Series Injection

Author: N.H. Crowhurst

Feedback circuits can be classified according to method of feedback injection

Another way of distinguishing between different types of feedback is in the way in which the feedback is injected at the input end of the amplifier. It can be injected either in series or in parallel (shunt) with the input resistance of the amplifier itself. The effective input resistance of the amplifier is affected by the method of injection. When feedback is injected in series with the input to the amplifier (as we have assumed it to be thus far because this system is the simplest to follow), we assume that only voltages are being considered. There are always small currents as well.

When the 10-millivolt input is applied, the input resistance to the amplifier will draw a current, depending upon its value. If the input resistance were 100,000 ohms, 10 millivolts applied across it would produce a current of 0.1 microampere. Due to the feedback of 9 millivolts, however, the input resistance of 100,000 ohms has only 1 millivolt across it, which means it will draw only .01 microampere. Thus, from the viewpoint of the 10-millivolt total input, the resistance appears to be not 100,000 ohms, but 1,000,000 ohms, or 1 megohm. We may conclude that series injection causes the input resistance to be multiplied by the feedback factor - in this case, 10.

Series injection causes the input resistance to be multiplied by the feedback factor.