Opamp Characteristics: Input Current & Output

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SUMMARY

The discussion focuses on the characteristics of operational amplifiers (op-amps), specifically regarding input current and output behavior. It is established that ideal op-amps have zero input current due to infinite resistance and infinitesimal capacitance, allowing them to maintain input voltage without current flow. However, real op-amps, particularly those with FET inputs, exhibit negligible input current while still producing output based on input voltage. The transient current during voltage changes and small steady-state currents due to leakage are acknowledged as practical considerations.

PREREQUISITES
  • Understanding of operational amplifier (op-amp) fundamentals
  • Knowledge of FET (Field-Effect Transistor) input characteristics
  • Familiarity with circuit theory, including resistance and capacitance
  • Basic principles of voltage and current relationships in electronic circuits
NEXT STEPS
  • Research the characteristics of FET input op-amps
  • Study the concept of input impedance in operational amplifiers
  • Learn about transient response in electronic circuits
  • Explore the impact of leakage currents in real-world op-amp applications
USEFUL FOR

Electronics engineers, students studying circuit design, and anyone involved in the design and analysis of operational amplifier circuits will benefit from this discussion.

kimmy510
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ideally input current of an opamp is 0. If input current is zero then how is that the opamp can give an output?
is it that in a real opamp a very small quantity of current flows?
 
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You are correct. However, there are op-amps with FET inputs which have virtually no input current.
 
Let me also comment...

The output is a function of the input voltage. The idealized Op Amp has infinite resistance and infinitesimal capacitance at the input so no current is required to bring it up or sustain a given input voltage. In reality you allow for positive capacitance (the FET gate capacitance e.g.) and you will have a short transient current as the voltage changes. Allow for finite resistance and you have a small steady state current to compensate for this leakage.

But in principle these can be as small as your like within the limits of your resources to build the OpAmp with the desired characteristics. There is no reason (in the classical world) to think current must flow for the OpAmp to read the input voltage and establish a proportionate output.
 

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