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Working principle of diode triode

1, diode working principle:

Crystal diode is a pn junction formed by P-type semiconductor and N-type semiconductor, and a space charge layer is formed on both sides of its interface to construct a self-built electric field. When there is no applied voltage, the diffusion current caused by the carrier concentration difference between the two sides of the pn junction is equal to the drift current caused by the self-built electric field, and it is in an electric equilibrium state. When the external DC bias exists, the mutual suppression of the external electric field and the self-built electric field increases the diffusion current of carriers and causes the forward current. When there is an external reverse bias, the external electric field and the self-built electric field are further strengthened, and a reverse saturation current I0 is formed in a certain reverse voltage range, which is independent of the reverse bias value. When the applied reverse voltage is high enough, the electric field intensity in the space charge layer of pn junction reaches the critical value, which leads to carrier multiplication, a large number of electron-hole pairs and a large reverse breakdown current, which is called diode breakdown phenomenon. Reverse breakdown of pn junction can be divided into zener breakdown and avalanche breakdown.

2, triode working principle:

Triode is a current amplifier with three poles, namely collector C, base B and emitter E, which can be divided into NPN and PNP. We will take the emitter amplifier circuit of NPN transistor as an example to illustrate the basic principle of triode amplifier circuit.

First, current amplification.

The following analysis is only for NPN silicon triode. As shown in the above figure, we call the current flowing from base B to emitter E as base current IB; The current flowing from collector c to emitter e is called collector current Ic. Both currents flow from the emitter, so the direction of the current is indicated by an arrow on the emitter E. The amplification function of the transistor is that the collector current is controlled by the base current (assuming that the power supply can provide enough current for the collector), and a small change in the base current will cause a big change in the collector current. And the change satisfies a certain proportional relationship: the change of collector current is β times that of base current, that is, the current change is amplified by β times, so we call β the amplification factor of transistor (β is generally much greater than 1, for example, tens or hundreds). If we add a small change signal between the base and the emitter, it will cause the change of the base current Ib. After the change of Ib is amplified, the Ic changes greatly. If the collector current Ic flows through a resistor R, then according to the voltage calculation formula U=R*I, the voltage on this resistor will change greatly. We take out the voltage across the resistor and get an amplified voltage signal.

Second, the bias circuit

When triode is used in practical amplifier circuit, it is necessary to add appropriate bias circuit. There are several reasons. First of all, due to the nonlinearity of transistor BE junction (equivalent to a diode), the base current can only be generated after the input voltage reaches a certain level (usually 0.7V for silicon tube). When the voltage between the base and emitter is less than 0.7V, the base current can be considered as 0. However, in practice, the signal to be amplified is often much less than 0.7V. Without bias, such a small signal is not enough to cause the change of base current (because when it is less than 0.7V, the base current is all zero). If we add a suitable current to the base of the transistor in advance (called bias current, the resistor Rb in the above figure is used to provide this current, so it is called base bias resistance), then when a small signal is superimposed with the bias current, this small signal will lead to the change of base current, and this current will be amplified and output to the collector. Another reason is the requirement of output signal range. If there is no bias, only the increased signal will be amplified, but not the decreased signal (because the collector current is zero without bias, it cannot be reduced). With bias, the collector will have a certain current in advance. When the input base current increases, the collector current increases, so that both the reduced signal and the increased signal can be amplified.

Third, the switch function

Let's talk about the saturation of triode first. As shown in the above figure, due to the limitation of resistance Rc (Rc is a constant value, the maximum current is U/Rc, where U is the power supply voltage), the collector current cannot be increased indefinitely. When the collector current can't increase continuously with the increase of the base current, the triode enters the saturation state. The general criterion for judging whether a triode is saturated is IB * β > IC. After entering the saturation state, the voltage between the collector and emitter of the triode will be very small, which can be understood as the switch is closed. In this way, we can regard the transistor as a switch: when the base current is 0, the collector current of the transistor is 0 (this is called transistor off), which is equivalent to the switch off; When the base current is so large that the transistor is saturated, it is equivalent to closing the switch. If the transistor mainly works in the off and saturation state, then such a transistor is generally called a switch tube.

Fourth, the working state.

If we use the bulb in the above picture instead of the resistor Rc, then when the base current is 0, the collector current is 0 and the bulb goes out. If the base current is relatively large (greater than the current flowing through the bulb divided by the amplification factor β of the triode), the triode is saturated, which means that the switch is closed and the bulb is lit. Because the control current only needs to be slightly more than one-third of the bulb current, small current can be used to control the passage of large current.