Transistors application in consumer electronics

Created: April/2020Last Updated: 5/26/2020

Introduction

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.

Background

Common Types

PN Diode

A diode is made of P and N junction.
Conduction starts when applied voltage is forward biased (i.e a sufficient positive voltage across PN junction)
Conduction stops when applied voltage is removed
Experience breakdown when large applied voltage id reverse biased (i.e a large negative voltage across PN junction)
- A commonly breakdown is called avalanche breakdown
Application
- Voltage clamp protection (zener diode)
- Reverse voltage protection (diode)
- Multiple power source (diode oring)
- Light emitting diode (LED)

Bipolar Junction Transistor (BJT)

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

NPN
- Current flows from collector to emitter
- Current controlled amplifier
  - Collector current conducts when the base current conducts
  - Collector current is a fixed multiplier (beta) of the base current
  - Vbe base to emitter voltage) is a diode constructing of a PN junction.
  - Base current conducts when Vbe is greater than PN junction's forward voltage
    - In practice emitter is tied to ground and pulling the base voltage high turns on the transistor, allowing current conduction between collector and emitter.
      - Note: In electronic system design, discrete PNP is operated in digital mode (i.e in the saturation region) where transistor is used as a on-off switch.
- Current flows from emitter to collector
PNP
- Current flows from emitter to collector
- Current controlled amplifier
  - Collector current conducts when the base current conducts
    - Collector current is a fixed multiplier (beta) of base current
    - Veb ( emitter to base base) is a diode constructing of a PN junction.
  - Base current conducts when Veb is greater than PN junction's forward voltage
    - In practice emitter is tied to a voltage rail and pulling base voltage low turns on the transistor, allowing current conduction between emitter and collector.
      - Note: In electronic system design, discrete NPN is operated in digital mode (i.e in the saturation region) where transistor is used as a on-off switch.

Metal Oxide Field Transistor (MOSFET)

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

NMOS
- Current flows from drain to source
- Voltage controlled amplifier
  - Drain to source current is controlled by gate to source voltage (VGS)
  - Transistor is turned on when VGS is greater that Vth (threshold voltage)
    - In practice source of NMOS is connected to ground, and fully turning on the transistor requires around pulling gate voltage high to be at least 2x Vth.
      - Note: In electronic system design, discrete NMOS is operated in digital mode (i.e in the linear region) where transistor is used as a on-off switch.
PMOS
- Current flows from source to drain
- Voltage controlled amplifier
  - Source to drain current is controlled by gate to source voltage (VGS)
  - Transistor is turned on when VSG is greater that Vth (threshold voltage)
    - In practice source of PMOS is connected to VDD, and fully turning on the transistor requires pulling gate voltage low to be around at least 2x Vth.
      - Note: In electronic system design, discrete PMOS is operated in digital mode (i.e in the linear region) where transistor is used as a on-off switch.

Junction Field Transistor (JFET)

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

Normally on without any voltage differential between gate and source
Voltage controlled amplifier

two types
- P
  - Turning P type JFET off requires applying positive VGS
- N
  - Turning N type JFET off requires applying negative VGS

Power Transistor

Silicon controlled rectifier (SCR)

Three terminal device, anode, gate, and cathode.

It's consisting of 3 PN junctions (PNPN)

Turning on the transistor requires applying a positive gate voltage.

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)

lower switch on time and high efficiency

Application

high frequency and high current devices such as variable frequency drive for electrical motor

Practical Design

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.

Q&A

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

Diode property can be used for direction control, voltage protection, power oring, and light emitting (LED)
BJT is current controlled amplifier. A biased base current is needed to turn on the transistor.
MOSFET is voltage controlled amplifier. A biased gate to source voltage is needed to turn on the transistor.
Power transistors, SCR and IGBT, are using in high current power applications such as inverter and frequency drive for motor control.
Reverse voltage protection circuit, Over voltage protection, soft start, load shift, and level shifter are common discrete circuit designs built using BJTs, MOSFETs, and diodes.

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:, https://www.falstad.com/circuit/