Transistors are basic devices in analog circuits. For electronics engineers, understanding the working conditions of transistors and judging the working state of transistors is very basic. This article will bring everyone together to learn or review.
First, the transistor working conditions
1, the collector resistance Rc:
In the common emitter voltage amplifier, a resistor Rc needs to be connected in series with the collector in order to take out the amplified signal voltage Use (dynamic signal) at the output of the transistor. In this way, when the collector current Ic passes, a voltage drop IcRc is generated at Re, and the output voltage is taken out between the transistors ce, that is, Usc=Uce=Ec-IcRc, so Use is also the same as IcRc with the input voltage Ui. The occurrence changes accordingly.
2. Collector power supply Ec (or Vcc):
Ec ensures that the collector junction of the transistor is in reverse bias, which causes the tube to operate in an amplified state, making the weak signal a strong signal. The source of energy is the maintenance of Ec, not the transistor itself.
3, base power Eb:
In order to make the transistor produce current amplification, in addition to ensuring that the collector junction is reverse biased, the emitter junction must also be forward biased. The function of Eb is to provide a forward bias voltage to the emitter junction and to match the proper base. Stage resistance Rb to establish a certain static base current Ib. When Vbe is small, Ib=O, only when Vbe exceeds a certain value (silicon tube is about 0.5V, the fistula tube is about 0.2V, called the threshold voltage), the tube starts to conduct, and Ib appears. Subsequently, Ib will increase as Vbe increases, but the relationship between Vbe and Ib is not a linear relationship: When Vbe is greater than 0.7V, Vbe increases a little bit further and Ib increases a lot. The fully conductive Vbe of the transistor is approximately equal to a constant (approximately 0.5V for a silicon tube and approximately 0.5V for a germanium tube).
4, base bias current resistance Rb:
Under the condition that the size of the power supply Eb has been determined, changing the resistance of Rb can change the quiescent current Ib of the transistor, thereby also changing the quiescent current Ic of the collector and the voltage drop Vce of the tube, so that the amplifier can establish a proper DC working state.
Second, the transistor status judgment
When the transistor operates in the amplification region, its emitter junction (between b and e) is a forward bias, and the collector junction (between b and c) is a reverse bias. For low-power NPN-type silicon, it appears as Vbe≈0.7V, Vbc 《0V (the specific value depends on the power supply voltage Ec and the value of the relevant component): For NPN-type manifolds, Vbe≈0.2V, Vbc《0V; PNP-type transistors, the above-mentioned voltage value of the opposite signs, namely low-power PNP-type silicon tube Vbe ≈ 0.7-0.7V, Vbc "0V, for low-power PNP-type helium tube, Vbe ≈ 0.2-0.2V, Vbc" 0V. If we find in the detection circuit the voltage between the transistors of the transistor is the above-mentioned value, it can be judged that the transistor operates in the amplification region, and this portion of the circuit composed of the transistor is an amplifying circuit.
In addition, in the oscillating circuit composed of transistors, the triode is also working in the amplification area, but since the output of the triode is fed through the selective-frequency resonant tank and fed back in phase between its b and C poles, the circuit starts to oscillate, then b, e The voltage Ube between the poles is less than 0.7V for the silicon tube (generally about 0.2V). If we detect Vbe "0.7V, and use a wire to short the inductance in the selective resonant circuit to make the circuit stop when Vbe is 0.7V, then we can judge the circuit as an oscillation circuit.
2, the work in the cut-off area to determine:
When the transistor operates in the cut-off region, both the emitter junction and the collector junction are reverse biased, and in an actual circuit, the emitter junction can also be zero biased. In this way, for low-power NPN triodes, Vbe ≤ ≤ 0, Vbc <<0V (specifically, the value is determined mainly by the power supply voltage Ec); for low-power NPN triodes, Vbe ≥ OV, Vbc ≥ 0V, Vce at this time Ec, if we detect that the voltage between the transistors in the circuit is the above, we can judge that the transistor works in the cut-off region.
3, work in the saturated zone judgment:
When the transistor operates in the saturation region, both the emitter junction and the collector junction are forward biased. For low-power NPN silicon tubes, Vbe is more than 0.7V (slightly larger than the value when working in the amplification area), Vbc†0V (not more than the value of Vbe); for low-power NPN-type silicon tubes, similarly Vbe≥ 0.2V (slightly larger than the value when working in the zoom area), Vbc"OV (not more than Vbe). For PNP-type transistors, the above voltage values ​​have the opposite signs, ie, low-power PNP-type silicon tubes, Vbe ≥ -0.7V, Vb 《 0V (not less than the value of Vbe, low-power PNP-type , tubes, Vbe -2 -2V, Vbc "0V (not less than the value of Vbe). Under normal circumstances, Vce ≈ 0.3V (silicon tube) or Vce ≈ 0.1V (manganese tube) at this time, if we detect that the voltage between the transistors of the transistor in the circuit meets the above In this case, the triode can be judged to work in a saturated area.
It is necessary to point out that in some electronic circuits, such as switching circuits and digital circuits, the triode operates between the cut-off region and the saturation region, as shown in the drawing. When point A is 0V, EB divides the base voltage by R1 and R2 to negative voltage, and the emitter junction is reversed; at the same time, the collector junction is also reverse biased, then the transistor T is cut off; when point A input is 6V, R1, The R2 voltage divider makes the triode emitter junction biased positively, generating a sufficiently large base current to make the triode saturated and conducting. The output L is about 0.3V, and the collector junction is also positive. When we detect whether the circuit is normal, we can make the voltage of 0V and 6V input to terminal A and measure the inter-electrode voltage between two cases respectively to see if it meets the above conditions of cut-off and saturation, so we can judge the circuit work. Is it normal?
Third, the summary
The transistor has three working areas, namely, amplification area, cut-off area and saturation area. When circuit design, according to the requirements of the circuit, the transistor works in different areas to form an amplifying circuit, an oscillating circuit, a switching circuit, etc. If the triode changes the original normal working state for some reason, it will make the circuit work abnormally; When the electronic product fails, it is necessary to analyze the fault. The first task is to check the working status of the triode as described above.
For the specific detection of the transistor, pay attention to two issues: First, it is best to use a digital multimeter with a large internal resistance to measure, in order to reduce the measurement error, while avoiding the change of the transistor's working status due to the small internal resistance of the multimeter during direct measurement. Second, it is best to measure the voltage of each pole of the transistor to ground, and then calculate the value of Ube.Ubc or Uce to avoid the possibility of inducing a circuit fault.
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