Transistors as Switches and Amplifiers
Controlling Large Signals with Small Ones
Transistor operation
A transistor lets a small input control a larger output. In BJTs, a base current controls collector current. In MOSFETs, a gate voltage creates a conductive channel. These devices are used either in switching mode or in linear amplification mode.
BJT vs MOSFET
BJT
- Current-controlled device
- Often used for linear amplification and simple switching
MOSFET
- Voltage-controlled gate
- Very efficient as a switch because gate current is tiny
Useful transistor terms
In many digital switching applications, why are MOSFETs popular?
MOSFETs are gate-voltage controlled and draw very little steady-state gate current, making them efficient switches.
Correct answer: They are voltage-controlled and efficient
What BJT terminal controls collector current?
A small base current controls a much larger collector current in a BJT.
Correct answer: base
A transistor used as a switch is typically operated in which region when fully on?
For switching, the goal is a low-loss on-state, which is the saturation region for a BJT.
Correct answer: Saturation
What is one benefit of using a transistor as an amplifier?
An amplifier can make a weak signal large enough to drive another stage or load.
Correct answer: it increases signal power
Designing a transistor switch
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Step 1: Determine the load current and supply voltage.
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Step 2: Select a transistor with adequate voltage and current ratings.
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Step 3: Provide proper base or gate drive from the control signal.
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Step 4: Add protection such as a flyback diode for inductive loads.