In this script, we will discuss the most important topic of Operational Amplifier, we have already discussed this topic in another script

Now we will discuss

  • Basic Op-Amp
  • Op-amp Salient features or op-amp ideal characteristics
  • Single-Ended Input
  • Double-Ended Input (Differential Input)
  • Double-Ended Output
  • Common-Mode Operation
  • What is a CMRR?

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An operational amplifier, or op-amp, is a very high gain differential amplifier with high input impedance and low output impedance. Typical uses of the operational amplifier are to provide voltage amplitude changes (amplitude and polarity), oscillators, filter circuits, and many types of instrumentation circuits. An op-amp contains a number of differential amplifier stages to achieve a very high voltage gain.

                               

                                              Figure:1.1 Basic Op-Amp

An operational amplifier (op-amp) is a circuit that can perform such mathematical operations as addition, subtraction, integration, and differentiation.
Figure 1.1  shows a basic op-amp with two inputs and one output as would result using a differential amplifier input stage. Each input results in either the same or an opposite polarity (or phase) output, depending on whether the signal is applied to the plus (+) or the minus (-) input


Op-amp Salient features or op-amp ideal characteristics:

  1. Infinite voltage gain (Infinite open-loop voltage gain)
  2. Infinite input impedance
  3. Zero output impedance
  4.  When the input voltage is zero then the Zero output voltage
  5. Infinite frequency bandwidth
  6. The infinite common-mode rejection ratio
  7. Infinite slew rate

Op-Amp different combinations:

 1. Single-Ended Input

Single-ended input operation results when the input single is connected to one input with the other input connected to the ground. Figure 1.2 shows the signals connected for this operation. 



                                                        Figure 1.1: a & b

We can see from this figure 1.2:a, that the input is applied to the plus(+) input, which results in an output having the same polarity as the applied input signal.


Figure 1.2b shows an input signal applied to the minus(-) input, the output then being opposite in phase to the applied signal.

2. Double-Ended Input (Differential Input)

There is another possible way to apply signals at each input. This type of configuration is called a doubled-ended operation.




Figure: 1.2 (a) Double Ended Input

In this figure, an input Vd is applied between the two input terminals. The resulting amplified output is a phase with that applied inputs. 


Figure: 1.2 (b) Double-Ended Input

This Double Ended Input features two different inputs between the inverting and non-inverting input, the difference signal is Vi1 - Vi2

Double-Ended Output

We know that a single output the op-amp can also be operated with the opposite output as shown in figure 1.3


Figure 1.3: Double-ended input with double-ended output

An input applied to either input will result in outputs from both output terminals, this output always being opposite in polarity.




Figure 1.4: Single-ended input with double-ended output

We can show a single-ended input with double-ended output. The signal applied to the plus input results in two amplified outputs of opposite polarity.



Figure 1.5: Differential-output

We can show the same operation with a single output measured between output terminals, not with respect to the ground. This difference in the output signal is 

V01-V02

it is also referred to as a floating signal since neither output terminal is the ground terminal and the difference output is twice as large as either V01 or V02 because they are of opposite polarity and subtracting them results in twice their amplitude.
E.g. 
     20V - (-20V)
    =40V


                                 Figure 1.6:  Differential input, differential output operations

We show differential input and differential output operations in figure 1.6. The input is applied between the two input terminals and the output taken from between the two output terminals.

Common-Mode Operation

When the same input signals are applied to both inputs, common mode operation results as shown in Figure 1.7

Figure 1.7:Common-mode operation


We can say ideally, that the two inputs are equally amplified and since they result in opposite-polarity signals at the output, these signals cancel, resulting in 0-V output. Practically a small output signal will result.

What is a CMRR?

The CMRR in an op-amp is a common mode rejection ratio. The op-amp has two input terminals which are positive and negative terminals and the two inputs are applied at the same point. This will give the opposite polarity signals at the output.  The overall operation is to amplify the deference signal while rejecting the common signal at the two inputs. Since noise is generally Common to both inputs, the differential connection tends to provide attenuation of this unwanted input while providing an amplified output of the difference signal applied to the inputs. This operating system is called common-mode rejection.

CMRR = Differential mode gain / Common-mode gain

Common Mode Rejection Ration of Op Amp

The CMRR is a differential connection in that the signals that are opposite at the inputs are highly amplified, whereas those that are common to the two inputs are only slightly amplified. The CMMR ratio can be applied to the operational amplifier. By using the condition of common mode rejection ratio, i.e. when both the input of the amplifier has the same voltages, then the output of the amplifier should be zero or the amplifier should be rejecting the signal.