Transistor critical saturation and deep saturation problems

For a single silicon tube, the correct condition for the critical saturation of the transistor should be VCE=VBE=, that is, the collector junction of the transistor VCB=0V; When the VCB > 0V, it means that the collector junction of the transistor is reversed, and the transistor is in the amplified state. When VCB < 0V, it means that the transistor collector junction is positively biased (VBC<=VBE=, and the transistor is in a saturated conduction state. The depth of saturation conduction of the transistor depends on the size of the collector resistance or base resistance, the larger the collector resistance or the smaller the base resistance, the deeper the saturation conduction depth of the transistor, and the condition for complete depth saturation of the transistor is vbc=vbe=, that is, vce=0

That's a good question, but I'm also curious. It's the second chapter of digital circuits.

DiodesThe saturation state is when the diode is saturated when the voltage no longer increases with the increase of current. Due to the R2 times short circuit, the VB voltage is about 12x10 (5+10)=8V,VE=,ICie= VCE=12-IC(2K+2K)<0, so it is saturated. VCE "i.e. saturated.

The diode has the characteristics of forward conduction, and the forward conduction current reaches a certain level, and the line becomes saturated, that is, saturated. This can be seen in the diode current and voltage curves. At this point, the voltage no longer increases with the increase in current.

How diodes work

The main principle of diodes is the use of p-n junctions.

The one-way conductivity of the P-n junction is added to the lead and package to become a diode. A crystal diode is a semiconductor made of p-type.

The p-n junction formed with n-type semiconductors forms a space charge layer on both sides of its interface, and a self-built electric field is built. When there is no applied voltage, due to the carriers on both sides of the pn junction.

The diffusion current caused by the concentration difference and the drift current caused by the self-built electric field are equal and are in an electrically equilibrium state.

When there is a forward voltage bias in the external area, the mutual suppression effect of the external electric field and the self-built electric field increases the diffusion current of the carriers and causes the forward current. When there is a reverse voltage bias in the outside world, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current that is independent of the reverse bias voltage value within a certain reverse voltage range. Limb pure.

The strength of the electric field in the charge layer of the pn junction space when the applied reverse voltage is high to a certain extent.

When the critical value is reached, the multiplication process of the carriers of the hungry bond is generated, and a large number of electron-hole pairs are generated, resulting in a reverse breakdown current with a large value, which is called the breakdown phenomenon of the diode. The reverse breakdown of the PN junction is divided into Zener breakdown and avalanche breakdown.

The transistor is a current-type control component, that is, a small current controls a large current, and it has three working zones: the cut-off area, the amplification area and the saturation area. The conditions of the cut-off zone (assuming that the pressure drop of the be is:

UBE, UBE, at this time IB=0, the internal resistance between EC is large (equivalent to an open circuit).

Amplification area: UBE, IB has current, satisfying IC= IB, IE=(+1)IB, that is to say, the larger the IB, the larger the IC, which is the amplification of the transistor, at this time, the transmitting junction is positively biased, and the collector junction is reversed.

Saturation region: When the base current continues to increase and the collector current no longer increases (tends to be stable), the transistor reaches saturation, and the collector junction of the triode is positively biased and the transmitting junction is positively biased when saturated.

When driving the relay, the transistor operates in a saturated state.

Only when the transistor is saturated and conducted, the voltage drop of the CE junction is close to 0V, and the voltage at both ends of the relay coil is 12V, and the relay can be engaged normally.

For example, the working voltage of the common radio circuit is 5 volts, the collector resistance is 1 thousand ohms, and the theoretical maximum IC is 5 milliamps, and then divided by the magnification to get the minimum IC entering the saturated area

You have to determine the amplification range of this transistor first, the general amplification range is 200-300, or 300-400, the current given in the saturated state is generally IB is about equal to 1 10 of the IC, in fact, to enter the saturated state, that is, to make the IC IB much smaller than the magnification.

When it is in the open state, the transistor is in the saturated state, and the voltage between UCE UBE and UCE is very small, which is generally less than the forward voltage drop (< of the PN junction

You can give the base a voltage large enough, say +5V

When the triode is saturated, the base voltage is indeed higher than the other two poles, the triode is mainly a current control element, the base current controls the collector current, when the base current increases, the collector current also increases rapidly, then the resistance on the collector voltage is also larger, the collector current is too large, and the resistance on the collector voltage is too large, which eventually leads to the collector voltage is lower than the base, which is what we call saturation flux. The collector is connected to the power supply voltage, and the voltage is almost all dropped on the resistor. The base voltage is lower than the resistor (load) voltage.

Therefore, it is not possible to say, "directly supply the base voltage to the load". The base voltage only plays a controlling role.

Supplementary answer to the question: The transistor works in the saturated state, whether it enters the saturated state from the amplified state first. The output level of the single-chip microcomputer is 5V, that is, to give a large enough current to the base, so that the transistor can quickly enter the saturation state.

Why not just feed the base voltage directly to the load? Do you want to use a triode? --It is to increase the driving capacity.

When the emission junction is conducted, how can the collector junction be conducted, because the collector potential is higher than the base pole. --The way semiconductor conducts electricity is different from conductor, which involves semiconductor physics, please refer to relevant books.

When the base current of the triode increases and the collector current does not increase with the increase is saturation, assuming that the load resistance is 1K, VCC is 5V, the maximum resistance through the current when saturation is 5mA, divided by the value of the tube (assuming = 100) 5 100 =, then the base current is greater than 50 A can be saturated.

1.In practice, ib* =v r is often used as a condition for judging critical saturation. The IB value calculated according to IB* =V R only makes the transistor enter the initial saturation state, which should actually be taken several times more than this value to achieve true saturation; The greater the multiplier, the deeper the saturation.

