Detailed explanation of the three states of a transistor

Detailed explanation of the three states of a transistor

The transistor is divided into three states: cutoff, amplification, and saturation

1. Cut off state: The transistor is in the off state, and Vce is approximately equal to the power supply voltage

2. Amplification state: The transistor is in current amplification state, 0V

3. Saturation state: The transistor is in a fully conductive state, Vce≈0V

The methods for determining the operating state of a transistor are divided into:

1. Current judgment method:

Ic_max>Ib * β amplification state

Ic_max ≤ Ib * β saturation state

Ic_max=Ib * β=0 cut-off state

2. Voltage judgment method:

Uce>0.3V amplification state

Uce=0.3V saturated state

Uce=VccOff state

1. Transistor amplification circuit (signal type transistor)

The most typical feature of a transistor amplifier circuit is: Ic max >β*Ib， That is to say, no matter how the Ib current changes, β * Ib never exceeds the maximum value of Ic, and the state at this time is the amplification state.

As shown in Figure 2, it is a transistor amplification circuit. So how to determine the actual parameters of the transistor amplification circuit?

Firstly, it should be clarified that Vbe=0.7V when the transistor is saturated and conducting, Vce=0.3V, Because β varies with temperature, humidity, and other factors, it is assumed here that β=100 for ease of calculation.

So Ic max=(5V Vce)/R1, first determine the current required on the IC based on the load to determine what R1 is. Assuming the IC needs 47mA, Ic max=(5V Vce)/R1=47mA. When Vce is minimized, Ic can reach the maximum current value, that is, Ic max=(5V-0.3V)/R1=47mA, R1=100 Ω. After determining the Ic current value and R1 resistance value, let's see how Ib is calculated because β=100, Ic=β*Ib， It can be calculated that Ib max=47mA/100=0.47mA, which means that when Ib<0.47mA, the transistor is in an amplified state.

Determine the resistance value of R2, as Vbe=0.7V when the transistor is conducting, and the input is a 3V square wave signal, Ib max=(3v-Vbe)/R2=0.47mA, At R2=4.89K, the transistor is in a critical saturation state. We choose a commonly used nominal resistance of 10K, that is, R2=10K. The actual Ib=(3V Vbe)/R2=(3V-0.7V)/10K=0.23mA, and Ib * β<Ic max meets the requirements. At this point, all parameters of the transistor amplification circuit have been obtained. In practical design, resistor selection needs to be far away from the critical saturation state.

Summary: In the transistor amplification state, the IC will increase and decrease with the increase and decrease of IB, that is, the IC is controlled by IB.

2 transistor saturation state (signal type transistor)

The most typical characteristic of a transistor saturation circuit is that Ic max ≤ β * Ib, where β * Ib exceeds the maximum value of Ic, resulting in a saturation state.

As shown in Figure 3, it is a transistor saturation conduction circuit. So how to determine the actual parameters of the transistor saturation conduction circuit?

When the transistor is saturated and conducting, Vce=0.3V, Vbe=0.7V, Assuming β=100, IC=5mA (the specific value of IC depends on the load), from Ic max=(12V Vce)/R4=5mA, R4=(12V-0.3V)/5mA=2340R is obtained. Taking a resistor R4=2K with a nominal value close to the calculated value, then the actual IC=5.85mA, Ib=Ic/β=5.85mA/100=0.0585mA, and IB=0.0585mA can be used to calculate the theoretical resistance of R5, which is 39.32K. Based on this, the transistor is at the critical saturation conduction moment, and the β value will change due to temperature influence. That is to say, when R5=39.32K, the transistor will be affected by external factors, sometimes saturated and conducting, sometimes in amplification state, while the amplification state will control. Poor manufacturing can lead to damage to the transistor, which is something we do not want to see.

If the current of Ib is much larger than 0.0585mA, then the transistor must be in a saturated conduction state. Assuming Ib=10mA, no matter how the temperature changes, the transistor will definitely be in a saturated conduction state. Of course, taking Ib so large must have its drawbacks, such as excessive power consumption. Although it satisfies the saturation conduction of the transistor, adding too much power consumption is also not acceptable. After multiple design verifications, it has been found that when Ib=1mA, the transistor is in a saturated conduction state, which is suitable for 90% of transistors. Empirical design does not consider special applications of transistors, because at this time, the transistor also follows the condition of Ic=β * Ib, β=100, Ic=100mA, When designing a transistor switch circuit in practice, the Ic current is not very large.

Therefore, when Ib=1mA, R5=(3V Vbe)/1mA=2300 Ω is obtained, and the nominal resistance R5=2K2 is taken. The actual Ib=(3V Vbe)/R5=(3V-0.7V)/2K2=1.05mA, and Ib * β ≥ Ic max meets the requirements. At this point, all parameters of the transistor saturation conduction circuit have been obtained.

Summary: In the saturated conduction state of the transistor, Ic does not increase with the increase of Ib, but decreases with the decrease, that is, Ic is not controlled by Ib.

3. Practical application circuit of switch diode

Selection of pull-down resistor for transistor in Figure 4: The resistance value is 2K. The resistance value of this resistor has been tested multiple times in practice. When designing, it is necessary to add a pull-down resistor. The transistor can still work without it, but adding it will ensure a more stable state of the transistor.

Why do we say that? Generally, the transistor is connected to the IO port of the microcontroller for switch control. If the IO port of the microcontroller has a high resistance state at this time, the level of the transistor IB connected is uncertain, which will cause the transistor to misguide. If a pull-down resistor is connected at this time, according to the internal resistance analysis method, IB will be given a stable state, which will not cause the transistor to trigger incorrectly. The function of the pull-down resistor is to quickly pull down the base through the pull-down resistor when there is no input or the input is in a high resistance state, ensuring that the transistor is in a stable cutoff state.

Summary: Transistors belong to the current flowing transistor category, and the relationship between Ib * β and Ic is explored. In practical use, it is important to pay attention to the fact that the base level must be given a certain state of either high or low.