E X P E R I M E N T P
Keele University Physics Laboratory
SCHOOL OF PHYSICAL AND GEOGRAPHICAL SCIENCES Experimental Scripts
20
Amplifiers and Negative Feedback
1. Introduction
Most of the experimental apparatus of modern physics makes use of electronic instruments of
some kind. Even if the primary object of study is not electrical in nature, such as acoustic phenomena or
chemical kinetics, the information collected is nevertheless usually converted into electrical signals which
are then processed and recorded electronically. The amplification of signals is an important feature of
such electronic instrumentation. So common is the need for amplification that integrated circuits to
perform this function are now mass produced at a very low unit cost. These integrated circuits are so
freely available and so flexible in their use that they meet the needs of most applications. Nowadays only
for very special requirements would amplifiers be individually designed and constructed from discrete
elements.
This experiment examines some of the circuits which can be constructed using a linear amplifier
having a high voltage gain, high input resistance (~ 1 M) and low output resistance (~100 ). The
amplifier is an inverting one, that is to say the change in the output voltage is of opposite sign to that of
the input voltage change (to within a 90° phase shift). Because of this it is possible to connect the output
to the input by suitable circuit elements so as to "feedback" some of the output to the input. Since the
voltage gain is negative this feedback is also negative - it acts to reduce the input change and so to modify
the function of the amplifier. When the amplifier has no feedback applied it is said to be in an open loop
configuration. The application of the feedback forms a closed loop configuration.
