Concept of Circuit common sense (1)-input and output impedance

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1. Input Impedance

An input impedance is the equivalent impedance of a circuit input. When a voltage source U is added to the input end and the current I at the input end is measured, the input impedance Rin = u/I. You can think of the input end as the two ends of a resistance. The resistance of this resistance is the input impedance.

The input impedance is no different from a common impedance element. It reflects the current obstruction.

For a voltage-driven circuit, the larger the input impedance, the lighter the load on the voltage source, the easier it is to drive, and it will not affect the signal source.;For a current-driven circuit, the smaller the input impedance, the lighter the load on the current source.. Therefore, we can think like this:If it is driven by a voltage source, the larger the input impedance, the better. If it is driven by a current source, the smaller the impedance, the better.(Note: It is only suitable for low-frequency circuits. In high-frequency circuits, impedance matching must be considered. In addition, if you want to obtain the maximum output power, you must also consider the impedance matching problem .)

2. output impedance

Either the signal source or the amplifier or the power supply have an output impedance problem. The output impedance is the internal resistance of a signal source. Originally,An ideal voltage source (including power supply) with an internal resistance of 0, OrThe impedance of the ideal current source should be infinite.. Special attention should be paid to the output impedance in circuit design.

In reality, voltage sources cannot do this. We often use an ideal voltage source to concatenate a resistor R to equivalent an actual voltage source. The resistance R in connection with the ideal voltage source is the internal resistance of the signal source/amplifier output/power supply.When the voltage source powers the load, there will be a current I flowing from this load, and the voltage drop of I × R will be generated on this resistance. This will cause the power supply output voltage to drop, thus limiting the maximum output power(For more information about why the maximum output power is limited, see "impedance matching "). Similarly, for an ideal current source, the output impedance should be infinite, but the actual circuit is impossible.

3. Impedance Matching

Impedance Matching refersSignal Source or transmission lineAndLoadA suitable matching method.

Impedance MatchingLow frequency and high frequencyDiscussion.

Let's start with a dc voltage source to drive a load. Because the actual voltage source always has internal resistance, we can equivalent an actual voltage source to an ideal voltage source in series with a resistance R. Assuming that the load resistance is R, the power source EMR is U, and the internal resistance is R, we can calculate the current flowing through the resistance R: I = u/(R + r). We can see that,The smaller the load resistance R, the larger the output current. The voltage on the load R is: uo = IR = u/[1 + (R/R)], we can see that,The larger the load resistance R, the higher the output voltage UO. Calculate the power consumption of resistance R as follows:

P = I2 × r = [U/(R + r)] 2 × r = u2 × R/(r2 + 2 × R + R2)

= U2 × R/[(r-r) 2 + 4 × R]

= U2 /{[(R-r) 2/R]+ 4 × R}

for a given signal source, its internal resistance r is fixed, while the load resistance r This is what we choose.

note [(r-r) 2/R] when r = r , [(r-r) 2/R] Minimum value 0 , the load resistance r maximum output power Pmax = u2/(4 × R) . That is, when the load resistance is equal to the internal resistance of the signal source, the load can obtain the maximum output power, which is one of the impedance matching we often call .

This conclusion is also applicable to low frequency and high frequency circuits.When the AC circuit contains capacitive or inductive impedance, the conclusion changes (for maximum output power)That is, the real part of the signal source must be equal to the load impedance, and the virtual part is opposite to each other.. In low-frequency circuits, we generally do not consider the transmission line matching problem, but only the signal source and the load, because the wavelength of the low-frequency signal is very long than that of the transmission line, the transmission line can be regarded as a "short line", and reflection can be ignored (it can be understood that, even if the line is short, the reflection is the same as the original signal ).

From the above analysis, we can conclude that if we need to output a large current, we should select a small load.RIf we need to output a large voltage, select a large load.RIf we need the maximum output power, select the resistance that matches the internal resistance of the signal source.R. Sometimes impedance mismatch has another meaning. For example, some output ends of the instrument are designed under specific load conditions. If the load conditions change, the original performance may not be achieved, this is also called impedance mismatch.

