Transistors application in consumer electronics


Not all electronic system design are fully using integrated circuits. Discrete transistors are commonly used in design to achieve simple function such as protection circuits and simple load control switch at a very low cost.


Common Types of BJT and FET Transistors

PN Diode

Bipolar Junction Transistor (BJT)

It is a 3 terminal device containing: base, emitter, and collector.

Metal Oxide Field Transistor (MOSFET)

It is a 3 terminal device containing: gate, drain, and source.

Junction Field Transistor (JFET)

It is a 3 terminal device containing: gate, drain, and source.

Power Transistor

Silicon controlled rectifier (SCR)

Three terminal device, anode, gate, and cathode.

It's consisting of 3 PN junctions (PNPN)

Insulated Gate Bipolar Transistor (IGBT)

Three terminal device, collector, gate, and emitter.

It's consisting of a MOSET control gate and a PNPN structure (same topology as SCR)


Let's Look at the IV Characterstics for each type of transistor

Explanation of the current-voltage (IV) curves for each circuit:

These IV curves provide insights into the behavior of each circuit component, allowing engineers to understand and design circuits based on their specific characteristics.

Practical Design using Discrete FETs in Consumer Hardware

Reverse voltage protection circuit

A PMOSFET can be used to turn on only when a positive input voltage applied to Source of PMOSFET referenced to Gate (which is grounded in this design) is positive, hence a negative input voltage will keep the PMOSFET off achieve reverse voltage protection.

The way the source becomes postive is Vin initially passes though the body didoe of the PMOS transistor from Drain to Source before turning to PMOS.

This approach has lower conduction loss compared to a single diode reverse protection design.

Over voltage protection circuit

Two PMOSFET plus a zener diode are used. The Zener diode clamps the gate voltage of PMOSFET to zener diode's breakdown voltage when Vin goes above that. When that happens, PMOS Q1 turns on, which in return pulls the gate of PMOS Q2 to Vin. Since VGS is ~ 0V,  Q2 turns off subsequently. As a result,  the load protected against an over voltage. The OVP trip voltage is roughly equal to zener diode's breakdown voltage plus threshold voltage of Q2.

Soft Start Circuit

Using the same PMOSFET as that of reverse voltage protection circuit but delaying the turn on time by adding a RC delay on gate.

The turn on delay is roughly equal to RC constant.

Load switch

Use NMOSSFET to turn on PMOS as a load switch. The reason to use a NMOST is to provide isolation between control signal (usually a GPIO) to the voltage rail that the PMOS is use to gate.

level Shifter

Use NMOSFET and its body diode to achieve bidirectional communication for open drain drivers.

When 3.3V driver pulls the interface low, the body diode conducts pulling source of NMOST low via the diode, then the low signal is read by 1.8V receiver.

It is often used for I2C level shifting.

When 1.8V driver pulls the interface low, VGS of the NMOS is 1.8V, and it turns on the transistor. As a result, the 3.3V receivers see a low voltage.

When either of driver releases the interface, the NMOS is switched off and both driver sees its respective pull up voltages, 1.8V or 3.3V.


How do you turn on a transistor?

Apply appropriate gate drive voltage. In practice, 1.8V is sufficient.

What mode operation is used on BJT and MOSFET in electronic system?

Transistor is generally used as on-off switches in electronic system, hence small signal model, miller effect, channel length modulation, load curve, etc. isn't not useful here since we are dealing with DC signals. We simply operate in the transistor as a switch.

What is difference between SCR and IGBT?

 SCR is slower than IGBT is turn of time but IGBT more efficient but less robust to over-current than that of SCR.

Summary & Conclusion

Transistors in electronic system design are mainly used as on-off switch to allow low cost circuit design. It's mainly purpose is for protections, delay, on and off switches, etc.

Further Practice

"Online Circuit Simulator:,