Transistor working principle diagram

good stuff.

How transistors work

Transistor is a current amplification device, there are three poles, respectively, called Collector C, base B, emission pole E. Divided into NPN and PnP two kinds. We only use the NPN transistor's common emitter amplification circuit as an example to illustrate the fundamentals of the transistor amplification circuit.

 

first, the current amplification

The following analysis is only for NPN-type silicon transistors. As shown above, the current from base B to emitter E is called the base current IB, and the current from the collector C to the emitter E is called the Collector current Ic. Both currents are in the direction of the emitter, so an arrow is used on the emitter e to indicate the direction of the current. The amplification of the transistor is: The Collector current is controlled by the base current (assuming that the power supply to the collector is large enough current), and the base current is very small changes, will cause a large collector current changes, and change to meet a certain proportion of the relationship: the change of the collector current is the base current change of β times, That is, the current change is amplified by the β-fold, so we call β the magnification of the transistor (β is generally far greater than 1, for example, dozens of, hundreds of). If we add a small change of signal between the base and the emitter, this will cause the base current IB to change, and the IB changes are amplified, resulting in large changes in the IC. If the collector current IC is flowing through a resistor R, then u=r*i can be calculated according to the voltage calculation formula, and the voltage on this resistor will vary greatly.  We take the voltage out of the resistor and we get the amplified voltage signal.

second, bias circuit

transistor in the actual amplification circuit used, but also need to add the appropriate bias circuit. There are several reasons for this. The first is the nonlinearity of the transistor be knot (the equivalent of a diode), and the base current must be large to a certain extent before it can be produced (for a silicon tube, 0.7V is usually taken). When the voltage between the base and the emitter is less than 0.7V, the base current can be considered to be 0. But the actual amplification signal is often much smaller than the 0.7V, if not biased, so small signal is not enough to cause a change in the base current (because less than 0.7V, the base current is 0). If we precede the base of the transistor with a suitable current (called bias Current, which is used to provide the current, so it is called the base bias resistor), then when a small signal is superimposed with this bias current, a small signal can cause a change in the base current, When the base current changes, it is amplified and output on the collector emitter. Another reason is the requirement of the output signal range, if there is no bias, then only the increased signal amplification, and the reduction of the signal is not valid (because there is no bias multiset electrode current of 0, can not be reduced). Coupled with a bias, the collector has a certain current, when the input base current becomes an hour, the collector current can be reduced, when the input base current increases, the collector current increases.  This reduces the signal and the increased signal can be amplified.

Third, switch action

Let's talk about the saturation of transistors. As shown above, because of the resistance RC limit (RC is a fixed value, then the maximum current is U/RC, where u is the supply voltage), the collector current can not be indefinitely increased. When the base current increases, the transistor becomes saturated when the collector current does not continue to increase. The general criterion for determining whether the transistor is saturated is: ib*β〉ic. After entering saturation, the voltage between the collector and emitter of the transistor will be very small, which can be understood as a switch closed. So we can use the transistor as a switch: when the base current is 0 o'clock, the transistor collector current is 0 (this is called the transistor cutoff), which is equivalent to the switch disconnection; When the base current is so large that the transistor is saturated, the switch is closed.  If the transistor mainly works in the cutoff and saturation, then such a transistor we generally call it a switch tube.

Iv. Working Status

If we change the resistor RC to a light bulb in the above figure, then when the base current is 0 o'clock, the collector current is 0 and the bulb is off. If the base current is large (greater than the current through the bulb divided by the transistor's magnification β), the transistor is saturated, equivalent to the switch closed, the light bulb is on. Since the control current only needs to be larger than the beta fraction of the bulb current, a small current can be used to control a large current.  If the base current increases slowly from 0, the brightness of the bulb increases as well (before the transistor is saturated).    For PNP transistors, the analytical method is similar, and the difference is that the current direction is exactly the opposite of NPN, so the arrow on the top of the emitter is reversed-turning into a forward direction.

test transistor of the formula transistor tube type and pin identification is an electronic technology beginner's basic skills, in order to help readers quickly grasp the method of testing, the author summed up four formulas: "Three upside down, find the base, PN Junction, fixed tube type, straight arrow, deflection large;   "Let's explain it in a sentence." One or three upside down, find the base you know, transistor is a semiconductor device with two PN junctions.  According to the two PN junction connection method, can be divided into NPN type and PNP type two different types of transistors, Figure 1 is their circuit symbol and equivalent circuits. Test transistor to use the Avometer ohm block, and select the Rx100 or rx1k stop. Figure 2 shows the equivalent circuit of the avometer ohm block.  The figure shows that the red pen is connected to the negative of the battery in the table, and the black pen is connected to the positive of the battery in the watch. It is assumed that we do not know whether the transistor being tested is NPN or PNP, and it is not clear what each pin is. The first step in the test is to determine which pin is the base pole. At this point, we take two electrodes (such as the two electrodes are 1, 2), with Avometer two pens to measure its positive and reverse resistance, observe the deflection angle of the needle, and then take 1, 32 electrodes and 2, 32 electrodes, respectively, the reverse measurement of their positive and negative resistance, to observe the angle of the needle deflection.  In these three reversal measurements, there are bound to be close to two measurement results: the reversal of the measurement of the hands of a large deflection, a small deflection; the rest of the time must be reversed before and after the measurement of the pointer deflection angle is very small, this time not measured that the foot is the base we are looking for (see Figure 1, Figure 2 is not difficult

 

second, PN Junction, fixed tube type

after finding the base of the transistor, we can determine the conductive type of the tube based on the direction of the PN junction between the base and the other two electrodes (Figure 1).   The multimeter of the black pen contact the base, the Red pen contact with the other two electrodes in any of the electrodes, if the table head pointer deflection angle is very large, then the test transistor is an NPN type tube; If the head pointer deflection angle is very small, then the tube is the PNP type.  Three, Shun arrow, deflection large to find the base B, the other two electrodes which is the collector C, which is the emitter e? Then we can use the method of measuring the penetration current ICEO to determine the collector C and emitter E. (1) For NPN transistor, the measuring circuit of the penetrating current is shown in Figure 3. According to this principle, with avometer black, red pen to measure the polarity between the positive and reverse resistance rce and Rec, although the two measurements of the multimeter pointer deflection angle is very small, but carefully observed, there will always be a slightly larger deflection angle, the current flow must be: Black-→c pole →b pole →e pole → Red pen,  The current flow is exactly the same as the arrows in the transistor symbol ("Straight arrow"), so at this point the black-out pen must be collector C, and the red pen must be the emitter E. (2) for the PNP type transistor, the principle is similar to the NPN type, its current flow must be: Black →e pole →b →c pole → Red pen, its current flow is also the same as the arrow in the transistor symbol, so at this time the black pen must be the emitter E, the red pen must be connected to the collector C (see Figure 1,   Figure 3). Four, can not be measured, if the moving mouth in the "straight Arrow, deflection large" measurement process, if the reversal of the two times before and after the measurement of the pointer deflection is too small to distinguish, it is necessary to "move the mouth." The specific method is: In the "straight Arrow, deflection large" two measurements, with two hands respectively pinch two pens and the combination of the pin, with the mouth containing (or with the tongue to resist) The base electrode B, still use "shun arrow, deflection large" method can be divided into separate collector C and emitter E. The human body plays the role of DC bias resistance, the purpose is to make the effect more obvious. Anyway, transistor .