Switching Circuits: MOSFETs, Transistors and Comparators

Circuits that switch

A switching circuit turns an output on or off in response to an input, without anyone pressing a button. It is the heart of every automatic system. This module works through the three main switching components: - The npn transistor: switched on by a small base current.\n- The n-channel MOSFET: switched on by a gate voltage.\n- The voltage comparator: switched by comparing an input with a reference.\n- Interfacing: how the switch drives an output such as a lamp or motor. Op-amp theory, voltage dividers, diodes and logic are covered in other modules. Here the focus is switching action.

Words for switching circuits

Five terms you need before you use them. Learn what each one means.

Match each switching part to how it turns on

The MOSFET against the npn transistor

Both are used as switches, but one is driven by a current and the other by a voltage, and that changes how you use them.

Which switches a comparator output?

A voltage comparator is being used as a switch. What makes its output change state?

• The input voltage rising past the reference voltage
• A large current flowing into the gate
• The base reaching about 0.7 volts
• The supply being switched off

Tap the two voltage-controlled ideas

Tap the TWO things that are controlled by a voltage rather than a current.

• A MOSFET turned on by its gate voltage
• A comparator switching at a reference voltage
• An npn transistor turned on by a base current
• A resistor limiting a current

Interfacing the switch to an output

A switching component rarely does anything useful on its own. Its job is to turn on an output stage, such as a lamp, a motor or a buzzer. The switch lets a small input control a much larger output current than the input could ever supply. When driving a lamp you add a series resistor to set a safe current, and when driving a motor you add a protection diode across the coil. Choosing the right component matters: a MOSFET is ideal when the driving signal can supply almost no current, while an npn transistor is a simple, cheap choice when a small base current is available.

Pick the true facts about switching circuits

Select the TWO statements that are true.

• A MOSFET is switched on by a voltage on its gate
• A comparator switches when its input passes the reference
• An npn transistor needs no base connection to switch
• A comparator adds its two input voltages together

Order how a light-activated switch works

Put the stages of an automatic light-activated switch in order.

• A sensor sub-system sets an input voltage that changes with light
• The input voltage is compared with a fixed reference
• When the input passes the threshold, the switch turns on
• The switching component drives the output stage
• The output, such as a lamp, turns on

A light that comes on at dusk

Follow how a dusk-sensing circuit switches a lamp on automatically as it gets dark.\n\nSensing the light: a light-dependent sensor sits in a divider, so the voltage at its junction rises steadily as the daylight fades. Setting the trigger point: a second, fixed divider sets a steady reference voltage, chosen to match the light level at dusk. The switching stage: a switching part continuously compares the sensor voltage with the fixed one. The moment the sensor voltage climbs past the fixed level, the part flips its output high. Driving the lamp: that high output turns on the stage that powers the lamp, so the light comes on just as dusk arrives, with no one touching a button.

Complete the switching facts

A current-controlled switch turned on by a small base current is a _____. A voltage-controlled switch turned on by its gate voltage is a _____. A part that switches its output by comparing two voltages is a _____. The input level at which the circuit changes state is the _____.

Work out the collector current

In a transistor switch, the base resistor has 6 volts across it and a resistance of 20 kilohms. The transistor current gain is 200. First find the base current, using the idea that current equals voltage divided by resistance, then multiply by the current gain. Give the collector current in milliamps.

Build a switching point

Choose the words that complete this statement about switching components.

Choose the switching component

Read each situation and choose the best switching component, then think about why.

• You need a switch that draws almost no current from a delicate sensor. Which component suits best?
• You want an output to turn on exactly when a sensor voltage passes a set level. Which part does this?
• An npn transistor is being used to switch a lamp. What must the base have?

Explain how switching circuits work

A classmate missed the lessons on switching circuits. Explain how they work using what this module has covered.

• Explain how an npn transistor works as a switch
• Explain how a MOSFET works as a switch
• Explain how a voltage comparator switches an output
• Explain one difference between a MOSFET and an npn transistor
• Finish by explaining how the switch drives an output such as a lamp