Op-amp Comparators

Non-linear applications of op-amp:

  • In this type of applications, output is not having a linear  relationship with input. Op-amp is operated either in open loop condition or with positive feedback.
  • Op-amp in open loop configuration goes into saturation. Op-amp requires more time to come out of the saturation, so these kinds of circuits are preferred at low frequency applications. By modifying the circuit we can also make circuit as non-saturating type and make it to use at high frequency.
  • In linear applications, we carried out mathematical analysis and it is based on two principles
  1. Input current to the op-amp is zero.
  2. Virtual ground.
  • The first assumption is also used in non-linear applications. The second assumption says that both the terminals are at the same potential (due to negative feedback). In non-linear applications no negative feedback is applied and hence this assumption is not used in non-linear applications.

Comparators:

  • Comparators are used in ADC & DAC. It is also used for generating waveforms (square & triangular). It has also to play decisive role in control circuits.
  • Comparator compares the two input signals out of which one is always fixed (i.e. reference signal). The two signals are always voltages. Thus comparator is a circuit which compares the input signal with the known reference voltage applied at other terminal and produces either high or low output voltage depending on which input is higher.
  • The output of the comparator is always in saturation (±Vsat) irrespective of whatever signal is applied as an input. The output state of the op-amp indicates whether input is greater than or less than the reference voltage. It only indicates; but it will not indicate the amount by which it differs.
  • In non-linear applications waveforms are drawn rather than mathematical analysis.
  • Comparators are divided into two basic types:
  1. Open loop comparator (Ideal comparator)
  2. Schmitt Trigger (with positive feedback)

Open Loop Comparator:

  • The op-amp in open loop configuration can be used as a basic comparator. The open loop comparators are divided into two types.
  1. Non-inverting Comparator
  2. Inverting Comparator.

Comparator Characteristics:

  • Following are some of the important characteristics of comparator.
  1. Speed of operation:

The switching between the two output states should be as fast as possible. Thus the output should respond the input changes quickly.

  1. Accuracy:

It is the smallest amount of input difference voltage required to make the output to change its state

  1. Strobe function:

To enable/disable the device, certain comparators are having a strobe terminal. When it is enable, output will respond to input. If it is disabling, output will not respond to input signal

  1. Latch: some of the comparators are having latching facility. The required output state is frozen in a latch flip flop.

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