Operational Amplifier: How Does It Work?

In summary, an Op-Amp is designed to amplify the input voltage differential with very little current flow through the inputs. The inputs are designed to be high impedance, allowing for a large power amplification. This "zero current" is an acceptable approximation and does not affect overall performance. However, some input current may be needed and can affect output errors, particularly if a large valued feedback resistor is used. The input differential is small but not zero, and a good text on op amps can provide more detailed explanations. The input current is not parasitic, as it is necessary for the voltage differential to develop, but good devices have low input currents and can produce minimal errors when designed correctly.
  • #1
ali297
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Hi, I have a quick question regarding Operational Amplifier's. I understand in an ideal Op-Amp the current through the + and - input terminals should be zero. What is the point of the Op-Amp if no current goes through the inputs? How does it work?

thanks
 
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  • #2
An Op-amp is designed to amplify the input voltage differential. Only the input voltage is relevant, any input current flow is purely parasitic.

Op-amps have separate power supply terminals (which are not always shown on all schematics) to provide the current (and power) that the Op-amp requires. Current through the input terminals is neither required or desirable.
 
  • #3
The "zero current" is not really zero. It means that the amplifier takes very very little current; a negligible amount compared with the current in the surrounding components. The op amp inputs are designed to be very high impedance and there is a lot of Power Amplification. (Not lots of Watts - just a massive gain from a really tiny signal to a moderate one)

There are many instances where, in order to design a circuit, a sub-circuit (perhaps a chip) can be assumed to have infinite or zero resistance. This is an acceptable approximation and gives the right answer for overall performance. It's a 'trick' that works.
 
  • #4
Some input current is needed, but if the source driving the op amp has a reasonably low output impedance, the op amp input current will not be a problem. It can be a problem if a large valued feedback resistor is employed. High values of input bias current can result in output errors.

Also, the input differential is quite small as well, but not zero. A good text on op amps can explain this in detail. I recommend reading peer-reviewed texts, then ask a specific question here if a point made in the text needs clarifying.

By the way, the input current is not "parasitic". The voltage differential could never develop unless charges are transported through the b-e junctions. Current is needed to obtain the diff voltage. But good bjt devices have a low input current. When the circuitry is designed right, the input current produces an error not too severe.
 

1. What is an operational amplifier (op-amp)?

An operational amplifier, also known as an op-amp, is an electronic circuit component that is used to amplify and process electrical signals. It has a high gain, meaning it can increase the amplitude of an input signal by a large factor. Op-amps are commonly used in a wide range of electronic devices, including audio equipment, signal processing circuits, and instrumentation.

2. How does an op-amp work?

An op-amp typically consists of three main components: an inverting input terminal, a non-inverting input terminal, and an output terminal. It also has two power supply terminals for providing positive and negative voltage. The input signals are applied to the inverting and non-inverting terminals, and the op-amp amplifies the voltage difference between these two inputs. The amplified output signal is then sent to the output terminal.

3. What is the ideal op-amp model?

The ideal op-amp model assumes that the op-amp has an infinite gain, infinite input resistance, and zero output resistance. It also has no bias current, no input offset voltage, and no output offset voltage. In reality, op-amps do not behave exactly like the ideal model, but they are designed to be as close to ideal as possible.

4. What are the different op-amp configurations?

There are three commonly used op-amp configurations: inverting, non-inverting, and differential. In the inverting configuration, the input signal is applied to the inverting terminal, and the output is inverted. In the non-inverting configuration, the input signal is applied to the non-inverting terminal, and the output is non-inverted. In the differential configuration, two input signals are applied to the inverting and non-inverting terminals, and the output is the difference between these two signals.

5. What are some common applications of op-amps?

Op-amps have a wide range of applications, including audio amplifiers, filters, oscillators, comparators, and voltage regulators. They are also used in instrumentation and control systems, such as in feedback circuits for stability and precision control. Op-amps are essential components in many electronic devices and play a crucial role in signal processing and amplification.

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