The BJTs and MOSFETs are two of the most commonly used components in electronics. BJTs are three-terminal devices used for switching and signal amplification. MOSFETs, while having four terminals (Drain, Source, Gate, and Body), often function as three-terminal devices in practical circuits, with the body typically tied to the Source. They are primarily used for switching applications but can also serve in amplification roles.
This article aims to provide a clear, practical comparison between BJTs and MOSFETs.
Before diving deeper, let’s review the basic concepts of BJTs and MOSFETs. Feel free to skip this section if you’re already familiar with them.
Table of Contents
BJT (Bipolar Junction Transistor)
A BJT is a three-terminal device (Emitter, Base, and Collector) that can be used as a switch or an amplifier. There are two main types of BJTs: NPN and PNP. An NPN transistor has a P-type semiconductor sandwiched between two N-type materials, while a PNP has an N-type semiconductor between two P-type materials.
BJTs are current-controlled devices, meaning that current must flow between the base and emitter to control the current between the collector and emitter. A small base current can control a much larger collector current. The ratio of collector current (IC) to base current (I_B) is the transistor’s current gain, known as beta (β).
BJTs are widely used in applications like audio amplifiers, power supplies, and digital circuits.
MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
A MOSFET has three main terminals: Gate, Drain, and Source, but can also include a fourth terminal, the Body (or Substrate), which is typically internally connected to the Source in most discrete MOSFETs.
MOSFETs are voltage-controlled devices. Applying a voltage to the gate allows control of a large current flow between the drain and the source. The VGS vs. ID curve provides insight into how varying gate-source voltage affects the drain current.
MOSFETs come in two main types: N-channel and P-channel, and each type can be further classified into enhancement mode and depletion mode:
- Enhancement mode: The MOSFET is normally off and requires a positive (N-channel) or negative (P-channel) gate-source voltage to turn on.
- Depletion mode: The MOSFET is normally on when the gate-source voltage is 0V and requires a negative (N-channel) or positive (P-channel) voltage to turn off.
MOSFETs generally offer advantages over BJTs in switching applications due to faster switching speeds and lower power consumption.
Differences between BJT and MOSFET
The following table summarises some of the major differences between BJT and MOSFET.
| Parameter | BJT | MOSFET |
|---|---|---|
| Full Form | BJT stands for Bipolar Junction Transistor. | MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. |
| Type | BJTs are divided into two types: NPN and PNP. | MOSFETs are divided into two types: P-Channel and N-Channel, with both enhancement and depletion versions. |
| Structure | Consists of emitter, base, and collector. | Consists of a metal gate, insulating oxide layer, and semiconductor channel. |
| Operation | Used for amplification and switching. | Used for amplification, switching, and power regulation in circuits. |
| Power Consumption | BJTs consume more power due to continuous base current requirements. | MOSFETs consume less power as they require minimal current at the gate during operation due to low RDS(on). |
| Polarity | BJT is a bipolar device, using both electrons and holes. | MOSFET is a unipolar device, using either electrons or holes. |
| Noise Immunity | Lower noise immunity. | Higher noise immunity. |
| Input Impedance | BJTs have low input impedance. | MOSFETs have very high input impedance. |
| Switching Speed | BJTs have a lower switching speed compared to MOSFETs. | MOSFETs have relatively high switching speeds. |
| Input Control | BJT is a current-controlled device. | MOSFET is a voltage-controlled device. |
| Voltage | BJTs are available with a range of voltage ratings, typically lower than high-voltage MOSFETs. | MOSFETs are available with a wide range of voltage ratings, often higher than typical BJTs. |
| Current | Moderate current-handling capability, depending on type. | High current-handling capability with low power loss. |
| Charge Carriers | In BJTs, both electrons and holes act as charge carriers. | In MOSFETs, either electrons or holes act as charge carriers depending on the type of channel (N-channel or P-channel). |
| Cost | Generally lower manufacturing cost. | Typically higher manufacturing cost, though modern advancements have reduced this difference. |
Using BJTs and MOSFETs in Practical Applications
BJTs and MOSFETs serve a crucial role in any modern-day application, and in this section of the article, we will try to understand the workings of those with some practical examples. At the end of the article, the reader will have a clear idea about BJTs and MOSFETs and how they affect modern-day electronics.
