This paper first describes the basic concept, performance index and composition principle of the amplifier circuit; then expounds the working principle and analysis method of the basic amplifier circuit; finally, it introduces the feedback of the amplifier circuit, multi-stage amplifier circuit, functional amplifier and so on.
First, the basic concept of amplifier circuit
The basic amplifier circuit is a kind of circuit and can be used in circuit construction. The input resistance of the basic amplifier circuit is very low, generally only a few ohms to tens of ohms, but its output resistance is very high.
The basic direct amplifier circuit can not only amplify the AC signal, but also amplify the DC signal and the signal that changes very slowly, and has the advantages of high signal transmission efficiency, simple structure and easy integration. This coupling method is often used in integrated circuits.
Second, the composition of the basic amplifier circuit
The function of the amplifier circuit is to amplify the electrical signal. Since it is an amplifier circuit, two requirements must be met:
One is to have the ability to amplify (voltage amplification or power amplification).
Second, the distortion should be small, that is to say, it is best to zoom in exactly the same proportion, just like a photo of you. After zooming in, it is still you.
There are many types of amplifiers, which are divided into low-frequency, intermediate-frequency, high-frequency and DC amplifiers according to frequency. Here, we mainly explain the low-frequency voltage amplifier, starting from the composition and working principle of the simplest triode amplifier circuit.
The analysis is carried out with the PNP transistor as the core component.
Ui is the input part, Uo is the output part, T represents the transistor, and the functions of C1 and C2 are to block direct traffic. The input signal generally has a certain waveform, and the output signal is also an amplified waveform signal. Adding C1 is to prevent the influence of a given power supply Ucc on the input signal Ui. The function of C2 is to isolate the DC signal of Ucc, and only the amplified input is taken. Signal.
The role of Ucc's power supply:
1. Power supply for
the circuit 2. Provide a suitable static operating point for the circuit
The role of RB: to provide a suitable quiescent current (IB) for the circuit.
The role of RC: to convert the amplified current signal into a voltage signal
.
Value range of each device
1. The triode amplifier circuit is the amplification of small signals, and the maximum current passing through the commonly used devices is only a few hundred milliamps, so the input signal should not be too large.
2. For the design of the triode amplifier circuit, the value of the base resistance Rb is generally tens to hundreds of thousands of ohms. The collector resistance Rc is generally several thousand to several tens of kiloohms.
3. C1 and C2 are used as the capacitors for blocking direct traffic, and the value is generally a few microfarads to tens of microfarads
3. Amplifier Analysis
Amplifier circuit performance indicators
Voltage amplification factor/voltage gain: the amplitude of the output voltage is greater than the amplitude of the input voltage. Au=Uo/Ui
Current Amplification/Current Gain: The output current is greater than the input current. Ai=Io/Ii
Input resistance ri: The signal frequency is not high, and the reactance effect is not considered, then ri=Ui/Ii
Output resistance ro: the equivalent resistance seen from the output end, generally the input signal is set to zero, and an external voltage source is connected to the output end to solve (see Thevenin Equivalent Circuit with Controlled Source Solution for details)
current magnification
The input current of the common emitter circuit is the base current IB, the output current is the collector current IC, and the current amplification factor β=△IC/△IB, usually the value of β is larger.
The input current of the common base circuit is the emitter current IE, the output current is the collector current IC, and the current amplification factor α=△IC/△IE. Since ΔIC is less than ΔIE, α is always less than 1.
The input current of the common collector is the base current lB, the output current is the emitter current IE, the current amplification factor K=△IE/△IB=(△IB+△IC)/△IB=1+β, it can be seen that its current amplification factor Also larger.
Voltage magnification
The input end of the common emitter circuit is actually the emitter junction of the triode. Since the triode is in the forward voltage working state, its input impedance is very low, and the collector junction at the output end is in the reverse voltage working state. The output impedance is very large. Since the current magnification of the common-emitter circuit is larger, the output current will generate a larger output voltage at the output end, so the voltage magnification of the common-emitter circuit is larger.
Although the current amplification factor of the common base circuit is less than 1, a larger collector load resistance RL and a suitable collector power supply EC can be selected, so that the IC remains unchanged after the resistance value of RL is increased, then the RL can still be obtained. Larger output voltage .. makes the voltage magnification much greater than 1.
The input end of the common collector circuit is the collector station, which is in a reverse voltage working state, so it has a high input impedance and a low output impedance, so that the voltage magnification of the common collector is always less than 1.
power magnification
These three circuits have the ability of power amplification. For the common base circuit, although its current amplification factor α<1, but the voltage amplification factor is larger, so there is still a power amplification factor. Among these three circuits, the power amplification factor of the common emitter circuit is the highest.
Welcome everyone to pay attention to the WeChat public account and learn more embedded knowledge with us
