//demo.component.html

<input  matInput name="A" #A="ngModel"
[(ngModel)]="A" placeholder="Enter A"
ngModel (keyup)="EventA($event,i,A)">

<input  matInput name="B" #B="ngModel"
[(ngModel)]="B" placeholder="Enter B"
ngModel (keyup)="EventB($event,i,B)">

<input  matInput name="C" #C="ngModel"
[(ngModel)]="C" placeholder="Enter C" disabled
ngModel (ngModelChange)="EventB($event,i,B)">

//demo.component.ts

EventA(i, a: NgModel) {
   this.C = 10;      //here value of C (ngModel) is changed. but ngModelChange did not get fired
}

EventB(i, a: NgModel) {
   //Some Code
}