Understand the working process of the triode through animations
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Understand the working process of the triode through animations

Posted Date: 2024-01-22

The current amplification effect of the transistor should be regarded as a difficult part of the analog circuit. I would like to use these animations to briefly explain why the small current Ib can control the size of the large current Ic, and the principle of the amplification circuit.

The triode here is also called a bipolar transistor, and is used in analog amplifier circuits and simple digital logic circuits. There are collector c, base b, emitter e, and two PN junctions: collector junction and emitter junction.

The collector area is relatively large, the base thickness is thin and the carrier concentration is relatively low. The picture below is an NPN transistor:

When the emitter junction is forward biased, the charge distribution will change and the width of the emitter junction will become narrower; it is equivalent to opening a door from e to b for electrons. When the collector junction is reverse biased, the charge distribution will also change, and the collector junction will The width will become wider.

It is equivalent to opening the door that prevents electrons from escaping from the c pole, as shown in the animation below:

The b electrode will be connected to a large resistor RB to limit the size of the current Ib. The excess electrons that run to the b electrode have to pass through the collector junction to form the current Ic, as shown in the animation below:

If the base voltage doubles, the charge distribution will continue to change, the width of the emitter junction will become narrower, the door will become wider, and more electrons will run to the b pole. As shown in the animation below:

Since RB is a large resistance, Ib is still very small even if doubled, so more electrons will pass through the collector junction, causing Ic to double. As shown in the animation below:

Two DC power supplies can be merged together, plus the small signal ui and two capacitors, and the amplifier circuit is obtained, as shown in the figure below:

If the resistor size is appropriate, this amplifier circuit can amplify the small signal ui into a large signal uCE with opposite phase, as shown in the animation below:

Red is the input terminal, and changes in ui will affect UBE. Think of the emitter junction as a small resistor, and the red Q point will move along the black line, and then draw the image of iB; according to iC=βiB, draw the image of iC, The ordinate has changed from μA to mA; and the output terminal has UCE=UCC-ICRC. When UCC and RC remain unchanged, UCE and IC are inverted.

Finally, let’s talk about the shortcomings of these animations:

The trumpet-like transistor is not my original creation, but the metaphor of the water tank can easily cause people to misunderstand that IC is the largest, but in fact IE is the largest current.

The animation completely ignores the thermal motion speed of electrons, which is much greater than the drift speed of electrons under the action of voltage.

The animation does not reflect energy levels, energy bands, Fermi distribution, etc.

Review Editor: Huang Fei


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