This article describes the role of the transistor base pull-down resistor and pull the resistor in the next two issues that should also be noted.
(1) To prevent the transistor from being affected by the noise signal and causing malfunction, make the transistor turn off more reliable! The base of the transistor can not be dangled. When the input signal is uncertain (such as when the input signal is in high impedance state) Can effectively ground.
In particular, when the GPIO is connected to the base, the internal of the GPIO is also in a power-on state when the IC of the GPIO is powered on. When the GPIO is powered on, the internal of the GPIO is unstable. Noise is easily generated and the malfunction is caused. This effect can be eliminated (if a sharp pulse occurs, the voltage is easily pulled down by the resistor due to the short time; if the high time is long, it can not be pulled low, that is, when it is normally high No effect)! But the resistance can not be too small, affect the leakage current! (Too small there will be a larger current flowing into the ground by the resistor)
(2) When the transistor switch, the ON and OFF time as short as possible, in order to prevent the OFF time, due to residual charge in the transistor caused by time lag, add a R between B, E play a role in the discharge. High-frequency, deep saturation, special attention. (secondary)
3) The base of the transistor plus resistor is mainly to set a bias voltage, so that there will be no distortion of the signal (this is very important when the input signal has AC: such as when the temperature rises, Ic will increase, resulting in Ie will Increases, then the voltage drop across Re also increases, and Vbe = Vb-IeRe, while Vb is essentially held down by the pull-down resistor, thus reducing Vbe. Of course this reduction is very good for 0.7v The small one is analyzed microscopically, and the decrease of Vbe makes Ib smaller, which results in the decrease of Ic, so that Ic is almost constant, which is also the principle of feedback control.
But also to prevent the input current is too large, add a resistor can be part of the current, so that it will not allow large current directly into the transistor damage it as for the discharge, generally used in the MOS tube, the transistor is not a big problem .
4) If the transistor is not connected to the pull-down resistor, the bias voltage can not be set. This will result in crossover distortion of the input signal, and excessive input current will cause large current to directly flow into the transistor and cause damage. Transistor When we always think it is always the internal diode effect, but this is the wrong understanding, it should be corrected. And MOS tube also need a bias voltage, and the pull-down resistor can play such a role, We generally call it GATE bias. Since the three internal MOS transistors are insulated from each other, so naturally there will be a capacitive effect, when the signal disappears when the internal equivalent capacitance can be discharged through the pull-down resistor. Or it will make a logical mistake.
Pull down the resistor should also pay attention to:
1 pull-down resistor can not be too large, otherwise it will cause the current into the base too small.
2 If it is a high-speed switching signal, try to connect a capacitor on the pull-down resistor to improve high-speed performance
Jilin Hongda Trading Co., Ltd , https://www.xuanyitruckpart.com