# In-depth explanation of the role of transistors and MOS transistor pull-down resistors

Briefly explain the triode, if the triode is working in the saturation region (fully turned on), Rce≈0, Vce≈0.3V, and this triode is a current-type drive component, so generally a current-limiting resistor is connected to the base, which is generally smaller than It is equal to 10K, but why is a resistor pulled down at the base? for example. The following figure is a circuit diagram of the temperature switch controlling the motor.

Briefly explain the triode, if the triode is working in the saturation region (fully turned on), Rce≈0, Vce≈0.3V, and this 0.3V, we think it is directly grounded. Then you need to make Ib greater than or equal to 1mA, if Ib=1mA, Ic=100mA, its magnification β=100, and the transistor is fully turned on. As shown in the figure below, it is an NPN transistor. The basic knowledge of triode reference article: Four-sentence formula, fun with triode!

The triode is a current-type drive component, so generally a current-limiting resistor is connected to the base, generally less than or equal to 10K, but why is a resistor pulled down at the base? for example. The following figure is a circuit diagram of the temperature switch controlling the motor.

As shown in the figure, the temperature switch controls the rotation and stop of the motor. The temperature switch is equivalent to a key switch. A switch is connected in series on the B pole, and the N tube can be used as a switch tube. The motor in the picture is a DC brushed motor. As long as the positive pole is connected to 12V and the negative pole is grounded, the motor will start to rotate.

When the temperature switch is turned on, the current flowing through loop I is

The transistor CE is fully turned on, Vce? 0.3V. At this time, the voltage drop across the motor is close to 12V, and it can rotate, because the impedance of the transistor be is much smaller than the 2K resistor R2 after it is turned on, so most of the current flows through the transistor. ; When the temperature switch is off, there is no current in ib and no current in ic.

Because the temperature switch is turned off at the moment, the current on the triode ib and ic cannot drop to zero at once, but slowly drop to zero. This is inevitable in the manufacturing process. During this time, the triode is Working in the magnified area, Z is susceptible to interference. Therefore, it is necessary to connect a pull-down resistor R2. This resistor firstly provides a discharge circuit for the triode, and secondly, provides an energy dispersion path for point A.

How to understand the discharge circuit?

As shown in the figure below, the parasitic capacitance of the triode, the actual manufacturing model of the triode, there are capacitors C1, C2, and C3 between the triodes BE, BC, and CE. On the one hand, the existence of these three capacitors is something we don’t need, on the other hand, it is unavoidable to overcome in the process, which is an inevitable phenomenon in the manufacturing process. We call this kind of capacitance generally stray capacitance, or parasitic capacitance.

Due to the presence of capacitors, the triode is bound to have a delay. When there is no current in ib, capacitor C1 begins to discharge to form loop I. At this time, the voltage at point B drops from 0.7V to 0V. It works in the amplifying zone and Z is easily interfered. Add a resistor R2 to both ends of C1. Part of the electricity will be discharged from the resistor R2, and the smaller the resistance, the faster the capacitor discharges. Therefore, the resistor R2 provides a path for the capacitor to release the charge, which greatly shortens the time that the triode works in the amplifying area.

How to understand the provision of a decentralized pathway for energy?

Why is the resistance R2 providing an energy dispersion path for point A? As shown in Figure 2, when the temperature switch is turned off, point A is floating at this time, and the voltage at point A is uncertain, and it is in a high impedance state (impedance is infinite), which is prone to mis-conduction, and it is also susceptible to interference from the surrounding environment. For example, static electricity, lightning strikes, etc. damage the device YJ.

When lightning strikes, high-voltage static, etc. occur in the use environment, pull down a resistor at point A to connect to the ground, and most of the current will flow into the ground along the resistor, providing a dispersed path for energy. If this resistor is not connected, when a lightning strike occurs, because the impedance on the left side of point A is infinite, and the right side of point A is connected to a transistor, the impedance is very low compared to the left side, so the current will all run in the direction of low impedance and flow into the transistor, causing current If it is too large, the YJ of the device will be damaged.

Due to space limitations, regarding the basic knowledge of MOS transistors, move here: Basic knowledge of MOS transistors.

There are two functions of the pull-down resistor:

To prevent the electric charge from having no discharge circuit under the action of static electricity, which may easily cause electrostatic breakdown

When the MOS tube is working in the switching state, it is continuously charging and discharging the Cgs. When the power is turned off, a part of the charge may be stored in the Cgs, but there is no release circuit. The electric field of the MOS tube grid still exists and can be maintained for a long time. The condition of the conductive channel has not disappeared. At the next startup, under the action of the conductive channel, the MOS tube immediately generates an uncontrolled huge drain current Id, causing the MOS tube to burn out.