Circuit parameters of real op amp

bigbat

Circuit parameters of real op amp [Copy link]

This is a voltage follower circuit. The circuit input is 0-10V (10K ohm variable resistor), the power supply is 0-15V (single power supply), and the op amp is LM358. My friend said this is the worst op amp on earth!

The question I want to ask today is to figure out what is wrong with this chip?

Question 1: Does the output of the circuit meet the design requirements? In other words, is the output between 0-10V?

Question 2: When the input is 0V, is the output voltage 0V, or is it a few uV or mV? And are all circuits similar?

Question 3: What is the minimum supply voltage? Is 12V OK? What is the minimum operating voltage of LM358?

Question 4: Is the output of the circuit linear? In other words, is the chip's amplification stable for input voltages between 0-10V?

Question 5: What is the output power of the circuit? How do you calculate the current that the circuit can output?

maychang

Question 1 and question 2 are the same thing. The output of this circuit cannot reach zero, the minimum is about tens of mV. As for whether it meets the design requirements, we don't know whether it meets the requirements when the circuit author designed it.

maychang

"Question 3: What is the minimum supply voltage? Is 12V OK? What is the minimum operating voltage of LM358? "

The maximum output voltage of LM358 is usually 2.5 to 3V lower than the power supply voltage when the load is light (the load resistance is large).

maychang

"Question 4: Is the output of the circuit linear? In other words, is the chip's amplification stable for input voltages between 0-10V?"

If the power supply voltage is a single 15V power supply, then except for the section where the input is close to zero (about tens of mV to one hundred mV), its linearity is relatively good, and the input and output usually differ by several mV.

maychang

"Question 5: What is the output power of the circuit? How do you calculate the current that the circuit can output? "

The maximum power that this circuit can output depends on the load resistance. However, the load resistance cannot be less than several hundred ohms, otherwise it will be limited by the output current of the op amp.

For the current that this circuit can output, see the op amp datasheet.

maychang

"My friend said this is the op amp with the worst parameters on earth!"

It is not the op amp with the worst parameters. There are worse ones than LM358. But LM358 is not far from the worst.

Gen_X

@ maychang's answer is very good! Support!

In addition: The first purpose of the op amp is to meet the application. Performance comes with cost issues.

Since 358 is a medium-speed, low-power op amp with a wide range of applications, its price/performance ratio is very good and its performance is not too bad.

In addition, it is sufficient for occasions that do not require zero-point offset, common-mode resistance, and low temperature requirements.

The same type of op amp, such as LM358, can also have very different performances:

Some can even only be used in AC amplification situations. DC stability, linearity and temperature resistance all need to be tested, otherwise scrap is inevitable.

Some (see suffix) have very good performance and good interchangeability.

Learning never ends! Let’s make progress together!

bigbat

Thanks @maychang , if the power supply in the circuit is changed to ±15V, when the input is 0V, is the output voltage closer to 0V than when it is +15V?