Transistors are the basic devices in analog circuits and are the basic building blocks for standard operation of computers, mobile phones, and all other modern electronic circuits. Because of its fast response speed and high accuracy, transistors can be used for a variety of digital and analog functions, including amplification, switching, voltage stabilization, signal modulation, and oscillators. Transistors can be packaged individually or in a very small area that can hold 100 million or more transistors as part of an integrated circuit. The transistor has three working states: amplification, saturation, and cut-off, so how to judge which working state it is in in the amplifying circuit? Let's take a look together:
1. Working conditions
1.1 Collector power supply Ec (or Vcc)
Ec ensures that the collector junction of the transistor is in reverse bias, so that the tube works in an amplified state, and the weak signal becomes a strong signal. The source of energy is the maintenance of Ec, not the transistor itself.
1.2 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 collector quiescent current Ic and the tube voltage drop Vce, so that the amplifier establishes a proper DC working state.
1.3 Collector resistance Rc
In a common emitter voltage amplifier, in order to extract the amplified signal voltage Use (dynamic signal) at the output of the transistor, a resistor Rc needs to be connected in series to the collector. In this way, when the collector current Ic passes, a voltage drop IcRc is generated on 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 Occurs and changes accordingly.
1.4 Base power Eb
In order to make the transistor have a current amplification effect, in addition to ensuring that the collector junction is in reverse bias, the emitter junction must also be forward biased. The function of Eb is to provide a forward bias voltage to the emitter junction and cooperate with an appropriate base. Level resistance Rb, in order to establish a certain static base current Ib. When Vbe is very small, Ib=0, only when Vbe exceeds a certain value (silicon tube is about 0.5V, germanium 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 linear: when Vbe is greater than 0.7V, Vbe increases a little bit, and Ib will increase a lot. The fully conductive Vbe of the transistor is approximately equal to a constant (about 0.5V for silicon tubes and about 0.5V for germanium tubes).
2. Judgment of working status
2.1 zoom area
When the transistor works in the amplifying region, its emitter junction (between b and e poles) is forward biased, and the collector junction (between b and c poles) is reverse biased. For low-power NPN silicon, it appears as Vbe≈0.7V, Vbc≤0V (the specific value depends on the power supply voltage Ec and the value of related components): For NPN-type germanium tube, Vbe≈0.2V, Vbc≤0V; for For PNP type transistors, the signs of the above voltage values are opposite, that is, low-power PNP-type silicon tubes Vbe≈-0.7V, Vbc≥0V; for low-power PNP-type germanium tubes, Vbe≈-0.2V, Vbc≥0V. If the voltage between the transistors is found to be the above value in the detection circuit, it can be judged that the transistor is working in the amplifying area, and this part of the circuit composed of the transistor is an amplifying circuit.
In addition, in the oscillation circuit composed of transistors, it also works in the amplifying area, but because the output of the transistor is fed back to its b and C poles in phase through the frequency-selected resonant circuit, the circuit oscillates, so between b and e poles The voltage Ube is less than 0.7V for silicon tubes (usually about 0.2V). If it is detected that Vbe≤0.7V, and the inductance in the frequency selective resonant circuit is short-circuited with a wire to stop the circuit from oscillating, Vbe≈0.7V, then the circuit can be judged to be an oscillating circuit.
2.2 Cut-off area
When the transistor works in the cut-off region, both the emitter junction and the collector junction are reverse biased. In actual circuits, the emitter junction can also be zero biased. In this way, for low-power NPN transistors, it appears as Vbe≤0, Vbc≤0V (the specific value is mainly determined by the power supply voltage Ec); for low-power NPN transistors, it appears as Vbe≥0V, Vbc≥0V, at this time Vce≈ Ec, if we detect that the voltage between the transistors in the circuit is the above situation, we can judge that the transistor is working in the cut-off region.
2.3 saturation zone
When the transistor works in the saturation region, its emitter junction and collector junction are both forward biased. For low-power NPN-type silicon tubes, it appears as Vbe≈0.7V (slightly greater than the value when working in the amplification zone), Vbc≥0V (not greater than the value of Vbe); for low-power NPN-type germanium tubes, similarly Vbe≥ 0.2V (slightly greater than the value when working in the amplified area), Vbc≥0V (not greater than the value of Vbe). For PNP type transistors, the sign of the above voltage value is opposite, that is, low power PNP type silicon tube, Vbe ≥ -0.7V, Vb ≤ 0V (not less than the value of Vbe; low power PNP type germanium tube, Vbe ≤ -2V, Vbc≤0V (not less than the value of Vbe). Generally, Vce≈0.3V (silicon tube) or Vce≈0.1V (germanium tube) at this time. If we detect that the voltage between the transistors in the circuit meets the above conditions , It can be judged that the transistor is working in the saturation region.
It should be noted that in some electronic circuits, such as switching circuits, digital circuits, etc., the transistors work between the cut-off zone and the saturation zone, as shown in the figure. When point A is 0V, EB makes the base at negative voltage through R1 and R2, and the emitter junction is reverse biased; at the same time, the collector junction is also reverse biased, then transistor T is cut off; when the input of point A is 6V, R1, The R2 voltage divider makes the transistor's emitter junction forward bias, and generates a large enough base current to make the transistor saturated and conduction. The output terminal L is about 0.3V, and the collector junction is also forward biased at this time. When detecting whether the circuit is normal, you can input the voltages of 0V and 6V to the A terminal, and measure the voltage between the transistors in the two cases to see if the above cut-off and saturation conditions are met, so that you can judge whether the circuit is working normal.
The above is the introduction to the judgment of the working state of the transistor. When the circuit is designed, the transistor can be operated in different areas to form an amplifying circuit, an oscillating circuit, a switching circuit, etc. according to the requirements of the circuit. If the transistor changes its original normal working state for some reason, the circuit will work abnormally. ; When the electronic product fails, it is necessary to analyze the failure at this time. The first task is to check the working state of the transistor according to the aforementioned method.
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