Tips for voltage followers (2), what are the main uses of voltage followers?

The application range of voltage followers in real life is far beyond imagination, but the general public does not know about voltage followers. In the previous article, the circuit of the voltage follower and the characteristics of the voltage follower were introduced. In this article, we will focus on the main purpose of the voltage follower. If you are interested in the content of this article, you may wish to continue reading the main text.

First, the introduction of voltage follower

The voltage follower is a common collector circuit, the signal is input from the base and the emitter is output, so it is also called the emitter output device. The base voltage and the collector voltage are in the same phase, that is, the input voltage and the output voltage are in the same phase, that is, the voltage amplification factor of the voltage follower is always less than and close to 1. When RF=0, R1=∞, that is, uo=ui, Auf=1, the output voltage follows the input voltage to make the same change, which is called a voltage follower.

Second, the characteristics of the voltage follower

The voltage follower has the characteristics of high input resistance and low output resistance. If you understand it at an extreme point, when the input impedance is high, it is equivalent to an open circuit to the front-end circuit. When the output impedance is very low, it is equivalent to a constant voltage source for the latter-stage circuit, that is, the output voltage is not affected by the impedance of the latter-stage circuit. . A circuit that is equivalent to an open circuit to the front-end circuit, and the output voltage is not affected by the impedance of the latter-stage, of course, has an isolation effect, even if the front-end circuit and the latter-stage circuit do not affect each other. The isolation function is to isolate the influence of the load on the input terminal.

3. Where is the main purpose of the voltage follower

1. Buffer

To a certain extent, it can avoid the signal loss caused when the output impedance is high and the input impedance of the next stage is small, which plays a linking role.

2. Isolation

Because the voltage follower has the characteristics of high input impedance and low output impedance, it presents a high-impedance state to the upper-stage circuit and a low-impedance state to the next-stage circuit. interaction between them. In the HIFI circuit, a voltage follower is included, which is placed between the front stage and the power amplifier to eliminate the interference of the back electromotive force of the speaker to the front stage, making the sound quality clearer.

3. Impedance matching, improve load capacity

The input high impedance and low output impedance characteristics of the common collector circuit make it possible to match the impedance in the circuit (the impedance is literally different from the resistance, only one resistance word is the same, and the other resistance word is the same. What? Simply put, impedance is resistance plus reactance, so it is called impedance; in a little bit, impedance is the sum of resistance, capacitive reactance and inductive reactance on a vector. In the world of direct current, the effect of objects on current obstruction is called Resistance, all substances in the world have resistance, but the difference in resistance value. Substances with low resistance are called good conductors, substances with large resistance are called nonconductors, and recently in the high-tech field called superconductors, It is a kind of resistance value close to zero. But in the field of alternating current, in addition to resistance, capacitance and inductance also hinder the flow of current. This effect is called reactance, which means resistance to current. effect.

The reactances of capacitors and inductors are called capacitive reactance and inductive reactance, respectively, referred to as capacitive reactance and inductive reactance. Their unit of measurement is ohms like resistance, and their value is related to the frequency of alternating current. In addition, capacitive reactance and inductive reactance also have the problem of phase angle, which has a relationship on a vector, so it is said that impedance is the sum of resistance and reactance on a vector), which can make the amplifier circuit of the latter stage better. Work. To give a typical example of application: the signal output of an electric guitar is high impedance. When connecting to a recording device or speaker, adding this voltage follower before the tone processing circuit will make the impedance match and the tone more perfect.

Fourth, the application of voltage follower

1. About the voltage follower in front of AD

The function of R25 is anti-reflection, the 5th and 6th corners of the op amp are theoretically the same voltage, and the input impedance is infinite! Then the current of the input signal mainly flows into the ground through R28, that is, the voltage of the input point is at WK-in If the point is formed, theoretically, no current will flow into R25. If there is no R25, the signal will be 100 % reflected to the WK-in. If the internal resistance of the signal source is very large, that is, the ability to carry the load is very poor, and the reflected signal A strong emission oscillation will be formed near the input point of R28, that is, the noise such as "echo" will be amplified and the quality of the output signal will be poor. The connection of R25 and C12 can effectively absorb the reflected signal at the 5pin. The high-frequency reflected signal is discharged to ground (AGND) through C12. R25 blocks the reflected signal at the 5pin input end.

So why is R25 20K ? This may be an empirical value. If R25 is too large, it will affect the signal strength of 5pin. After all, the op amp is not ideal, and it will also reflect a large number of signals. If it is small, it will not block the reflected signal like a wire. . Usually it will take 2-3 times of R28. The selection of R28, R25, and R27 is related to the working impedance of the operational amplifier. D2, D3 electrostatic clamp, 100ohm resistance is not impedance matching! Usually circuits have internal resistance, the general internal resistance of general digital circuits is about 100ohm, that is, in the case of VCC 5V, the maximum output current looks like 50mA, so A 100ohm resistor is often used in the circuit to de-reflect. The maximum output power in such a circuit is impedance matching! The current is also the largest.

2. Skillfully use LM324 op amp to build a voltage follower

Let's first make a simple sketch with the LM324 voltage follower, the picture is as follows:

The above circuit diagram actually shows that the im324 voltage follower needs very professional electronic knowledge to complete the design of the circuit. In the following introduction in this article, you can see that when the signal is within 10K (-3DB), the characteristics are OK, 10k After that, the op amp characteristics dropped sharply. cause waveform distortion. . . In addition, the slew rate of this op amp is 0.3V/us. When the input signal VPP is 10MS, the output is amplified by 1000 times, and its peak value is 5V. By SR=2πf*v. Available f is around 10K. Once again, the above problems are explained. If the voltage board tests BG, this will not pass as shown in the figure:

The voltage diagram of this lm324 voltage follower has the characteristics of high internal frequency compensation DC voltage gain (about 100dB), wide power supply voltage range: single power supply (3-32V) dual power supply (±1.5-±16V) OPA637 , as for the parameters Needless to say, you can tell the difference by looking at the price. The amplifier circuit made feels very simple, and the effect is also very good.