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REASON OF REJECTION OF COMMON MODE OUTPUT RESISTANCE

The ultimate effect we are going to try this is real amplifier common mode rejection. It is always a problem linked to the characteristic of differential amplifier. We treat now because the operational amplifier is a differential amplifier when studied in open loop.

A represents the gain differential operational amplifier. If you do the next mount

output Vo should be zero because

however, to put it into practice notes that Vo is not zero, and also to increase VCM also increases the value of Vo.

All this requires us to rethink the ideal case: now the output could be expressed as AVi more something that depends on VCM that could be expressed in the form ...

where K is a constant that is easily obtained by making V d = 0 and measuring V or so that K = V or / V CM . All this we can rewrite it again in another way if we understand the process differently. Thus, we assume that V or is due to a differential voltage V d between terminals and a common mode voltage V CM which is the voltage level reference that have applied the two terminals and which is superimposed on the V d . Now V or will ...

where we represent the voltages applied in the form

If we have a differential assembly is

where

that term will have to add a term in the form GCMVCM.

These terms, called common mode of symmetry displayed by problems in the circuit and we will understand how an error amplifier.

We want to know if the common-mode terms are large or not, if A CM much or little influence on the output, or even if is too big or too small compared to differential mode. In general we know that if A CM big the error is too large and if A CM is very small then the amplifier will not error. Thus A CM might be an appropriate parameter to determine the error due to common mode. However, there is right and that to know his real influence is to compare it with the term A d V d . To achieve a good standard, we define the reject reason common mode, which we denote by CMRR as the ratio of differential gain and common ...

depend on the quality of the device and since G d >> G CM high values \u200b\u200bare expected this parameter, the order of 10 ⁵ or 10 ⁶ . To work with more manageable values \u200b\u200bredefine this parameter expressing its value in decibels

this parameter tends to have higher values \u200b\u200bof 100 and the higher its value, better present the amplifier as a differential amplifier, having greater ability to reject common mode signals. In closed-loop amplifiers, CMRR typical values \u200b\u200bare around 120, 130 dB.

If we model this error as something external to the amplifier in the form Vo = GdVd + GCMVCM we add a source at the entrance VCM to produce a voltage output increased GCMVCM, so ...

is clear then that we can model the input error if we put a voltage equal to VCM / CMR in the form ...

in the output have to

In this configuration this error is not very important because CMR is much greater than V.