Chapter 4 - Switching circuits

What are three common switching devices in electronics?

NPN transistor

What is an NPN transistor used for in a switching circuit?

Acts as a current-operated switch to control larger current in collector circuit with a small base current

What are the three terminals of an NPN transistor?

Emitter

How is an NPN transistor connected in a switch circuit?

Emitter to 0V, base to input (through resistor), load in collector branch to +Vs

At what input (VIN) does an NPN transistor switch on?

VIN ≥ 0.7V (for silicon transistor)

What is the collector current formula for a saturated transistor?

IC = hFE × IB until saturation; after that, IC is determined by the load

What is hFE?

Current gain of a transistor (IC/IB)

What is the typical base-emitter voltage for an NPN transistor when ON?

0.7V

What happens if VIN < 0.7V in a transistor switch?

Transistor is off; no current flows through collector

What practical outputs can a transistor switch control?

Lamps, LEDs, buzzers, motors, relays, solenoids

What happens to the output device if transistor is OFF (VIN low)?

No current flows in output; device is off

Why is a base resistor needed in transistor switches?

To limit base current so transistor isn’t damaged

What is a MOSFET in electronics?

Voltage-operated switch (very high input impedance—almost no gate current)

What are the three terminals of an n-channel MOSFET?

Gate

When does an n-channel enhancement MOSFET turn ON?

When the gate-source voltage (VGS) is above threshold (often > 2V)

What is the MOSFET current equation?

ID = gM × (VGS – 3)

What happens to the MOSFET when gate voltage is zero?

OFF (no drain current flows)

What is a key advantage of a MOSFET switch over a transistor?

Handles high currents with no gate loading; suitable for direct connection to logic circuits

Why do MOSFETs need careful handling?

Very high input resistance makes them sensitive to static damage

What does a voltage comparator do?

Compares two input voltages; switches output to high if non-inverting (+) input is higher than inverting (–) input

How does a comparator turn slow sensor signals into rapid switching?

Output snaps sharply between LOW (0V) and HIGH (supply voltage)

Where is a reference voltage connected on a comparator?

Usually to the inverting (–) input

How does a comparator improve LDR or thermistor circuits versus using just a transistor?

Comparator switches output sharply at threshold, avoiding transistor partially ON (overheating)

Why don't comparators drive high-power outputs directly?

Low output current (typically ≤ 50mA); needs output driver (transistor/MOSFET) for motors, lamps, etc

How do you make a comparator circuit adjustable?

Use a variable resistor or potentiometer to set the reference voltage or sensor divider

In a comparator circuit, what happens if + input voltage rises above – input?

Output jumps to supply voltage (HIGH)

What is a common IC used as a comparator at GCSE?

LM358 (op-amp acting as comparator)

How do you connect an LED as an output to a comparator?

LED (with series resistor) between output and 0V or supply (check max output current)

Give a real-world example of a transistor switch application.

Automatic lamp turning on in darkness using LDR + transistor switch

Why is saturation region important in transistor switching?

Transistor is fully ON (VCE ≈ 0), output device gets full current, less transistor heating

What happens if transistor is in the linear region (not fully ON or OFF)?

Partial voltage across the collector-emitter, less current to output, transistor may overheat

How would you connect a comparator and MOSFET for high power switching?

Comparator output to gate of MOSFET; MOSFET source to 0V, drain to output device

Why use a MOSFET or transistor as an interface?

To allow low-power logic or sensor circuit to control a high-power output

What is VGS for an n-channel MOSFET to be ON?

Above its threshold, typically > 2V (check datasheet)

For what purpose do we use a comparator with a temperature sensor?

Sharp switching at a set temperature threshold (e.g., turns on a fan, alarm)

What is the main safety point with MOSFETs?

Protect from static electricity (wear wrist strap/ground touch metal)

In which units is gM (transconductance) measured?

Siemens (S)

How do you calculate required base current for NPN transistor?

IB = IC / hFE (choose IB slightly higher to ensure saturation)

Why might you use a MOSFET over a transistor for switching?

Higher current capacity, easier to drive from logic

If sensor output is slow-changing, what problem may occur with a transistor switch?

Output device may be dim or noisy; transistor not fully ON or OFF

How do you set threshold voltage in a comparator circuit?

Change the voltage divider on the reference input or use a potentiometer

How could you use a MOSFET to switch a motor with digital control?

Apply digital HIGH to gate, MOSFET turns ON, connects motor from +Vs to 0V

How does a comparator "digitalize" an analogue input?

It gives pure HIGH or LOW output, not a gradual intermediate level