Really learn to understand the transistor primer (classic) __ electronic circuit

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BJT can be viewed functionally as a two-diode tandem in back-to-back.

However, in the actual manufacturing process,

When we compare transistors to two diodes, we refer to the base-emitter diodes and the base-collector diodes.

NPN type diode and PNP type diode

Depending on the unidirectional conductivity of the diode, the following example is the first to have an electric current, and the second to have no current (reverse bias on the base-collector diode).

Now we're thinking about the following transistor's connection, noting that both the base and the collector have power.

when both the base electrode and the collector are connected, the current in the circuit reflects the key characteristics of the transistor. Called the transistor action (if the transistor's base current is flowing, the collector current is also flowing), the current is as shown in the figure:

We note from the diagram that the base current causes the current of the collector to flow (if there is no base current and no collector current). There is no current between the base pole and the collector electrode.

One of the transistor properties is that the ratio of---collector current to base electrode current is constant. The collector current is larger than the base electrode current. The ratio of two currents is called the transistor's current gain.

Changing the beta or changing the value of RB can change the collector current when the base current is large enough that the voltage of the collector is 0 at the given set electrode resistance and supply voltage. The current of the collector is the largest, and this state is called saturation.

Transistor conduction (ON)

Because the transistor switch is functionally equivalent to a closed switch. So the collector voltage and emitter voltage are the same. The emitter voltage is zero after the transistor conduction. In practical applications, there is a small pressure drop between the collector of the transistor and the emitter electrode. This discussion can be ignored, so the collector voltage is recorded at 0v.

If the VS and RC are known. To find the value of RB at the time of transistor conduction.

The collector current can be obtained according to the pressure drop of the collector-emitter to 0. Based on the relationship between the collector current and the base current, the base electrode current is obtained. The Vs-0.7 voltage is the voltage of RB. According to Ohm's law, RB can be obtained.

The transistor is turned off. If the transistor is disconnected, then no current flows through the load (i.e., the collector current is 0).

When the base electrode current is zero, the current of the collector is zero when the base current is not set. When the base is not connected to the voltage, in order to ensure that the transistor turn-off, you can add a resistor in the circuit (R2), through the resistor, the base is connected to the 0V. Thus the base is unlikely to present a current. (since no current flows through the R2, the R2 voltage is equal to 0V) without pressure drop. The value of R2 in the actual circuit is 1kω-1mω.

Analysis of the transistor's conduction and turn-off

When the switch is in a, the existence of the base is very current, then the transistor conduction (on)

When the switch is at B, there is no base current, then the transistor is off (off)

Many types of electronic circuits contain multiple switching transistors, and a transistor controls the conduction and shutdown of other transistors.

Analysis process: When the switch is located in a, the generation of base current, Q1 conduction, so that the Q1 of the collector of the current through the emitter to the ground, not through the Q2 of the base, so the Q2 base is no current, so the Q2 off, the light bulb off. Similarly, when the switch is located at B, the Q1 base has no current, and the Q1 shuts off. The Q1 collector current flows through the Q2 base, Q2 leads, and the bulb brightens.