Transistor amplifier circuit common base amplifier circuit common collector amplifier circuit

Transistor amplifier circuit common base amplifier circuit common collector amplifier circuit

Friends who have studied analog electronics should not be unfamiliar with the amplification circuit of transistor or field-effect transistor (the amplification circuit mentioned in this article refers to voltage amplification). This is the key point in analog electronics, but it is also a difficult point. It is important to know for yourself, but if you cannot understand what is going on, it is okay. This time, taking the three configurations of transistor amplification circuit as an example, I will briefly explain the amplification factor calculation formula of amplification circuit.

The basic configuration amplification circuit composed of transistors can be divided into three types, namely common emitter amplification circuit, common base amplification circuit, and common collector amplification circuit.

1. Common emitter amplifier circuit

The circuit schematic is as follows:

① The magnification factor 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 bias voltage for the transistor, which should be ignored here, but should be considered in practice). Due to the circuit amplification characteristics of the transistor, Re needs to be amplified by beta times when converted to the input terminal, resulting in a high input impedance. ③ Output impedance: Zout=Rc. In order to reduce the current of the transistor and lower power consumption, Rc is generally taken at a large value. ④ Frequency characteristics: Due to the Miller effect, the parasitic capacitance between the base and collector of the transistor will be amplified by A times in the amplification region and reflected to the input terminal, resulting in poor frequency characteristics and inability to amplify high-frequency signals.

2. Co amplifying circuit

The input resistance of a common amplification circuit is very high and the output resistance is very small, but it only has current amplification capability and no voltage amplification capability. Generally, it is close to but less than 1. The AC path of a common amplification circuit is as follows

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At first glance, it looks very similar to a collector amplifier circuit with no resistance at the emitter, except that the position of the AC ground is different. You can compare it yourself for easy differentiation. The formula for communication magnification is：

From the formula, it can also be seen that the voltage amplification factor cannot be greater than 1. Usually, 1+β is very large, and then multiplied by a large resistor Re ', so this result is close to 1. In this formula, there is only one Re 'that is different from the formulas for the two amplification circuits above. Its value is equal to Re and is denoted by R L, while the other letters represent the same meaning as above.

3. Common base amplifier circuit

The common base amplifier circuit has a small input resistance, a large output resistance, and good frequency characteristics. However, the common base amplifier circuit only has voltage amplification capability and does not have current amplification capability. Let's take a look at the AC circuit diagram directly

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Does this circuit look strange? The current flowing from the emitter to the collector is a common base amplification circuit (although the current flow itself feels strange, it is indeed like this). Through the introduction of the common emitter circuit, I believe everyone should be familiar with the four components on the schematic. Let's take a direct look at the formula of this circuit：

In fact, the magnitude of this voltage amplification factor is the same as when the emitter of the common emitter amplifier circuit is not connected to a resistor, except for 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 amplification factor of the transistor, R L 'is equal to R c and R L is added, 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.

How to distinguish the three working states of a transistor

Simply put, determining the working state can be based on the size of Uce. If Uce is close to the power supply voltage VCC, the transistor will operate in the load stop state. The load stop state means that the transistor is basically not working, and the Ic current is small (about zero). Therefore, R2 has no current flowing and the voltage is close to 0V, so Uce is close to the power supply voltage VCC.

If Uce is close to 0V, the transistor is operating in a saturated state. What is a saturated state? That is to say, when the Ic current reaches its maximum value, even if Ib increases, it cannot increase any further.

The above two states are generally referred to as switch states. In addition to these two states, the third state is the amplification state, where Uce is usually measured to be close to half of the power supply voltage. If Uce is biased towards VCC, the transistor tends to the load stop state. If Uce is biased towards 0V, the transistor tends to the saturation state.

Principle of Stable Operation of Amplification Circuit

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

2: The key component of the R4 feedback resistor amplification circuit is to increase the resistance of R4 appropriately. The larger the feedback, the better the stability. Therefore, it is necessary to make reasonable choices based on the actual circuit design. In order to reduce the loss of AC energy on R4, C3 capacitor is added to bypass the AC to ground, which can improve the AC gain of the amplification circuit.

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