Operating principle of Transistor
The transistor is a current amplifier, which has three poles, namely the collector C, the Base B, and the emission pole E. It can be divided into two types. The basic principle of the transistor amplifier circuit is described in this example.
I. Current Amplification
The following analysis is only applicable to the silicon transistor. As shown in, we call the current from base B to the emission pole e the base current IB, and the current from the collector C to the emission pole e is called the collector current IC. The direction of the two currents is the outbound emission pole, so an arrow is used on the emission pole e to indicate the direction of the current. 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 to the Collector), and the base current changes very little, this will cause a large change in the collector current, and the changes meet a certain proportional relationship: the variation of the collector current is Beta times of the conversion of the base current, that is to say, the current change is amplified by beta, so we call beta the magnification of the transistor (beta is generally much larger than 1, such as dozens or hundreds ). If we add a small signal of change to the gap between the base pole and the transmit pole, this will cause the change of the base pole current Ib. The change of IB is amplified, resulting in a great change of IC. If the collector current IC flows through a resistor R, the voltage on the resistor changes greatly according to the formula u = r * I. We can extract the voltage from the resistor to obtain the enlarged voltage signal.
Ii. bias circuit
When the transistor is used in the actual amplification circuit, a suitable bias circuit is also required. There are several reasons for this. First, because of the non-linearity (equivalent to a diode) of the be junction of the transistor, the base current must be generated after the input voltage reaches a certain level (for silicon tubes, usually take 0.7 V ). When the voltage between the base pole and the launch pole is less than 0.7v, the base pole current can be considered as 0. However, in reality, the signal to be amplified is often far smaller than 0.7v. without bias, such a small signal will not be enough to change the baseline current (because when the signal is smaller than 0.7v, the base current is 0 ). If we add a suitable current to the base pole of the transistor in advance (called the offset current, the resistance Rb is used to provide the current, so it is called the base offset resistance ), when a small signal is combined with the offset current, the small signal will lead to changes in the base current, and the changes in the base current will be amplified and output on the collector. Another reason is the requirement of the output signal range. If there is no offset, only the increased signal is amplified, and the reduced signal is ineffective (because the collector current is 0 when there is no offset, ). When the input base current changes to an hour, the collector current can be reduced. When the input base current increases, the collector current increases. In this way, the reduced signal and the increased signal can be enlarged.
Iii. Switch Functions
Next we will talk about the saturation of the transistor. As shown in the figure above, the maximum current is u/RC, where U is the power supply voltage, because it is limited by the resistance RC (RC is a fixed value ), the collector current cannot be infinitely increased. When the base current increases and the collector current cannot continue to increase, the transistor enters the saturation state. The general criterion for determining whether the transistor is saturated is IB * Beta> IC. After entering the saturation state, the voltage between the collector and the transmitting pole of the transistor will be very small. It can be understood that a switch is closed. In this way, we can use a transistor as a switch: when the base current is 0, the collector current of the transistor is 0 (this is called the end of the transistor), which is equivalent to disconnecting the switch; when the base current is large, when the transistor is saturated, the switch is closed. If the transistor mainly works in the cutoff and saturation states, we generally call it a switch.
Iv. Working Status
If we replace the RC resistor with a light bulb in the above figure, when the base current is 0, the collector current is 0, and the light bulb is off. If the base current is large (greater than the current flowing through the light bulb divided by the amplification factor beta of the transistor), the transistor will be saturated, which is equivalent to closing the switch and then the light bulb will be on. Since the control current only needs to be larger than the beta fraction of the light bulb current, a small current can be used to control a large current of the current. If the base current increases slowly from 0, the brightness of the light bulb will also increase (before the transistor is saturated ).
How does a transistor work?