BJTs and MOSFETs play crucial roles in modern electronic circuits. This section provides practical examples of their use, helping readers understand their impact on contemporary electronics.
Applications of BJT as a switch
BJTs are commonly used in switching applications. Below are practical examples of BJTs configured as switches:
Primary Switch in SMPS: BJTs are sometimes used as primary switches in self-oscillating circuits within power supplies, particularly in the range of 10 to 25 watts. While BJTs can be used for such applications, MOSFETs are more commonly favored today due to their higher efficiency and faster switching speeds.
Basic Configuration: When an NPN transistor is used as a switch, it turns on when a positive voltage is applied to the base, allowing current to flow from the collector to the emitter and through the load. For a PNP transistor, applying a negative voltage (or a base voltage lower than the emitter) turns it on, allowing current to flow from the emitter to the collector.
Relay Driver: One common use of a BJT is in relay driver circuits. A BJT can control the switching of a relay, which can then be used to power larger loads, such as motors or light bulbs. It’s important to add a flyback diode across the relay coil to protect the BJT from voltage spikes.
LED Driver: BJTs can be configured as LED drivers, capable of driving high-power LEDs. Using a PWM signal, the brightness of the LED can be adjusted by varying the duty cycle of the signal, controlling the average current through the LED.
Motor Driver: BJTs can be employed to control the RPM of motors by modulating the power supplied to the motor using PWM signals or a specially designed motor driver circuit.
High power LED driver circuit with BJT
In this example, we explore a high-power LED driver circuit. Suppose you need to drive a 12V, 5A LED using an ESP8266/ESP32 microcontroller. Which component would be best: a BJT or a MOSFET? And which would be the most cost-effective choice for your design?
In the circuit example, the BJT driver uses a TIP41C transistor, while the MOSFET driver employs an IRF540. While both components are common, the IRF540 has a gate threshold voltage (V_GS(th)) of around 4V, which means it won’t fully turn on when driven by a 3.3V logic signal from the ESP8266/ESP32. This results in the MOSFET remaining in a non-conductive state, and the LED will not light up.
Solution: To drive a MOSFET with a 3.3V logic signal, use a logic-level MOSFET like the IRLZ44N, which can fully turn on at lower gate voltages. This ensures that your circuit works correctly, but it may increase costs compared to using a standard BJT.
For a 5A load driver circuit like this, a BJT such as the TIP41C can be a cost-effective and simple choice. However, BJTs require more base current and might have higher power dissipation. If your design prioritizes high-speed switching, lower power loss, or higher efficiency, a logic-level MOSFET would be the better choice despite the slightly higher cost and potential circuit complexity.
Conclusion: Choose a BJT for cost-sensitive, simpler designs, especially when base current requirements can be met easily. For high-speed or more efficient designs, opt for a suitable logic-level MOSFET.
Single stage class-A amplifier circuit
The simplest form of an amplifier is a Class-A amplifier circuit. This type of circuit uses a single transistor in its output stage, with a load resistor connected between the collector of the transistor and the power supply (Vcc). When the transistor is conducting, it draws current through the load resistor, causing a voltage drop and resulting in an output signal that is inverted relative to the input.
The efficiency of a Class-A amplifier is quite low, typically around 10-30%, and in practice, it often falls closer to 10-20%. This inefficiency results in significant heat dissipation, requiring a large heat sink to cool the transistor.
Why use a Class-A amplifier? Despite its poor efficiency, a Class-A amplifier is valued for its high linearity and low distortion, making it ideal for high-fidelity audio applications where sound quality is paramount.
Improving Class-A Circuits with Darlington Configuration: To enhance the current-handling capability of a Class-A amplifier, a Darlington transistor configuration can be used. This setup replaces a single transistor with two transistors connected in a way that increases current gain and input impedance. However, it’s important to note that this does not improve the inherent efficiency limitations of a Class-A amplifier.
