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Triode Working Principle

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Guidance: Triode, also known as semiconductor triode, bipolar transistor and crystal triode, is a kind of current-controlled semiconductor device. Its function is to amplify weak signals into larger amplitude electrical signals, and also to be used as contactless switches.  

What is Triode

Triode, also known as semiconductor triode, bipolar transistor and crystal triode, is a kind of current-controlled semiconductor device. Its function is to amplify weak signals into larger amplitude electrical signals, and also to be used as contactless switches. Crystal triode is one of the basic components of semiconductor. It has the function of current amplification and is the core component of electronic circuit. The triode is made of two PN junctions which are very close to each other on a semiconductor substrate. The two PN junctions divide the whole semiconductor into three parts, the middle part is the base area, the two sides are the emitter area and collector area, and the arrangement is PNP and NPN.

Common transistors are 9012, s8550, 9013 and s8050. The main function of transistors in the application circuit of single-chip computer is switching. Among them, 9012 and 8550 are PNP transistors, which can be used in general. Among them, 9013 and 8050 are NPN transistors, which can be used in general.

Distinguishing pin: the transistor is facing itself. The pins are e b c from left to right. One end of the arrow in the schematic diagram is e, and the other is c, which is connected with the resistance. The arrow points inward to PNP (9012 or 8550) and outward to NPN (9013 or 8050).

How to distinguish the type of transistor and the three electrodes e (emitter), B (base) and C (collector)

  • (1) Judging the type of base B and transistor by pointer multimeter: Put the ohm of multimeter at “R&TImes; 100” or “R&TImes; lk”, assume that one of the poles of triode is “base”, and connect the black-watch pen to the hypothetical base, then connect the red-watch pen to the other two poles, if the resistance values measured twice are all equal. If the base is very small (or about several hundred to several thousand euros), then the hypothesis is correct, and the transistor tested is NPN type; if the resistance values measured twice are very large (about several thousand to tens of thousands of euros), the hypothesis is correct, and the transistor tested is PNP type. If the resistance values measured twice are large or small, then the original assumption of the base is wrong, then we must re-assume that the other electrode is the “base” and repeat the above test.
  • (2) Judgment of collector C and emitter e: Still block the ohm of pointer multimeter at “R&TImes; 100” or “R&TImes; 1k”, take NPN tube as an example, connect the black-watch pen to the hypothetical collector c, connect the red-watch pen to the hypothetical emitter e, and hold the B and C poles by hand (can not make the B and C contact directly) through the human body, Similar to the bias resistance between B and C, read out the resistance value shown in the head of the meter, and then retest the two pens back-to-back. If the resistance value measured at the first time is smaller than that measured at the second time, the original hypothesis is valid, because the current through the multimeter is large and the bias is normal.
  • (3) Measuring the gear of the diode with digital multimeter can also detect the PN junction of the transistor. It is convenient to determine the quality and type of the transistor. However, it should be noted that, unlike the pointer multimeter, the digital multimeter red pen is the front end of the internal battery. Example: When the red pen is connected to the hypothetical base and the black pen is connected to the other two poles successively, if the table shows that the forward voltage drop of the through silicon tube is about 0.6V, the hypothetical base is correct, and the tested transistor is NPN type. Digital multimeter usually has a gear (hFE) to measure the magnification of transistor. When using it, first confirm the transistor type, and then insert the e, B and C pins of the transistor into the transistor jack corresponding to the digital multimeter surface plate respectively. The table shows the approximate value of hFE.

Working Principle of Triode

The triode is a current amplifier with three poles, called collector C, base B and emitter E. They are divided into NPN and NPP. We will illustrate the basic principle of the transistor amplifier circuit by taking the common emitter amplifier circuit of NPN transistor as an example.

