op-amps

Op-amp is a shortening of the words operational and amplifier. The word operational is used because they were first used to perform mathematical operations!


An op-amp is a DC-coupled, high-gain, voltage amplifier, with a differential input. Let’s break that down, to understand what this means.

DC-coupled

DC-coupled means that the op-amp can work down to frequencies of 0 Hz, as there are no internal capacitors between stages, which would make it AC-coupled. This means that the op-amp can still produce gain at 0Hz.

High-gain

Gain is the ratio between the signal level at its input and that of its output. Therefore, high-gain means that a small signal at its input can cause a large change at its output. It can use a small voltage difference at its input to control a larger voltage at its output. Sometimes 100,000s of times larger. In this way an op-amp is configured to produce an output voltage (relative to ground) that can be hundreds of thousands of times larger than the potential difference between its inputs.

Voltage amplifier

The voltage at the output is a function of the voltage between its two inputs. Therefore it is known as a voltage amplifier.

Differential input

Differential input means that the op amp responds to the difference between its two inputs. This is in contrast to single-ended signalling, which responds to the difference between an input and ground.


Symbol

Figure 1 shows the symbol for an op-amp. There are two inputs, one output and power supply connections. The input labelled + is called the non-inverting input. The input labelled – is called the inverting input. One of the power supply connections is for connecting to + and the other to -. op-amps are often connected to a supply voltage that has a + and – rail, as opposed to + and ground. It is however possible to configure an op-amp to work of such a power supply.

Figure 1 op-amp symbol

The two DC inputs usually determine the limit of the output. They may connect to positive voltage and ground or matching + and – power supplies. One way to achieve this is to connect a voltage divider made of resistors between a power supply and the op amp. On its own the two voltages at the input are compared (v- – v+ = vdiff) and that determines if the output saturates towards the + or – supply voltage. If the op amp is connected to a 9v power supply. If the voltage at the + input is larger, the output would go towards the + power supply voltage. If the voltage at the – input is larger, the output would go towards ground. When the output reaches either of these points, it reaches saturation and is at the maximum rated gain.

You can calculate the differential input voltage by using this formula:

Vout = AOL(V+ – V-)

AOL is the open-loop gain of the op-amp, which means that there is no feedback loop present.


Ideal vs real


Construction

Internally op-amps are mainly made up of transistors and resistors, see Figure 2. A typical opamp has two inputs (inverting and non inverting), two DC power leads (+ and -), output terminal and sometimes a few other specialised leads used for fine tuning.


Figure 2 Circuit diagram of a 741 op-amp

Open and closed loop

Open-loop amplifier

The magnitude of the open-loop gain is usually very high and therefore even a small difference between V+ and V- drives the output near to the supply voltage. In a situation where the output voltage is equal to or greater than the supply voltage is referred to as saturation of the amplifier. During the manufacturing process it is difficult to control the exact magnitude of the open-loop gain and therefore makes an open-loop amplifier impractical as a differential amplifier! Without any negative (and sometimes positive) feedback, op-amps act as comparators. If the inverting input is held at ground and the input voltage applied to the non-inverting input is positive, the output will be at the maximum positive. If the input voltage is negative, then the output will be at the maximum negative.

Closed-loop amplifier

To enable an op-amp to act in a predictable manner, we used negative feedback. The introduction of negative feedback allows control of the circuit’s gain, instead of using the op-amps characteristics. By using negative feedback, the characteristics of the circuit are determined by external components, having little effect from the op-amp. The op-amp amplifies the difference in voltage between the two inputs. This is called the differential input voltage.


Other uses

Op-amps were originally used in analogue computers, to perform mathematical operations.

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