In high-frequency circuits, we must also consider the reflection problem. When the signal frequency is very high, the signal wavelength is very short. When the wavelength is shorter than the transmission line length, the reflection signal overlay the original signal and changes the shape of the original signal. If the characteristic impedance of the transmission line is not the same as the load impedance (that is, the load impedance does not match), reflection is generated at the load end. Why does the resolution of the reflection and feature impedance occur when the impedance does not match? The solution of the second-order partial differential equation is involved. Here we will not elaborate on it, if you are interested, please refer to the transmission line theory in the electromagnetic and microwave books. The Characteristic Impedance of a transmission line (also called the characteristic impedance) is determined by the structure and material of the transmission line, and is independent of the length of the transmission line, as well as the amplitude and frequency of the signal.

For example, the characteristic impedance of the commonly used closed-circuit television coaxial cable is 75Ω In some RF devices, the commonly used feature impedance is 50Ω . Another common transmission line is Characteristic Impedance 300 Ω The flat parallel lines, which are common in television antenna frames used in rural areas, are used as feeder for eight-wood antennas. Because the input impedance of the RF input of the TV is 75Ω , So 300 Ω It will not match. In reality, how does one solve this problem? I wonder if you have noticed that there is a TV attachment 300 Ω To 75Ω (A plastic package with a circular plug at one end, which is about as big as two thumbs ). It is actually a transmission line Transformer 300 Ω The impedance is transformed 75Ω In this way. It should be emphasized that the characteristic impedance is not a concept of resistance that we generally understand. It has nothing to do with the length of the transmission line and cannot be measured using an ohm table. In order not to generate reflection, the load impedance and the characteristic impedance of the transmission line should be equal. This is the impedance matching of the transmission line. What will happen if the impedance does not match? If the signal does not match, reflection will be formed, and the energy transmission will not pass over, reducing the efficiency. A standing wave will be formed on the transmission line (a simple understanding is that the signal is strong in some places and the signal is weak in some places ), as a result, the effective power capacity of the transmission line is reduced; power transmission fails, and may even damage the transmission device. If the High-speed signal line on the circuit board does not match the load impedance, it will produce shock and radiation interference.

When the impedance does not match, how can we make it match? First, you can consider using a transformer for impedance conversion, as in the example in the TV set above. Second, you can consider using series/The method of parallel capacitance or inductance, which is often used in RF circuit debugging. Third, you can consider using series/Parallel resistance. Some drivers have relatively low impedance and can be connected to a suitable resistance to match the transmission line. For example, high-speed signal lines may sometimes concatenate a resistance of dozens of euros. The input impedance of some receivers is relatively high. You can use the method of parallel resistance to match the transmission line. For example,485Bus receiver, often connected in parallel with data cable Terminals120Euclidean matching resistance.

To help you understand the reflection problem when the impedance does not match, Let me give two examples: You are practicing boxing-sandbags. If it is a sandbag with proper weight and hardness, you will feel comfortable. However, if one day I made a sandbag, for example, if it was replaced by Isha, you would still use the previous force to fight it, your hand may be unable to handle it-this is the case of heavy load, which will produce a lot of elastic. On the contrary, if I replace it with something very light, you may leave it empty and your hands may not be able to stand it-this is the case where the load is too light. In another example, I don't know if you have ever had this experience: You just can't see the stairs./When you think there are still stairs, there will be a feeling of "load mismatch. Of course, this example may not be appropriate, but we can use it to understand the reflection when the load does not match.

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Q: What is a current controller?
A: If the output parameter of the device is related to the input current parameter, the device is called a "Current Controller Device ".

The current controller device inputs the current signal, which isLow impedance input requires high driving power. For example, the bipolar transistor (BJT) is a current controller device, and the TTL circuit is a current controller device..

Q: What is a voltage controller?
S: If the output parameter size of the device is related to the input voltage parameter size, the device is called a voltage controller device ".

The "voltage controller" inputs a voltage signal, which isHigh impedance input requires only a small driving power. For example, the field effect transistor (FET) is a voltage controller, and the mos circuit is a voltage controller.

Q: Why is BJT a current controller while FET and MOS a voltage controller?
S: BJT is throughBase CurrentTo control the collector current to achieve amplification, while FET & MOS rely on ControlGate voltageBJT is the current controller, while FET and MOS are the voltage controller.