Example of a Practical Amplifier Circuit: A common example is a 100W audio amplifier circuit using a Darlington pair for the preamp stage to boost current before driving the power stage. In such circuits, the Darlington configuration ensures higher current gain, enabling the preamp to drive the final stage more effectively.
Differential Amplifier Stage: In more complex amplifier designs, such as a 100W MOSFET amplifier circuit, a differential amplifier (e.g., transistors Q1 and Q2) can be used in the input stage to provide initial signal amplification, followed by a Darlington pair (e.g., Q3 and Q4) in the preamp stage for further current amplification.
Most common application of MOSFETs with example
Previously, we discussed why BJTs may be preferred in some applications. Now, we will look at practical applications of MOSFETs.
For high-power loads, MOSFETs are often a better choice than BJTs due to several advantages:
- High input impedance: This reduces the loading on the driving circuit.
- Low on-state resistance: This leads to minimal voltage drop and reduced heat generation.
- High switching speed: MOSFETs can switch on and off rapidly, making them ideal for high-frequency applications.
- Low gate current requirements: Only a small amount of current is needed at the gate to turn on the MOSFET.
However, MOSFETs also have disadvantages, such as higher gate threshold voltages in non-logic-level types, which can make them incompatible with low-voltage logic signals from microcontrollers.
Example Application: Consider the N-channel and P-channel MOSFET driver circuits using the IRF540 (N-MOS) and IRF9540 (P-MOS). If you want to drive these MOSFETs with a 3.3V logic level from a microcontroller (e.g., an ESP8266 or ESP32), their high gate threshold voltage means they won’t fully turn on. This results in inadequate conduction and potential circuit failure.
Solution: One solution is to use a logic-level MOSFET that fully turns on at lower gate voltages (e.g., IRLZ44N). However, these MOSFETs can increase the Bill of Materials (BOM) cost. A more cost-effective approach is to use a BJT driver to boost the 3.3V signal to a higher voltage that can drive the gate of a standard MOSFET like the IRF540.
This configuration reduces BOM cost but has one drawback: the logic is inverted. This means when the input signal from the microcontroller is high, the MOSFET may turn off, and when the input is low, the MOSFET may turn on.
Conclusion: For cost-sensitive designs where logic-level MOSFETs are too expensive, using a BJT to drive the gate of a standard MOSFET can be a practical alternative. However, consider the logic inversion and potential circuit complexity when choosing this approach.
Application of MOSFET in an SMPS circuit
In this example, we look at a simplified block diagram of an SMPS circuit where a MOSFET is used as the primary switching element. MOSFETs are favored for their fast switching speed, low on-resistance, and ability to handle high voltages and currents. In an SMPS circuit, a MOSFET switches the high-voltage DC input to create a high-frequency AC signal. This high-frequency operation allows for a smaller transformer size, making the power supply more compact and efficient.
Why MOSFETs Are Preferred in SMPS: MOSFETs are often chosen over BJTs for SMPS designs due to their lower power dissipation and higher overall efficiency. Efficiency can be further improved by using a proper gate driver circuit, which ensures rapid switching, reduces switching losses, and simplifies circuit design.
Synchronous Rectification with MOSFETs: In many high-power applications, such as EV chargers and high-power SMPSs (e.g., 250V to 1000V), MOSFETs are used as replacements for output diodes in a technique called synchronous rectification. This method significantly reduces power losses associated with the voltage drop of traditional diodes, thereby improving the efficiency of the power supply.
Conclusion: Both MOSFETs and BJTs serve vital roles in electronic circuits, particularly for switching and amplification tasks. The choice between using a MOSFET or a BJT depends on the specific requirements of the application:
- MOSFETs are the preferred choice when high input impedance, low on-resistance, and fast switching are important.
- BJTs are advantageous when stability and linearity are critical, as they offer reliable performance in analog circuits.
Selecting the right component involves balancing factors such as power requirements, switching speed, cost, and complexity.
