What is an Operational amplifier (Op-Amp) ?
Op Amp is a short name for operational amplifiers. It amplifies the difference between two input terminals and forces the output terminal to reflect the amplified voltage. In electronics, it is used for addition, subtraction, integration, differentiation, logarithms, gain, buffer (to amplify power), etc. That is how the name “operational” is justified.
Operational Amplifier Symbol
Most common op-amps have two input terminals (inverting and non-inverting terminal), one output terminal and two power terminals. Power terminals are VCC and VEE. The simplest form of relationship between the input and output is:
Op-amps are a fundamental building block in analog electronic design, and their versatility and predictable behavior make them essential in applications ranging from audio amplification to instrumentation and control systems.
Mode of operation of an opamp
An opamp can be configured in two ways: Comparator mode and Feedback mode. The most common mode is feedback mode. Details are mentioned below:
Closed loop or Feedback mode
The most widely used operational amplifier setup is the closed-loop configuration. In this arrangement, a feedback path connects the op-amp’s output to its inverting input. This feedback link can be a direct short or through a resistor. By using feedback, the amplifier’s gain and other characteristics can be precisely controlled. Common closed-loop types include the inverting amplifier, non-inverting amplifier, voltage follower, and differential amplifier—each serving distinct functions in electronic circuits. In this mode, the principle of the virtual short applies.
Open loop / comparator mode
An operational amplifier (op-amp) can be used in a comparator mode when it is configured to compare two input voltages and produce a digital output based on the comparison. In this mode, the op-amp’s output will typically be in one of two states, either high (usually close to the positive supply voltage, VCC) or low (usually close to the negative supply voltage, VEE). This mode is commonly referred to as open-loop operation, and it’s different from the typical linear amplifier mode where op-amps are used with negative feedback to amplify signals with gain.
Pinouts of popular opamps
Model of an operational amplifier
A simple model can be constructed for an opamp as shown above. Both the inputs see Zin as input impedance. The output is determined by a voltage-controlled voltage source (VCVS). The output of the VCVS is determined by the difference of Vip and Vim. Also, it offers a gain of A. An output impedance Zout is added in the series of the VCVS.
Operational Amplifiers Parameters
Here are some key characteristics and features of operational amplifiers:
- High Gain (A): Op-amps have a very high voltage gain, typically in the range of 10,000 to 100,000 or more. This means that even small input voltage differences can result in large output voltage changes. Ideally, an opamp should have infinite gain.
- Differential Inputs: Op-amps have two input terminals, often referred to as the non-inverting (+) and inverting (-) inputs. The output voltage is proportional to the voltage difference between these two inputs.
- Bandwidth, (BW) – In opamps, the bandwidth is the frequency till the opamp has gain greater than unity. Ideally, it should be infinite.
- High Input Impedance (Zin): Op-amps typically have a very high input impedance, which means the input terminals draw very little current from the input sources, making them suitable for connecting to various sensors and signal sources. Ideally, opamp should have infinite input impedance (drawing zero current from source).
- Low Output Impedance (Zout): Op-amps usually have a low output impedance, allowing them to drive low-impedance loads such as speakers or other amplification stages without output voltage division/degradation. Ideally, an opamp should have zero output impedance.
- Common mode rejection ratio (CMRR): An ideal opamp amplifies only difference voltage while having zero gain for the common mode input. In reality, there is a very small gain for common mode inputs. Common mode rejection ratio is the ratio is the ratio of differential gain over common mode gain. It should be as high as possible. In ideal opamps, it should be infinite.
- Power Supply: Op-amps require a dual power supply (positive and negative voltage) or a single-supply voltage, depending on the specific op-amp design. The power supply voltage range is usually specified in the datasheet.
Golden Rules of Operational Amplifiers
- Op Amp tries to keep both the inputs voltage same (virtual short)
- No current flow into the input terminals.
