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Introduction
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
Background
Common Types of BJT and FET Transistors
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
Let's Look at the IV Characterstics for each type of transistor
Explanation of the current-voltage (IV) curves for each circuit:
Explanation of the current-voltage (IV) curves for each circuit:
PN Diode: The IV curve for a PN diode shows the relationship between the voltage applied across the diode (V) and the resulting current flowing through it (I). The curve indicates that when the diode is forward biased (positive voltage applied across the PN junction), the current increases exponentially with the voltage. However, when the diode is reverse biased (negative voltage applied across the PN junction), the current is essentially zero. This characteristic behavior of the PN diode makes it useful for applications such as voltage clamping and reverse voltage protection.
Bipolar Junction Transistor (BJT): The IV curve for a BJT represents the relationship between the base-emitter voltage (Vbe) and the resulting collector current (Ic). The curve shows that when a positive voltage is applied between the base and emitter, the base-emitter junction becomes forward biased, allowing current to flow from the collector to the emitter. The collector current is typically a fixed multiplier (beta) of the base current. The BJT operates as a current-controlled amplifier, with the current gain determined by the transistor's characteristics.
MOSFET: The IV curve for a MOSFET illustrates the relationship between the gate-source voltage (Vgs) and the resulting drain-source current (Ids). For an NMOS (N-channel MOSFET), when the gate-source voltage is greater than the threshold voltage (Vth), the MOSFET turns on, allowing current to flow from the drain to the source. The drain-source current increases linearly with the gate-source voltage. A similar behavior occurs for a PMOS (P-channel MOSFET), but with opposite voltage polarities.
Junction Field Transistor (JFET): The IV curve for a JFET represents the relationship between the gate-source voltage (Vgs) and the resulting drain-source current (Ids). The curve shows that when a positive voltage is applied between the gate and source (for P-type JFET), or a negative voltage is applied (for N-type JFET), the JFET turns on, allowing current to flow from the drain to the source. The drain-source current decreases linearly as the gate-source voltage is varied.
Silicon Controlled Rectifier (SCR): The IV curve for an SCR shows the relationship between the gate voltage (Vg) and the resulting current flow. The SCR acts as a controlled rectifier, allowing current flow only when a positive gate voltage is applied. When the gate voltage exceeds a certain threshold, the SCR turns on and conducts current. Below the threshold, the SCR remains in a non-conducting state.
Insulated Gate Bipolar Transistor (IGBT): The IV curve for an IGBT illustrates the relationship between the gate-emitter voltage (Vge) and the resulting collector current (Ic). The curve shows that when a positive voltage is applied between the gate and emitter, the IGBT turns on, allowing current to flow from the collector to the emitter. The collector current increases as the gate-emitter voltage increases.
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
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.
Q&A
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
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
Further Practice
"Online Circuit Simulator:, https://www.falstad.com/circuit/
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