Download Amplifiers and Negative Feedback: An Experimental Study on Closed-Loop Configurations and more Study notes Engineering Physics in PDF only on Docsity! 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. E X P E R I M E N T P Keele University Physics Laboratory SCHOOL OF PHYSICAL AND GEOGRAPHICAL SCIENCES Experimental Scripts 21 2. Closed loop configurations 2a. Multiplying Amplifier In the circuit of fig. 1 the loop is closed by a resistor, R2 connecting the output to the input and the voltage generator is connected to the input through another resistor R1. Fig. 1 Multiplier circuit If the amplifier has a sufficiently high input resistance the current from the generator I, flows through R1 and R1. Thus V V IR V V IR V A V s o i o i i = - = - = - 1 2 ( ( . 1 1 .a) b) (1.c) where A is the open loop voltage gain measured previously. Eliminating I from these equations gives If A R R >> / 2 1 then Vi ~ 0 and i s 1 2 0 s 1 2 V = V 1 + R R (A + 1) V = -A V 1 + R R (A + 1) (2.a) (2.b) o s 2 1 V V = - R R (3)
