Shunt Injection

Author: N.H. Crowhurst

Shunt injection

In shunt injection we must also consider the input current as well as the input voltage. Again assuming an input resistance of 100,000 ohms and an input current of 0.1 microampere, this would produce an input of 10 millivolts. However, the feedback current will provide 9/10 of the total input current in this case, or .09 microampere, leaving a current through the input resistance of only .01 microampere. This current would produce a voltage drop of only 1 millivolt.

Thus, although the input current is still 0.1 microampere, the input voltage is only 1 millivolt instead of the 10 millivolts it would be in the absence of feedback. The effective resistance now is 10,000 ohms instead of 100,000 ohms, and shunt injection has divided the input resistance by the feedback factor.

We can now analyze some practical feedback circuits to see how they classify under these different distinctions.