Op Amp Basics: Why Does It Not Input Current?

In summary, the op amp is easy to learn because the input voltage must be zero due to its nearly infinite amplification. However, the input bias current, though small, must be taken into account when choosing resistor values for the surrounding circuit. Op amps existed before solid state electronics and are still used today, even in satellites. The high input resistance is achieved through negative feedback in the emitter follower building block circuit, also known as the source follower or cathode follower.
  • #1
Harrison G
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Ah, the op amp... so easy to learn. Yet i have a question:
Why the op amp does not input any current? Is it made by some sort of CMOS technology?
 
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  • #2
The ease of learning in my past (long ago) was in remembering that the input voltage has to be zero because the amplification should be considered nearly infinite...
And that was "all I needed to remember". (so I did :smile:)
 
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  • #3
Find an opamp datasheet
look for parameter "Input Bias Current"
you'll find it small but nonzero.
The circuit designer must be aware of its magnitude when he chooses resistor values for the circuit with which he intends to surround his opamp. He must not use resistor values large enough that "input bias current" would cause more than a negligible voltage drop .

http://www.st.com/web/en/resource/technical/document/datasheet/CD00000490.pdf
upload_2016-3-7_5-6-35.png
 
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  • #4
Harrison G said:
Is it made by some sort of CMOS technology?

Op amps existed before solid state electronics back in the paleolithic age of vacuum tubes.
 
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  • #5
Thanks all for the feedback :-D
 
  • #6
gleem said:
Op amps existed before solid state electronics back in the paleolithic age of vacuum tubes.
Vacuum tubes are used in satellites in the form of traveling wave tubes.
Regarding the input resistance, the building block circuit used in an op-amp is the emitter follower, so it gives a high input resistance due to negative feedback. Emitter follower/source follower/cathode follower have the same action.
 

1. What is an op amp and how does it work?

An op amp, short for operational amplifier, is an electronic component that amplifies the difference between two input voltages. It consists of a differential input stage, a gain stage, and an output stage. The inputs are connected to the differential input stage, which amplifies the difference between them. The gain stage then amplifies this difference, and the output stage provides the final amplified output signal.

2. Why does an op amp have a high input impedance?

An op amp has a high input impedance because it is designed to have minimal impact on the circuit it is connected to. This means that it draws very little current from the input signal, allowing the signal to pass through without being affected. This high input impedance also allows for a wide range of input voltage levels to be accurately amplified.

3. How does an op amp maintain a virtual ground at its inverting input?

The inverting input of an op amp is connected to ground through a feedback resistor. This creates a voltage divider between the inverting and non-inverting inputs, causing the inverting input to maintain a virtual ground. This means that the voltage at the inverting input will be equal to the voltage at the non-inverting input, which is typically set to ground.

4. What is the purpose of a bias current compensation circuit in an op amp?

Bias current compensation circuits are used in op amps to minimize the input current that flows into the inputs. This is important because any input current can cause an error in the output voltage, especially in high-impedance circuits. The bias current compensation circuit typically uses a feedback loop to cancel out any input current, resulting in a more accurate output voltage.

5. How does an op amp handle common-mode input voltage?

An op amp is designed to have a very high common-mode rejection ratio (CMRR), which means it can effectively reject any input voltage that is common to both inputs. This is achieved by the differential input stage, which amplifies only the difference between the two inputs. Any common-mode input voltage is then rejected by the gain stage, resulting in a very small output voltage. A high CMRR is important for minimizing noise and interference in the output signal.

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