Feedback

Feedback occurs in one form or another in every amplifier and can take the form of desired feedback where the response can be intentionally altered or undesired feedback such as that owing to stray capacitance as mentioned in chapter 1. Undesired feedback can be minimised with correction circuitry or intentional feedback in the form of connection between the output and input via one of many coupling circuits. The amplifier described in section 3.6 contains no intentional feedback and incorporates many undesirable features such as noise, distortion, nonlinearity and mismatch of impedances. The circuit of section 3.9, however, incorporates negative feedback which reduces the effect of these inherent faults. A further form of feedback exists, positive feedback, which again can be inadvertant and give rise to unwanted oscillations or can be intentional to cause an amplifier to oscillate under controlled conditions. This chapter outlines the basic principles of negative and positive feedback and serves as an introduction to the operational amplifier of chapters 6 and 7 where one or both forms of feedback are always incorporated in the circuit design.