Types of Op amp: Classification on Feedback Topology & Operation

By Neeraj Dubey|Updated : September 13th, 2022

Operational Amplifiers, also referred to as Op amp, are one type of differential amplifier that amplifies the difference of the signals provided at its input. There can be multiple classifications of types of Op amp based on their configuration, input terminal, and operation. There is also a distinction between different ICs of Op amp according to their designs and characteristics, which are taken care of in the manufacturing process.

We will review various classifications, see how they affect the operation, and briefly discuss their performance and response. First, one should know the basics of operational amplifiers and their characteristics for a better understanding. Each Op amp type has implications and scope of applications utilized for common operational and electrical applications.

Table of Content

What are the Different Types of Op amp?

Types of Op amp means discussing different configurations and arrangements that operational amplifiers can be connected to get the desired application. The classification is generally done on two basis-

  • Based on Feedback Topology
  • Based on Operation

Types of Op amp Based on Feedback Topology

Feedback is the influence of output on the input of a system. Based on the type of feedback connection, the system response can be varied, and even the system's stability can be altered. There can be two types of feedback- positive and negative. The positive feedback is related to oscillatory circuits, and the negative feedback is by the amplifier circuits, which are generally damped and stable systems. Types of Op-amp related to feedback can be categorized as follows:

Op amp with no Feedback

Op amp without feedback element or path does not receive any correction and continues to work as a basic comparator circuit. The input difference will drive the response to a positive or negative saturation due to a very high open loop gain. The response time and slew rate are both low for comparators, making them suitable for digital outputs.

Op amp with Positive Feedback

A positive feedback op amp tends to remain in whichever output state it is already in. It latches between saturated positive or saturated negative states. An effective comparator circuit can use this hysteresis to its advantage. Furthermore, it can be used in wave generators, as positive feedback adds oscillatory nature to the circuit. A generic positive feedback Op amp is shown in the figure below, where the input can be connected at the terminals. If inverting input is connected to the ground, we will have a bistable multivibrator since it will drive the response to positive or negative saturation.

Op amp with Negative Feedback

Negative feedback results in a condition of equilibrium (balance) since the output is directed in a certain direction. By balancing both input voltages, the circuit keeps its output voltage from saturating in the negative feedback case. Instead, it drives it to an appropriate level to balance the two inputs.

Types of Op amp Based on Operation

We can move ahead and classify them based on the input, output, and operations in four main ways:

  • Voltage amplifiers (voltage as input, voltage as output)
  • Current amplifiers (current as input, current as output)
  • Transconductance amplifiers (voltage as input, present as output)
  • Transresistance amplifiers (current as input, voltage as output)

Two possible configurations depending on the terminal where we will provide the input. However, the response and gain can be found similarly using KVL, KCL, and ideal Op amp assumptions.

Voltage Amplifiers

A voltage amplifier will take voltage input and produce a voltage output. It can be of two types depending on which terminal we have applied the input to, the inverting terminal or the non-inverting terminal. In this types of Op amp, the gain expression can be easily derived using the basic KVL and KCL in association with the assumptions of an ideal case.

Voltage Amplifier Circuit

Voltage Amplifier Circuit

Voltage Amplifier Gain

Vout = Vin (-Rf/Rin)

G = Vout/Vin = -Rf/Rin

Current Amplifier

A current amplifier will take current input and produce a current output. Below we can see the basic circuitry for a current amplifier and calculate the gain. The circuit diagram and gain for this types of Op amp are discussed below.

Current Amplifier Circuit

current amp opamp

Current Amplifier Gain

Iout = -Vout /RL

Vout = Iin (Rf)

G = Iout/ Iin= -1/Rf

Transconductance Amplifier

A transconductance amplifier will take a voltage input and produce a current output. Below we can see a basic diagram for the circuit. In this types of Op amp, the gain can be found by using the ideal case's response and simple KVL at the output.

Transconductance Amplifier Circuit

transconductance amp opamp

Transconductance Amplifier Gain

Iout = (v+ - v-).gm ; where gis the transconductance of the amplifier

Vout = Iout.RL

G = Vout/(v+ - v-) = gm. RL

Transimpedance Amplifier

A trans-impedance amplifier is a types of Op amp that will take current input and will produce voltage output. For example, a basic trans-impedance or trans resistance amplifier is shown in the below figure; considering the ideal case, the input current to the device would be zero, and the inverting terminal would be virtually ground; this will give us an output expression of a simple ohms law and would be the current times the feedback resistance.

Transimpedance Amplifier Circuit

Transimpedance Amplifier Circuit

Transimpedance Amplifier Gain

Vout = -Iin.Rf

G =Vout/Iin= -Rf

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FAQs on Types of Op amp

  • The types of Op amp can be classified into different categories, based on inputs, based on feedback topology, and based on operation and response.

    Based on Inputs:

    • Inverting Op amp
    • Non-Inverting Op amp

    Based on feedback topology

    • Open loop Op amp
    • Closed loop Op amp - Negative feedback, Positive feedback

    Based on operations and response

    • Voltage Amplifier
    • Current Amplifier
    • Trans-impedance Amplifier
    • Trans-conductance Amplifier
  • An open loop Op amp works as a basic comparator, while with feedback loops the response is either constrained or exaggerated based on the type of loop. There are two types of Op amp with lop, positive feedback, and negative feedback. Positive feedback exaggerates the response and forces it to saturate, while negative feedback constrains it to only respond to the differential inputs.

  • Yes, Op amps work with DC as well as Alternating signals. They are constructed with 3 terminals, two for inputs and one for the output.

  • Trans-conductance amplifier with an Op amp can be considered as a types of Op amp when we are classifying based on the operation and response. A transconductance amplifier takes voltage as the input and delivers current at the output and the gain or the ratio of output to input gives the trans-conductance gain of the Op amp circuit.

  • The most common Op amp IC is IC 741, some other types of Op amp ICs available are LM324, LM339, CA3130 etc.

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