What is an Op-Amp? Operations Amplifier Circuits Explained

What is an Op-Amp?

You’ve probably heard the term “op-amp” thrown around in electronics jargon, but what are these components? Operations amplifiers — op-amps for short, are integrated circuits, constructed mostly out of transistors and resistors. These integrated circuits multiply an input signal to a larger output. You can use these components with voltage and current in both DC and AC circuits. 

Karl D. Swartzel Jr. invented the first op-amp in 1967, and he originally conceived them to do mathematical operations in analog computers — thus the “operation” part of their name. We now use op-amps in many other applications, and they form the basis of many modern analog electronic circuits.

What Does an Op-Amp Do?

At their most basic, an op-amp takes a differential signal — the voltage difference between the V+ and V- pins — and outputs a voltage proportional to this difference through the Vs+ and Vs- power supply. You can see the Vs+ and Vs- power supply in the image below. Many simplified representations of this component omit the Vs+ and Vs- nodes and only show the V+, V-, and Vout pins.

0918_Opamp2_3

Fig 1: Op-Amp circuit diagram symbol

This open loop operation typically results in a device gain (known as the open loop gain or AOL) of 100,000 or more. Even a tiny difference in the voltage on the non-inverting (+) and inverting (-) pins works out to an output of nearly the supply voltage when the + input voltage is greater than the — input. This configuration acts as a comparator, turning a potentially varying input signal to a steady on/off output.

Closed-Loop Op-Amps

We usually use op-amps in a closed-loop configuration, with the output voltage feeding back (as feedback) into the inverting input to form a more controllable signal amplification. The simplest way to accomplish this is to use a buffer circuit, where the output feeds back into the inverting input with no resistors or other components.

To understand how this operation works, here are the two op-amp golden rules:

1. The output attempts to make the voltage difference between the inputs zero

2. The inputs draw no current

Here’s how to construct a closed-loop op-amp:

1. Feed the input voltage into the + input

2. Connect the – to the amplifier’s output

3. The output should go to the same value as the + input to keep both equal

This configuration can be useful for weak signals that require an amplified current before triggering another device.

0918_Opamp3_2

Fig 2: Op-amp with a feedback loop and voltage divider

In the image above, we took the closed-loop configuration concept further. If you want the output voltage to be a different value than the input, add a pair of resistors to form a voltage divider for the feedback loop. Amplification is thus based on the voltage we see at the node between these two resistors, calculated by the following formula:

Vin- = Vout * Rg / (Rg + Rf)

By this formula’s logic, we can state the following:

Vout = Vin- * (Rg + Rf)/Rg

 Vout = Vin- *(1 + Rf/Rg)

The 1 + Rf/Rg term is the closed-loop gain (ACL) of the circuit. If the resistors stay the same as Vin increases or decreases, Vout will vary proportionally by a factor of ACL up to the supply voltage.

Other configurations are also available, including feedback to the inverting pin, and using a voltage divider circuit to allow an op-amp to provide negative and positive voltage.

Op-Amp Differences

Beyond being able to connect and use op-amps in different ways, you can select op-amps with a variety of specifications to fit your application, including variations on:

- Offset voltage

- Maxim supply voltage

- Gain-bandwidth product

You can find a wide variety of other op-amps available in different packages and with different specifications. Take, for example, the LTC2063 from Analog Devices, a low-supply current op-amp available in a variety of packages -- great for a variety of applications.

Related news articles

Latest News

Sorry, your filter selection returned no results.

We've updated our privacy policy. Please take a moment to review these changes. By clicking I Agree to Arrow Electronics Terms Of Use  and have read and understand the Privacy Policy and Cookie Policy.

Our website places cookies on your device to improve your experience and to improve our site. Read more about the cookies we use and how to disable them here. Cookies and tracking technologies may be used for marketing purposes.
By clicking “Accept”, you are consenting to placement of cookies on your device and to our use of tracking technologies. Click “Read More” below for more information and instructions on how to disable cookies and tracking technologies. While acceptance of cookies and tracking technologies is voluntary, disabling them may result in the website not working properly, and certain advertisements may be less relevant to you.
We respect your privacy. Read our privacy policy here