Question One: The Current Limiting Problem of Transistors

As described in the title: In the current limiting circuit shown below, I would like to ask how each transistor works and how it limits the current when the input current changes.

Note: A transistor is a semiconductor device composed of three differently doped semiconductor materials, usually n-type, p-type, and n-type. Its structure can be divided into three parts: emitter, base, and collector. Among them, the emitter and collector are n-type semiconductors, and the base is a p-type semiconductor, forming two pn junctions. When the transistor is working normally, the pn junction between the emitter and base is in a forward biased state, while the pn junction between the base and collector is in a reverse biased state.

The application environment of transistors includes the following aspects:

Operating Temperature: The operating temperature range of transistors is usually between -55°C and +150°C. In practical applications, it is necessary to select the appropriate transistor according to the specific working environment and requirements.

Operating Voltage: The operating voltage of transistors is usually between tens of volts and hundreds of volts. In practical applications, it is necessary to select the appropriate transistor according to the requirements of the circuit.

Operating Frequency: The operating frequency range of transistors is usually between several kilohertz and several gigahertz. In high-frequency applications, transistors with high-frequency characteristics need to be selected.

Operating Current: The operating current of transistors is usually between several milliamperes and several amperes. In practical applications, it is necessary to select the appropriate transistor according to the requirements of the circuit.

Transistors are widely used in electronic circuits for amplification, switching, voltage regulation, oscillation, and other aspects. In amplification circuits, transistors can amplify small signals into large signals, thereby achieving amplification functions. In switching circuits, transistors can control the on-off state of the circuit, realizing the control function of the circuit. In voltage regulation circuits, transistors can stabilize the voltage to ensure the stability of the circuit. In oscillation circuits, transistors can realize the oscillation function of the circuit, thereby generating and processing signals.

Question Two: Is MOSFET Amplification the Same as Transistor Amplification?

Transistor amplification is easy to understand, where the input current is amplified, and the waveform phase remains unchanged, with the peak-to-peak value increasing.

However, I do not understand MOSFET amplification. Since there is no current flowing into the gate, what does it amplify?

It seems that the MOSFET is just a switch controlled by the gate voltage, or it is controlled by a variable resistor.

Previous Answers:

Previous Answer One: For ARM and microcontrollers, the IO is generally 3.3V. If you are controlling 12V or 24V without a transistor, a single MOSFET won't work and might damage the CPU. Adding a transistor will solve this. Also, when using a P-type MOSFET as a power switch, if there is no transistor OC gate, pay attention to a few things:

• The gate control voltage should not be lower than the controlled power supply. The P-type MOSFET Vgs >= Vds to turn off. If you use 1.8V to directly control the P-type MOSFET switch at 3.3V, there is usually a problem of not turning off.

• Before giving power to the IC, the pin is not necessarily high impedance. The controlled power supply may leak into the IC through the control pin, resulting in minor functional abnormalities or, in severe cases, burning. This problem has occurred in many cases.

• The gate control signal usually comes from a microcontroller (MCU). The pin state is uncertain before reset is completed. The moment the system is powered on may cause the power to briefly turn on, which can easily cause system misoperation or functional abnormalities, such as flashing indicator lights or screen backlight, motor rotation, etc.

Previous Answer Two: First, if the load is high and the selected transistor power is too small, only 2 watts, consider changing to a higher power rating. Second, the MOSFET gate drive voltage and current are too small, consider re-optimizing the circuit. Third, the MOSFET switching speed is slow, leading to excessive switching loss and temperature rise, causing the power of the transistor to further decrease after the temperature rises.