Is source impedance the input resistance of the op amp?

乱世煮酒论天下

Is source impedance the input resistance of the op amp? [Copy link]

Let's look at two definitions first: The input resistance parameter Ri is defined as the DC resistance between the two input terminals when either input terminal is grounded (of course, there are distributed parameters at high frequencies). The input resistance is the resistance between the two input terminals, but one of the input terminals is required to be grounded. The input resistances of the two input terminals are recorded as Rp and Rn respectively.

The differential input resistance, Rid, is defined as the small signal resistance between two ungrounded input terminals. When the signal source impedance is high, the input impedance becomes a design issue because the input is a load to the signal source.

In an op amp circuit, when the input terminals are connected to gnd, a voltage will be generated at the output. According to the characteristics of an ideal op amp, when the input terminals are connected to the same potential, there should be no voltage output at the output terminal. In fact, there will be input offset current at the op amp input terminals, and the two input terminals will generate input bias current. Assume that the positive input offset current is 40pA and the negative input offset current is 60pA. It is precisely because the input resistance values of the two input terminals are different that there will be voltage at the output terminal, so it is necessary to choose an op amp with consistent input impedance as much as possible. It can be seen that the direct manifestation of the bias current of the op amp is the output voltage, and the output voltage is related to the input resistance Ri, so does the source impedance of the op amp refer to the input resistance Ri?

maychang

[Is the source impedance the input resistance of the op amp? ]

You seem to be mixing up several unrelated things. The offset voltage and offset current of an op amp are not the same thing, and the offset current and bias current are not the same thing. The source impedance of an op amp is not the same thing as the input resistance of an op amp.

maychang

Maybe you taught yourself electronics, and not in the order it should be.

In another thread, you asked "Are phase compensation and frequency compensation for closed-loop systems or open-loop systems?", while in this thread you asked about offset voltage and offset current, etc. Offset voltage, etc. should be something you learn long before learning about frequency compensation.

It is recommended not to jump around when studying electronic technology textbooks, and try to study in sequence.

maychang

[Is the source impedance the input resistance of the op amp? ]

Source impedance is the internal resistance of the power supply (power supply includes those with relatively high internal resistance and those with relatively low internal resistance, including DC and AC power supplies, including those that mainly provide energy and those that mainly provide signals). The input resistance of the op amp circuit is the load of the signal source.

乱世煮酒论天下

maychang published on 2024-6-21 14:37 [Is source impedance the input resistance of the op amp? ] Source impedance is the internal resistance of the power supply (speaking of power supply, including those with relatively high internal resistance and relatively low internal resistance, including DC and...

Which power supplies have relatively high internal resistance and which have relatively low internal resistance? Is it relatively low if it is close to an ideal voltage source, and relatively high if it is close to an ideal current source? I also saw an introduction to the output port of a signal source in the introduction to the use of an oscilloscope. The typical value of the signal source internal resistance is 50 ohms.

maychang

Talking about the world in a chaotic world published on 2024-6-21 21:46 Which power supplies are considered to have relatively high internal resistance and which are considered to have relatively low internal resistance? Is it close to an ideal voltage source? Is it close to an ideal current source?

It can be said that if the output voltage is marked, the internal resistance is relatively low, and if the output current is marked, the internal resistance is relatively high. For example, ordinary dry batteries are always marked with a nominal voltage of 1.5V, and the internal resistance of dry batteries is relatively low (the internal resistance of dry batteries will become higher after they are used up).

maychang

Talking about the world in a chaotic world published on 2024-6-21 21:46 Which power supplies are considered to have relatively high internal resistance and which are considered to have relatively low internal resistance? Is it close to an ideal voltage source? Is it close to an ideal current source?

The internal resistance of an ideal voltage source is zero, and the internal resistance of an ideal current source is infinite. Of course, the internal resistance of a power source close to an ideal voltage source is relatively low, and the internal resistance of a power source close to an ideal current source is relatively low.

maychang

Talking about the world in a chaotic world published on 2024-6-21 21:46 Which power supplies are considered to have relatively high internal resistance and which are considered to have relatively low internal resistance? Is it close to an ideal voltage source? Is it close to an ideal current source?

[I also saw an introduction to the output port of the signal source in the introduction to the use of the oscilloscope. The typical value of the signal source internal resistance is 50 ohms]

The internal resistance of the signal source is 50 ohms. This is a high-frequency signal source. The internal resistance of 50 ohms is for use with coaxial cable.

The internal resistance of a low-frequency signal source may be 600 ohms or several ohms. The internal resistance of a low-frequency signal source requiring power output may be as low as less than 1 ohm.