor.

The option '9.5 to 40 V' is the input voltage range, not the regulated output. '0 to 40 V' and '0 to 27 V' are wrong because the 723 cannot regulate down to zero; its lowest output is about 2 V.

Answer: B

The output can be increased by increasing ground pin voltage.

A fixed regulator such as the 78xx holds a fixed voltage between its output and its ground (common) pin. If the ground pin is lifted above circuit ground by a resistor, a Zener diode or a potential divider, the output rises by the same amount: V_out = V_reg + V_{ground pin}.

An external pass transistor increases the current the regulator can deliver, not its voltage. Cascading two regulators only drops the voltage further, and a voltage doubler at the input raises the input, which the regulator then removes, so the output stays fixed.

Answer: C

Load regulation should be as low as possible.

Load regulation measures how much the output voltage changes as the load current varies from no load to full load:

\[ \text{LR} = \frac{V_{NL} - V_{FL}}{V_{FL}} \times 100\% \]

A good regulated supply keeps its output steady whatever the load, so this change, and the percentage, should be as small as possible, ideally zero. A high value means poor regulation. Regulation that varies with load impedance or is fixed at some intermediate value is not a design goal.

Answer: A

The minimum input-output voltage for the LM317 is 3 V.

The LM317 adjustable regulator needs a minimum difference (dropout) between its input and output to keep regulating. The datasheet specifies line regulation for an input-output differential from 3 V up to 40 V, so designers keep at least about 3 V of headroom.

1 V is below that headroom, and 5 V or 10 V are larger margins than the minimum the device needs, so they are not the minimum value.

Answer: C

The sequence is transformer, rectifier, filter, regulator.

The transformer first steps the AC mains voltage down to a suitable level. The rectifier converts that AC to pulsating DC. The filter (usually a capacitor) smooths out the ripple. Finally the regulator holds the output constant against changes in load and mains.

Each other order is physically impossible: the regulator and filter need DC at their input, and the transformer must act on the AC from the mains, so it has to come first and the regulator last.

Answer: B

The bypass capacitor gives a very high ripple rejection ratio.

In the LM317 the reference voltage sits between the output and the adjustment terminal, and the adjustment pin is fed from a resistive divider. Ripple on the output is fed back through that divider and amplified along with the reference. A capacitor of about 10 μF from the adjustment pin to ground bypasses the ripple, and the datasheet shows the ripple rejection rising from about 65 dB to about 80 dB.

The purpose is the opposite of low ripple rejection. Transient response is handled mainly by the output capacitor, and the capacitor does not create a high impedance.