What is input and output impedance?

There will be resistors in the circuit, so do you know what input and output impedance is? I am always confused and not very clear about what input and output impedance is. Today I will share what an expert has summarized about input and output impedance. One thing?

Input impedance is the ratio of input voltage to current, that is, Ri = U/I. Under the same input voltage, if the input impedance is very low, a larger current needs to flow, which will test the current output capability of the front stage; and if the input impedance is very high, then only a small current is required. This reduces a lot of burden on the current output capability of the front stage. Therefore, try to increase the input impedance in circuit design.

Let’s talk about the output impedance. It can be regarded as the internal resistance r at the output end, which can be equivalent to the series connection of an ideal signal source (power supply) and this internal resistance r. Combining it with the input impedance of the lower-level circuit, it is equivalent to a loop composed of an ideal signal source (power supply), internal resistance r and lower-level input impedance Ri. The internal resistance r will play the role of voltage divider in the loop. The larger r is, the greater the voltage will be allocated to it, and the smaller voltage will be allocated to the lower-level circuit; conversely, the smaller r will be, the greater the voltage will be allocated to the lower-level circuit, and the higher the efficiency of the circuit. Therefore, of course, it is better to design the output impedance r as small as possible.

Looking back, since the larger the input impedance, the better, wouldn’t it be best if we try to design it to be very large? Otherwise, when the input impedance is very large, the loop current will be very small, and In actual circuits, the current path is easily disturbed (crosstalk from other signals, or electromagnetic radiation from the air). At this time, as long as a small disturbance is superimposed on the loop current, it will seriously interfere with the signal quality. Therefore, unless it can be ensured that the signal is well shielded from external interference, the input impedance should not be designed to be too large. It is said, it is said that the input impedance is generally designed to be 47K. Of course, anything close to this value of tens of K should be fine~

The person said that the input impedance of the device I chose is very small, or the output impedance is very large, what should I do? This is simple, just add a voltage follower before the input or after the output.

I have to add that what I said earlier refers to the voltage signal, and the current signal needs to be looked at the other way around. If it is a current signal (current source), then the smaller the input impedance of the next stage, the smaller the load of the previous stage; and the larger the output impedance of the previous stage, the more current will enter the next stage. Rather than being consumed within this level. For the output impedance r of the current signal (current source), it should be equivalent to the ideal current source connected in parallel. The input impedance of the next stage is then connected in parallel to the top. The basic knowledge is not solid, so you should check it in a book. It is required that the output voltage does not change due to load changes, and the output impedance should be as small as possible. It is required that the output current does not change due to load changes, and the output impedance should be as large as possible. Not all situations require the output impedance to be as small as possible.

Output impedance is independent of power.

"Impedance matching" is a very confusing concept in circuits. It is best not to use this concept.

1. Under what circumstances should the input impedance be as large or as small as possible? Why should the output impedance be as small as possible? What is the relationship between output impedance and power?

There are prerequisites for determining the input and output impedance. It is meaningless to say whether it should be as large or as small as possible without preconditions. Generally speaking, if the voltage characteristics are emphasized, higher input impedance and lower output impedance are usually required; correspondingly, if the current characteristics are emphasized, lower input impedance and lower output impedance are usually required. Higher output impedance. Also note that dynamic impedance is usually discussed and DC bias is ignored.

2. What is the relationship between input and output impedance and impedance matching? Should high and low frequencies be considered?

The current and voltage in the circuit are the "right and left arms", and neither one can survive without them. This concept is particularly important when the electrical dimensions (wavelength) and circuit dimensions are similar. For example, in high-frequency circuits, isolated currents and voltages are often replaced by a seemingly special "power wave". In principle, impedance matching is proposed for "power waves".

Although impedance matching must be considered when the electrical size (wavelength) and circuit size are similar (generally set as the wavelength is less than ten times the circuit size), usually only the "wiring" in the circuit - the transmission line is considered. Therefore, matching only considers the connection between devices, that is, the impedance matching between the device output and input, and the device is still regarded as a lumped parameter. Of course, when it comes to the microwave band, the situation may become more complicated.

