Correctly understand the amplification region, saturation region, and cutoff region of the transistor

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Correctly understand the amplification region, saturation region, and cutoff region of the transistor [Copy link]

Analog circuits are very important , and the application of transistors in analog circuits is of paramount importance. Being able to correctly understand the amplification region, saturation region, and cutoff region of the transistor is a sign of understanding the transistor.

Many beginners think that the transistor is a simple combination of two PN junctions, as shown below:

This idea is wrong. The combination of two diodes cannot form a transistor. Let's take the NPN transistor as an example, as shown below:

The two PN junctions share a P region (also called the base region), which is made extremely thin, only a few microns to tens of microns. It is through this region that the two PN junctions are organically combined into an inseparable whole. There is a mutual connection and mutual influence between them, which makes the triode completely different from the characteristics of two separate PN junctions. Under the action of an external voltage, the triode forms a base current, a collector current, and an emitter current, becoming a current amplifier device.

The current amplification of the transistor is related to its physical structure. The physical process inside the transistor is very complicated, so beginners do not need to explore it in depth for the time being. From an application perspective, the transistor can be regarded as a current distributor. After a transistor is made, the proportional relationship between its three currents is roughly determined, as shown in the following figure:

β and α are called the current distribution coefficients of the transistor, and the β value is more familiar to everyone, and it is called the current amplification factor. If one of the three currents changes, the other two currents will also change proportionally. For example, the change in base current ΔI b = 10 μA, β = 50, according to the relationship ΔI c = βΔI b, the change in collector current ΔI c = 50×10 = 500μA, achieving current amplification.

The transistor itself cannot turn a small current into a large current. It only plays a controlling role, controlling the power supply in the circuit and providing the three currents Ib, Ic and Ie to the transistor in a certain proportion. For easy understanding, we still use water flow to compare current, as shown in the following figure:

These are two water pipes, one thick and one thin. The thick pipe is equipped with a gate, and the amount of water in the thin pipe controls the degree of opening of the gate. If there is no water flow in the thin pipe, the gate in the thick pipe will be closed. The more water is injected into the thin pipe, the wider the gate will open, and the more water will flow through the thick pipe. This reflects the principle of "controlling the big with the small, and controlling the strong with the weak". As can be seen from the figure, the water in the thin pipe and the water in the thick pipe merge into one pipe at the lower end.

The base b, collector c and emitter e of the triode correspond to the thin tube, thick tube and the tube where the thin and thick tubes meet in the figure. As shown in the figure below:

If a certain voltage is applied to the triode, currents Ib, Ic and Ie will be generated. Adjusting the potentiometer RP changes the base current Ib, and Ic also changes accordingly. Since Ic = βIb, the very small Ib controls Ic, which is β times larger than it. Ic is not generated by the triode, but is provided by the power supply VCC under the control of Ib, so the triode plays the role of energy conversion.

With the above knowledge, it is easy to understand what the textbook says:

The emitter is forward biased and the collector is reverse biased , and the transistor is in the amplification state;

The emitter is forward biased and the collector is forward biased and operates in the saturation region;

The emitter is reverse biased and the collector is reverse biased and works in the cut-off region;

The emitter is reverse biased and the collector is forward biased, working in the reverse amplification state.

First assume that it is in the saturation region, and when calculating the voltage across CE, use 0.3 volts as the criterion for judging the saturation region and amplification region (less than that is the saturation mode, greater than that is the amplification mode); when the voltage between CE is infinite, it is the cutoff region!

Another explanation: three states of transistors

The three states of the transistor are also called three working areas, namely: cut-off region, amplification region and saturation region.

(1) Cut-off region: The transistor works in the cut-off state. When the emitter junction voltage Ube is less than the on-state voltage of 0.6-0.7V, the emitter junction is not turned on and the collector junction is in reverse bias, with no amplification effect.

(2) Amplification region: When a forward voltage is applied to the emitter of the transistor and a reverse voltage is applied to the collector to turn it on, Ib controls Ic. Ic and Ib are approximately in a linear relationship. A small signal current is added to the base, causing a large signal current to be output from the collector.

(3) Saturation region: When the collector junction current IC of the transistor increases to a certain extent, even if Ib is increased, Ic will not increase, and it exceeds the amplification region and enters the saturation region. When saturated, Ic is maximum, the internal resistance between the collector and the emitter is minimum, the voltage Uce is only 0.1V~0.3V, Uce<ube, and the emitter junction and the collector junction are both at forward voltage. The transistor has no amplification effect, and the collector and emitter are equivalent to short circuits. It is often used in switch circuits with cutoff.

It is mainly determined by the bias conditions of the two pn junctions:

The emitter junction is forward biased, and the collector junction is reverse biased - amplification state;

The emitter junction is forward biased, and the collector junction is also forward biased - saturation state;

The emitter junction is reverse biased, and the collector junction is also reverse biased - cut-off state.

The transition between these states can be controlled by the input voltage or the corresponding input current. For example, in the amplification state, as the input current increases, when the voltage drop of the output current on the load resistor is equal to the power supply voltage, the power supply voltage completely falls on the load resistor, so the collector junction becomes 0 bias, and then becomes positive bias - that is, the amplification state changes to the saturation state. When the input voltage is reverse biased, the emitter junction and the collector junction become reverse biased, and no current passes, which is the cut-off state.

The difference between forward bias and reverse bias: For NPN transistors, when the emitter is connected to the positive pole of the power supply and the base is connected to the negative pole, the emitter junction is forward biased, otherwise it is reverse biased; when the collector is connected to the negative pole of the power supply and the base (or emitter) is connected to the positive pole, the collector junction is reverse biased, otherwise it is forward biased. In short, when one side of the p-type semiconductor is connected to the positive pole and the other side of the n-type semiconductor is connected to the negative pole, it is forward biased, otherwise it is reverse biased.

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