2.The larger the collector resistance, the easier it is to saturate;

3.The phenomenon of the saturation region is that the two PN junctions are positively biased, and the IC is not controlled by IB.

When the base current of the triode increases and the collector current does not increase with the increase, the saturation current is determined by the collector resistance and the emitter resistance, the size of the saturation current has nothing to do with the triode, generally when the CE voltage is small, the triode is saturated.

Whether it directly enters the saturation state after starting depends on the specific circuit, and amplification, saturation zone and amplification, saturation state are different concepts.

Shallow saturation occurs when the base current multiplied by the magnification is equal to or slightly greater than the collector current, and deep saturation is much greater than the collector current. Deep saturation in switching circuits affects speed.

The transistor has a current amplification effect, but it must be matched by an external circuit condition, that is, the external circuit satisfies the "emitter e forward bias". collector reverse bias". The output characteristic channel of the transistor can be divided into three zones, namely the amplification zone, the saturation zone, and the cut-off zone.

In the saturation zone, the saturation pressure drop is generally small (less than 1 volt).

First of all, distinguish the chip type, whether the output is an open collector output; u1 plus diode.

There is no mistake in your understanding, and you have worked hard to understand this level. But there are still a few problems, mainly in the following ways:

1. Too much concern about the "positive deviation of the pole junction". In fact, in the saturation region, even if the pole junction is positively biased, the forward conduction voltage of the pole junction has not yet been reached. However, most people will be misled by "positive bias".

2. The meaning of saturation: the collector current increases with the increase of the base current, when the collector current increases to a certain extent, and then increases the base current, the collector current no longer increases with the increase, this phenomenon is called saturation. And "if the transistor works in a saturated state, then it is a double junction positive bias" is a phenomenon or causal relationship, and it is not an explanation.

The essence of saturation is that the IC is detached from the linear relationship with IB due to the positive bias of the collector junction (please review the triode structure).

3. The saturation state of the transistor includes the state where the IC tends to 0, which please experience and understand.

4. By adding forward conduction bias to the transistor transmission junction and positive bias to the collector junction, the triode must be in the saturation region (it must not be in the amplification area, including the case where the IC is zero).

The above key points cannot be made up by making up for them. Of course, people who don't learn can't see this problem.

Impressions: The emitter ensures that the positive deviation is lower, and the inflow of base current will pull the voltage of the collector relative to the base, which will pull the direction of the collector's positive and negative deviation, resulting in saturation, criticality, and amplification.

But why does the book always say that the saturated state collector current is the largest, according to the characteristic curve, shouldn't the state collector current be enlarged to the maximum?

Congratulations, you have a deeper understanding of the working state of the transistor source, your understanding is correct, in fact, in addition to the positive deviation of the transmission junction and the positive deviation of the collector junction mentioned in the book is to judge whether the transistor is in a saturated state, there is also a method that can also be used to judge whether the transistor is in a saturated state, that is, the collector junction is biased, and the transmission junction is positively biased.

Understood correctly.

Complex! The collector current increases with the increase of the base current, when the collector current increases to a certain extent, and then increases the base current, the collector current is no longer increased with the increase, the tube is saturated, as the base current continues to increase, it will also reach a state called deep saturation, the voltage drop of the tube is very low, if there is no device to limit the current in the circuit at this time, it is very easy to burn the transistor.

Triode complex.

Saturation is explained in conjunction with the actual circuit.

It will be easier to make (that is, the diagram of the basic co-emission amplification circuit in the book.) When the signal increases from zero, the transmitting junction is positively biased, and the collector junction is reversed under the action of the external power supply VCC, and the circuit is in an amplified state, and with the increase of the signal, IB increases, IC= IB, IC increases, and the voltage at both ends of RC also increases, VCC is certain, and the voltage added to both ends of the collector junction slowly decreases (UCE decreases), and at a certain time, the collector junction voltage decreases to zero, and the transistor will be saturated at this time, because the collector junction is about to be reversed, no matter how large IB is, IC will not increase again.

The most important thing about saturation is that UCE is reduced to a certain extent, and saturation will be reached, so the book says that UCE UBE, UBE UON, and transistors are in a saturated state.

I don't know if I'm right, but I can discuss it.

In order to increase IB, UB must be increased, assuming that UC remains unchanged, then UBC will increase inwardly (the original capacity UBC is less than zero, here it gradually approaches to zero and is greater than zero, and the collector junction is positively biased when UBC is greater than zero), so that the flow of electrons injected into the base region from the emitter will be reduced to the collector under the suppression of UBC (assuming that it is already greater than zero), that is, IC will gradually decrease to zero.

If the UB increases, the UC also increases and the UBC is always less than zero, that is, the collector junction is always inverted, so that no matter how much the IB increases, the IC will always be equal to a certain multiple of IB and will not be saturated.

Extend a question, please:

If a large current is poured into the collector IC when the transistor is in the amplified state, is IC= *IB still valid?

Also, is UCE only related to the VCC that supplies power? Is it a controlled quantity or a controlled quantity? What exactly does it have to do with it?

I think your understanding of the circuit is a bit naïve, a definite connected circuit, it can only have one state at a time, for example, if you know the current in this place, you can use the physical resistance to calculate everything about the circuit, but you discuss the absolute corresponding changes of current and voltage separately (of course, the device is ideal), your foundation is not solid. I think this problem As long as the device is ideal, you can follow the actual test results or computer simulation results to reason about the process, use your brain imagination, simplify the model, and generalize... And so on, this is the right way (maybe in the end you will find that the result is not so important or find something that shocks the world, haha.) )