How do bjts work

Remember that a typical silicon diode needs about 0. So, with less than 0. If we were to apply a negative voltage to the NPN BJT in figure 4 the situation worsens; the p-n junction between the base and emitter widens. Applying a voltage of at least 0. As the voltage increases the depletion zone narrows and electrons head toward the positive base.

Remember that the emitter is heavily doped and contains an excess of free electrons. Increasing the base voltage increases this effect. During this time the BC diode is actually reversed biased, but the electric field set up by the collector supply voltage pulls the electrons through. It is this that makes the BJT act as a variable resistor; the wider the depletion region of the BC diode, the more resistance the device offers.

Figure 5: how a BJT works on a low-level. Note the picture uses electron flow rather than conventional current flow. The small white circles in the base are holes.

Some electrons will combine with the holes in the base. The only way to get rid of the charge is to draw electrons from the base.

So, to maintain collector current all electrons that fall into holes need to be drained. This is why the base current controls the collector current.

Before we go any further, it may be useful to define some important terms concerning the operation of a bipolar junction transistor. Saturation refers to a region of operation where maximum collector current flows and the BJT acts like a closed switch from collector to emitter. In this mode the BC junction forward biases and maximum current flows.

The collector current cannot increase any more, even with an increase in base current. Cutoff refers to a region of operation where the BJT acts like an open switch, though a very small leakage current which we can usually neglect still flows. Finally, the active mode or active region describes operation in which the BE junction is forward biased, and the BC junction is revered biased.

In this mode, the BJT is equivalent to a variable resistor between the emitter and collector. If we want to use a bipolar junction transistor for amplification, we need to operate it in this mode or region.

Figure 6 graphically depicts these modes of operation with a family of collector characteristic curves. The cutoff region lies along the horizontal axis. The saturation region is the area to the left of the near-vertical line. The straight, horizontal lines represent the active region. Equation 1 gives the total current flowing in the emitter of a BJT. Since the base current I B is very small compared to the collector current I C , it is usually neglected and equation 2 suffices for most applications.

We already know that a small base current causes a larger collector current to flow. Beta is also sometimes referred to as h FE. Every single BJT has its own unique beta value ranging from 20 to Equation 3 gives the formula for current gain as it relates to total beta and total collector current.

First, the BJT needs to be in the active region of operation for it to apply. For this to be the case there are two rules that we need to heed.

Rule 1: The voltage at the collector must be at least a few tenths of a volt higher than the voltage at the emitter. Rule 2: There is a voltage drop of about 0. Using an NPN transistor as an example, this means that the base voltage must be at least 0. Semiconductors 2. Diodes 3. Bipolar Junction Transistors 4. Field Effect Transistors 5. Opto-Coupled Devices 6. Transistor Faults. Module 3. Fig 3. This is one of the most important parameters when doing a BJT design, as it tells you how much the output signal will be amplified.

If we take the 2-diode model for the BJT, we have up to 4 different possibilities depending on the polarity of each junction. These are known as the 4 transistor operation zones. Your email address will not be published.

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