Introduction
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.
Most common op-amps have three signal terminals and two power terminals. Signal terminals include an inverting input terminal (Vim), a non-inverting input terminal (Vip), and an output terminal (Vout). 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 common configuration of the operational amplifier is closed loop configuration. A closed-loop configuration of an operational amplifier (op-amp) involves feedback from the op-amp output to the inverting terminal of the operational amplifier. This means there is a connection from the output to the inverting terminal of the op-amp, which can be either a short or a connection through a resistor (or any other component). This allows us to control the gain and other characteristics of the amplifier circuit. The most common closed-loop configurations are the inverting amplifier, non-inverting amplifier, voltage follower, and differential amplifier. These configurations provide specific functions and are widely used in various electronic circuits. In this mode, the concept of virtual shorts is applicable.
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
Properties of common opamps
A simple model can be constructed for an opamp as shown in Fig 4. 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.
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 usable gain. Ideally, it should be infinite.
- High Input Impedance (Zin): Op-amps typically have a very high input impedance, which means they 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.
- Linear Operation: Ideally, op-amps operate linearly within a specified range. This means that the output voltage is directly proportional to the input voltage within this linear region. The linearity of an opamp is characterized by harmonic distortion. Ideally, an opamp should be linear and have zero harmonic distortion.
- Single-ended Output: Op-amps typically have a single output terminal, which provides the amplified version of the input signal.
- Power Supply Requirements: 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.
Basic opamp 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.
Opamp buffer or voltage follower
When the output is connected to the inverting terminal of the opamp, it is in negative feedback. Using the virtual short in negative feedback, we can say that the inverting terminal (and the output terminal) equals the non-inverting terminal.
Inverting gain operational amplifier
We observe that the output (Vout) is connected to the inverting terminal of the opamp using RF. The non-inverting terminal (Vip) is not connected to the output by any means. Instead, it is connected to the ground (zero). This means that the opamp is in negative feedback. Again, using the concept of virtual short in negative feedback circuits, the inverting terminal is also forced to zero. This means that a current Vin/RI flows through the RI resistor. The same current has to flow through the RF resistor. So, the output voltage is Vout=-(RF/RI)Vin.
Non-inverting gain operational amplifier
We again observe that the inverting terminal (Vim) is connected to the output (Vout) using RF, and the non-inverting terminal is not connected to the output by any means. This means a negative feedback configuration. In negative feedback configuration, we can use the concept of virtual short. The opamp forces the inverting terminal to be Vin due to negative feedback. This means a current Vin/RG is flowing through RG. The output voltage can be calculated using KVL. The output voltage (Vout) is Vin(1+RF/RG).
Operational amplifier as comparator
An operational amplifier (op-amp) can be used as a comparator, although it’s not its primary function. A comparator is a circuit component that compares two voltages and outputs a digital 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 of the op-amp to ensure it works properly with your input voltages.
Operational amplifier classifications
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.