Operational Amplifier Configurations
Op-amps can be used in a variety of configurations, such as inverting, non-inverting, voltage followers, integrators, differentiators, and more. The inverting and non-inverting configurations provide gain at the output. Buffer configurations are used to isolate the input circuit from the output circuit without providing any gain. It is also called power amplifier configuration.
Opamp buffer or voltage follower
In voltage follower configuration, the output is connected to the inverting terminal of the opamp. Therefore it is in negative feedback. Using the golden rules of opamp, we can say that the inverting terminal (and the output terminal) equals the non-inverting terminal. Therefore the output voltage follows the input voltage. This configuration is used for impedance transformation. Due to zero output impedance of opamp, the signal can be applied to a heavy load (low value of resistance/impedance).
Inverting gain operational amplifier
In inverting gain configuration the output (Vout) is connected to the inverting terminal of the opamp using RF. The non-inverting terminal (Vip) is connected to the ground (zero). The opamp is in negative feedback. Using the golden rules of opamps, the inverting terminal is becomes zero. Using KCL, the output :
$$V_{out}=-\cfrac{R_F}{R_I}V_{in}$$
Non-inverting gain operational amplifier
In non-inverting gain configuration, the inverting terminal is connected to the output (Vout) using RF establishing negative feedback. The non-inverting terminal is connected to the input. Using golden rules of opamps, the inverting terminal’s voltage is same as the non-inverting terminal (input). Using KCL, the output can be expressed as:
$$V_{out}=-\left(1+\cfrac{R_F}{R_G}\right)V_{in}$$
Operational amplifier as comparator
An operational amplifier (op-amp) can be used as a comparator, although an opamp is suboptimal for comparator applications. A comparator compares two voltages and outputs a high or low signal, indicating which is larger.
Remember, when using an op-amp as a comparator, make sure its slew rate is sufficient for the application to ensure fast switching. Also, consider the common-mode input voltage range and maximum input differential voltage of the op-amp to ensure it works properly with your input voltages.
Operational amplifier classifications
In real life, opamps have lot of non-idealities. To achieve some parameters better, some other parameters are traded-off. For example, to achieve the best speed to power ratio, input bias current is allowed by using BJT based opamps.
Classification based on Technology:
- Bipolar Junction Transistor (BJT) Op-Amps: These op-amps use bipolar junction transistors as their active elements. They are known for their high-bandwidth to quiescent current ratio, low offset voltage, and good linearity with manageable input impedance. These are able to provide high currents at the load. The input bias current is non-zero.
- Junction Field-Effect Transistor (FET) Op-Amps: FET-based op-amps use field-effect transistors as the amplifying devices. They are known for their high input impedance and low input bias currents. They offer higher bandwidth than CMOS opamps for the same Quiescent current. However, in comparison to BJT opamps, these take more Quiescent current and are costlier. The input bias current is zero.
- CMOS Op-Amps: Complementary Metal-Oxide-Semiconductor (CMOS) op-amps use CMOS technology, making them suitable for low-power applications. They typically have very high input impedance and bandwidth (because of transistor scaling). However, the load current capability is lower in CMOS opamps than in BJT opamps for the same power consumption. The input bias current is zero.
Classification based on Functionality:
- General-Purpose Op-Amps: These are versatile op-amps designed for a wide range of applications. Examples include the LM741 and LM324.
- Precision Op-Amps: Precision op-amps are characterized by low offset voltage, low drift, and high common-mode rejection ratio (CMRR). They are used in applications where accuracy and stability are critical.
- High-Speed Op-Amps: These op-amps are designed for high-frequency applications, such as in video amplifiers and RF circuits. They have fast response times and wide bandwidths.
- Low-Noise Op-Amps: Low-noise op-amps are designed to minimize electrical noise and are commonly used in audio and sensor applications.
- Rail-to-Rail Op-Amps: These op-amps are designed to operate with input and output voltages that can approach the power supply rail voltages, making them suitable for single-supply applications.
- Current Feedback Op-Amps: These op-amps use a current feedback architecture instead of voltage feedback, which can provide high bandwidth and fast transient response.
References
- A very nice article compiled by ADI – Op Amp History