Current amplification

The following analysis is only for NPN-type silicon triodes. As shown in the figure above, the current from base B to emitter E is called base current Ib, and the current from collector C to emitter E is called collector current Ic. Both directions of the currents flow out of the emitter, so an arrow is used on the emitter E to indicate the direction of the current. The amplification function of the triode is that the collector current is controlled by the base current (assuming that the power supply can supply enough current to the collector), and a small change in the base current will cause a great change in the collector current, and the change satisfies a certain proportional relationship: the change of the collector current is the base current. The beta-fold of the rheological change, that is, the current change is amplified beta-fold, so we call beta the magnification of the transistor (beta is generally much larger than 1, for example, tens or hundreds). If we add a variable small signal between the base and emitter, it will cause the change of base current Ib, which will be amplified and lead to a great change of Ic. If the collector current Ic flows through a resistor R, then the voltage on the resistor can be calculated according to the voltage calculation formula U=R*I, and the voltage on the resistor will change greatly. We take the voltage from this resistor and get the amplified voltage signal.

Bias Circuit

When the transistor is used in the actual amplifier circuit, a suitable bias circuit is needed. There are several reasons. First, because of the nonlinearity of the transistor BE junction (equivalent to a diode), the base current must be generated only after the input voltage is large enough (for silicon transistors, 0.7V is often used). When the voltage between the base and emitter is less than 0.7V, the base current can be considered as 0. But in practice, the signal to be amplified is often much smaller than 0.7V. Without bias, such a small signal is not enough to cause the change of the base current (because when the base current is less than 0.7V, the base current is 0). If we add a suitable current (called bias current) to the base of the transistor beforehand (the resistance Rb in the figure above is used to supply this current, so it is called base bias resistance), then when a small signal is superimposed on the base current, the small signal will cause the base current. Change, and the change of base current, will be amplified and output on the collector. Another reason is the requirement of output signal range. If there is no bias, only those increased signals will be amplified, but the reduced signals will not be valid (because the collector current is 0 when there is no bias, and can not be reduced any more). When the input base electrorheological current is small, the collector current can be reduced; when the input base current increases, the collector current increases. Both the reduced signal and the increased signal can be amplified.

Switching Function

Let’s talk about the saturation of the transistor. As shown in the figure above, the collector current cannot be increased indefinitely due to the limitation of resistance Rc (Rc is a fixed value, then the maximum current is U/Rc, where U is the power supply voltage). When the base current increases, the collector current can not continue to increase, the transistor will enter a saturated state. The general criterion for judging whether the transistor is saturated is Ib*beta_Ic. After entering the saturation state, 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 zero, the collector current of the transistor is 0 (this is called the triode cut-off), which is equivalent to the switch off; when the base current is so large that the transistor is saturated, it is equivalent to the switch closing. If the transistor works mainly in the cut-off and saturated state, then such a transistor is generally referred to as a switching transistor.

Operative Mode

If we change the resistance Rc into a bulb in the figure above, when the base current is zero, the collector current is zero and the bulb goes out. If the base current is large (greater than the current flowing through the bulb divided by the amplification factor beta of the transistor), the transistor will be saturated, equivalent to the closure of the switch, and the bulb will be on. Since the control current is only a little larger than the beta of the bulb current, a small current can be used to control the on-off of a large current. If the base current increases slowly from 0, the brightness of the bulb will also increase (before the transistor is unsaturated).


10 thoughts on “Triode Working Principle

  1. Detailed content, got it.

  2. Excellent

  3. Can triodes be usedas amplifiers?

    1. Of course it can.

  4. What is the contrast between the triode ( ohmic ) area and the immersion ( direct ) locale in mosfet ?? It’s befuddling since the two areas are straight however for what reason does mosfet function as a speaker in immersion not in triode ? Isn’t triode equivalent to the dynamic straight district in Bjt ? On the off chance that it isn’t, at that point what is the triode area and what is it utilized for ??

  5. This is presumably the most faltering inquiry of all the time here, however… I can’t understad lattice cathode “negative” polarization.

  6. Thank you for you content. I have learned it.

  7. Why is one transistor active and the other triode?

  8. Vcc = IC. Rc + UCE, as we all know, Vcc is a constant voltage source, IC increases, Rc voltage increases, in order to maintain Vcc unchanged, UCE has to be reduced, so I want to ask is the resistance between CE changed with it? If it doesn’t change, it’s not against Ohm’s law to keep Vcc unchanged.

    1. Between C and E of the transistor, you can understand it as an adjustable constant current source. In this case, the internal resistance of the constant current source varies with the voltage at both ends. (If the internal resistance of a constant current source is defined as the ratio of terminal voltage to current)

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