In the selection of drive for microcontroller I/O ports, why are bipolar transistors generally chosen over MOSFETs?

Firstly, let's take a look at the principles and characteristics of these two devices.

Bipolar Transistor

A bipolar transistor is a type of transistor that has three terminals: the base, the emitter, and the collector.

The current flowing between the collector and the emitter is controlled by the current flowing into the base. The control circuit of a bipolar transistor is relatively simple, as it only requires a base current to control the collector current.

MOSFET

A MOSFET is a type of field-effect transistor that has three terminals: the source, the drain, and the gate.

The current flowing between the source and the drain is controlled by the voltage applied to the gate. However, compared to a bipolar transistor, the control circuit of a MOSFET is more complex, as it requires a gate-source voltage to control the drain current.

So why are bipolar transistors generally chosen over MOSFETs? We can analyze this based on the following four aspects (but not limited to):

Drive Capability: Bipolar transistors have a much higher drive capability than MOSFETs. The base drive voltage of a bipolar transistor only needs to be higher than the base-emitter voltage (usually 0.7V) to turn it on, while the gate drive voltage of a MOSFET must be higher than the threshold voltage Vgs. In some scenarios where the load current is large, the drive capability of a MOSFET may be affected. Therefore, in applications requiring high drive capability, bipolar transistors have the advantage.

Current Amplification Capability: A MOSFET is a voltage-controlled device that can convert an input voltage signal into a current signal. The amplification capability of a MOSFET depends on the length and width of its conducting channel, as well as the gate-source voltage. A bipolar transistor is a current amplification device that can amplify a small current into a larger one.

In an NPN bipolar transistor, if the base current is Ib, the collector current Ic can reach up to about 200 times Ib (assuming Ib is 1mA, then Ic can reach 200mA). This means that through a bipolar transistor, we can amplify the small current output from the microcontroller I/O port into a larger one to drive a larger load.

Cost Perspective: The manufacturing process of a bipolar transistor is relatively simple and low-cost. In contrast, the manufacturing process of a MOSFET is more complex and expensive. Therefore, in projects with limited budgets, a bipolar transistor would be a more ideal choice!

Application Scenarios: Since a bipolar transistor is a current-controlled device with high input resistance and low output resistance, it is suitable for use in low-impedance loads (such as LEDs, resistor loads, etc.). Additionally, bipolar transistors have a large current amplification capability, making them suitable for driving large current loads. However, bipolar transistors have relatively high power consumption and are not suitable for use in low-power scenarios.

MOSFETs are voltage-controlled devices with low input resistance and high output resistance, making them suitable for use in high-impedance loads (such as inductors, capacitor loads, etc.). Additionally, MOSFETs have low power consumption, making them suitable for low-power scenarios. However, MOSFETs have a small current amplification capability and are not suitable for driving large current loads.

Recommendation: The choice between a bipolar transistor and a MOSFET for microcontroller I/O port drive mainly depends on the load characteristics, drive capability requirements, and circuit operating voltage. When selecting a drive circuit, it is necessary to choose the appropriate driving device based on the load characteristics and circuit operating voltage.