Triode amplifier circuit, common base amplifier circuit, common collection amplifier circuit The key point, but also the difficulty, I know it is very important, but I don't understand what's going on. It doesn't matter this time, I will take the three configurations of the triode amplifier circuit as an example to briefly explain the calculation formula of the amplification factor of the amplifier circuit.

The basic configuration amplifying circuit composed of triodes can be divided into three types, namely common-emitter amplifying circuit, common-base amplifying circuit and common-collecting amplifying circuit.

1. Common emission amplifier circuit

The schematic diagram of the circuit is as follows:

①. The magnification is: A=-Rc/Re. Design the values of Rc and Re according to the requirements. ②. Input impedance: Zin = beta * Re. (R1 and R2 provide a bias voltage for the triode, which is ignored here, of course, it should be considered in practice). Due to the circuit amplification characteristics of the triode, the conversion of Re to the input end needs to be amplified by beta times, so the input impedance is high. ③. Output impedance: Zout = Rc. In order to reduce the current of the triode and reduce the power consumption, Rc generally takes a large value. ④. Frequency characteristics: Due to the Miller effect, the parasitic capacitance between the base and the collector of the triode will be enlarged by A times and react to the input end in the amplification area, so the frequency characteristics are poor and high-frequency signals cannot be amplified.

2. Common collector amplifier circuit

The input resistance of the common-collection amplifier circuit is very large, and the output resistance is very small, but it only has the ability to amplify the current and not the voltage, which is generally close to but less than 1. The AC path of the common-collector amplifier circuit is as follows.

At first glance, it feels very similar to the common-collection amplifier circuit with no resistor on the emitter. The difference is that the location of the AC ground is different. You can compare it yourself to make it easier to distinguish. The formula for AC magnification is:

It can also be seen from the formula that the voltage magnification cannot be greater than 1, usually 1+β is very large, plus a large resistance Re' is multiplied behind it, so this result is close to 1. In this formula, only one Re' is different from the formulas of the above two amplifier circuits. Its value is equal to Re and RL is added. The meanings of other letters are the same as above.

3. Common base amplifier circuit

The input resistance of the common-base amplifier circuit is small, the output resistance is large, and the frequency characteristics are good, but the common-base amplifier circuit only has the ability to amplify the voltage and not the current. Just look at the circuit diagram of the AC path.

Does this circuit look strange? The current flows from the emitter to the collector and flows out of the common base amplifier circuit (it feels strange to see the current flow itself, but it is really like this), through the introduction of the common emitter circuit, I believe You should be familiar with the four components on the schematic diagram. Let's take a look at the formula of this circuit:

In fact, the magnitude of this voltage magnification is the same as that of the common emitter amplifier circuit when the emitter is not connected to the resistor, but the difference between the two is a negative sign. The meaning of each letter in the formula is the same as that of the common emitter amplifier circuit. That is, β represents the magnification of the triode, R L' is equal to R c and RL is attached, R b b' is composed of the base lead resistance and the base body resistance, and the calculation method of R b'e is also the same as that of the common emitter amplifier circuit. same.

How to distinguish the three working states of the triode

In simple terms, the working state can be judged according to the size of Uce. Uce is close to the power supply voltage VCC, then the triode works in the load-stop state. The load-stop state means that the transistor basically does not work, and the Ic current is small. (about zero), so R2 is close to 0V because there is no current flowing, so Uce is close to the power supply voltage VCC.

If Uce is close to 0V, the transistor is working in a saturated state. What is a saturated state? That is to say, the Ic current has reached the maximum value, and even if Ib increases, it cannot increase any more.

The above two states are generally referred to as switching states. In addition to these two states, the third state is the amplification state. Generally, the measured Uce is close to half of the power supply voltage. If the measured Uce is biased to VCC, the triode tends to be in a load-stop state; if the measured Uce is biased to 0V, the triode tends to be saturated.

The stable working principle of the amplifier circuit

1: As the temperature rises, the collector Ic increases, the emitter current Ie increases, and the voltage IeR4 across R4 increases. Ub is provided by the voltage divider resistor, and Ub remains basically unchanged. Since Ube=Ub-IeR4, Ube will decrease, the corresponding base current Ib will decrease, and the increase of collector Ic will be suppressed, thereby stabilizing the DC operating point of the collector current, thereby reducing the adverse effect of temperature rise on the circuit.

2: R4 feedback resistor is the key component of the amplifier circuit, appropriately increase the resistance value of R4, the greater the feedback, the better the stability. Therefore, a reasonable choice should be made according to the actual circuit design. In order to reduce the loss of AC energy on R4, the C3 capacitor is added to bypass the AC to the ground, which can improve the AC gain of the amplifier circuit.

3: The current negative feedback bias circuit has good temperature stability. As long as the appropriate bias resistor value is selected and a reasonable DC operating point is designed, the amplifier circuit can work stably and reliably. Therefore, more bias circuits are used in amplifier circuits.

If you want to use the triode well, then the above three basic configuration amplification circuits must be mastered