The smaller the output impedance, the stronger the load capacity, the larger the input impedance, the better the isolation effect from the external circuit. The purpose of impedance matching is to eliminate the influence between various circuit functional modules. Simply put, in a radio frequency circuit, to obtain maximum power, the load impedance and the Thevenin equivalent impedance of the source need to be in a conjugate relationship. In this way, the circuit reactance is zero, the real parts are equal, and maximum power is obtained.

Input and output impedance, usually what we can easily obtain is a voltage source, such as an audio power amplifier circuit, which requires a large input impedance and a small output impedance, so the global negative feedback of the circuit is exclusively voltage series negative feedback. Of course, optical communication applications are often current-type, and the situation is different at this time. In short, the form of negative feedback used is always related to the input and output impedance.

Impedance definition

In a circuit with resistance, inductance and capacitance, the resistance to alternating current is called impedance. Impedance is often expressed as Z. Impedance consists of resistance, inductive reactance and capacitive reactance, but it is not a simple sum of the three. The unit of impedance is ohms. In direct current, the effect of an object on the current obstruction is called resistance. All materials in the world have resistance, but the difference in resistance value is just the difference. Materials with very low resistance are called good conductors, such as metals; materials with extremely high resistance are called insulators, such as wood and plastic. There is also a conductor in between called a semiconductor, and a superconductor is a substance with a resistance value of almost zero. However, in the field of alternating current, in addition to resistance that blocks the current, capacitance and inductance also block the flow of current. This effect is called reactance, which means resistance to current. The reactances of capacitors and inductors are called capacitive reactance and inductive reactance respectively, referred to as capacitive reactance and inductive reactance. Their measurement unit is the same as resistance in ohms, and their value is related to the frequency of alternating current. The higher the frequency, the smaller the capacitive reactance and the larger the inductive reactance. The lower the frequency, the larger the capacitive reactance and the smaller the inductive reactance. In addition, capacitive reactance and inductive reactance also have phase angle issues, which have a vector relationship. Therefore, it is said that impedance is the vector sum of resistance and reactance. For a specific circuit, the impedance is not constant, but changes with frequency. In a series circuit of resistors, inductors, and capacitors, the impedance of the circuit is generally greater than the resistor. That is, the impedance is reduced to a minimum value. In a parallel circuit of inductors and capacitors, the impedance increases to a maximum value at resonance, which is the opposite of a series circuit.

1. Input impedance

Input impedance refers to the equivalent impedance at the input end of a circuit. Add a voltage source U to the input end and measure the current I at the input end. Then the input impedance Rin is U/I. You can imagine the input terminal as the two ends of a resistor. The resistance of this resistor is the input impedance.

Input impedance is no different from an ordinary reactive component. It reflects the amount of resistance to current flow. For voltage-driven circuits, the greater the input impedance, the lighter the load on the voltage source, so it is easier to drive and will not affect the signal source; while for current-driven circuits, the smaller the input impedance, the The load on the current source is lighter. Therefore, we can think of it this way: 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: only suitable for low-frequency circuits, in high-frequency circuits In the circuit, the impedance matching issue must also be considered.) In addition, if the maximum output power is to be obtained, the impedance matching issue must also be considered.

2. Output impedance

Regardless of the signal source, amplifier or power supply, there is always the problem of output impedance. Output impedance is the internal resistance of a signal source. Originally, for an ideal voltage source (including power supply), the internal resistance should be 0, or the impedance of an ideal current source should be infinite. Output impedance requires the most special attention in circuit design. But the voltage source in reality cannot do this. We often use an ideal voltage source connected in series with a resistor r to equate to an actual voltage source. This resistor r in series with the ideal voltage source is the internal resistance of the (signal source/amplifier output/power supply).

When this voltage source supplies power to the load, a current I will flow through the load, and a voltage drop of I×r will occur on the resistor. This will cause the power supply output voltage to drop, thereby limiting the maximum output power (for why the maximum output power is limited, please see the "Impedance Matching" question below). Similarly, for an ideal current source, the output impedance should be infinite, but this is not possible in the actual circuit. The above is the relevant analysis of input and output impedance. I hope it will be helpful to everyone when designing.