Primary Electronics for Sensors

Amplification by Operational Amplifiers. Although operational amplifiers (OAs) contain numerous electronic components, they can be handled themselves like a single component. Working with integrated OAs is so easy that untrained persons can solve their problems in this way. Sometimes, some do-it-yourself electronics is a better solution than to look for specialized commercial instruments. It is sufficient to understand the graphic symbol of the OA and its inherent functionalities. 

Experiments with OAs can start already with the basic circuit in Fig. 2.61, where the terminals -i-17b and -17b are connected with two 9-volt batteries to 

Indeed, commercially available OAs correspond to the criteria given above to such an extent that the circuit can be considered an ideal component For most of the applications which deal with chemical sensors, the series 080 OA types are useful. They are produced by many different manufactiuers. Since they are inexpensive, some loss by electric shock can be tolerated. Experiments can be done easily by means of commercially available OA sockets moimted on a piece of printed board material. 

In common OA applications, negative feedback is necessary. In the voltage follower (Fig. 2.62, left), a connection is made between Out and In2. As a result of feedback, all points Out, In2 and Ini assume equal potential in relation to the reference. No voltage amplification can be stated however, now we have a voltage soiuce (of a magnitude equal to the input voltage studied) that can 

The current follower (Fig. 2.62 right) seems to solve an imsolvable problem. It can measure the current flowing in a short-closed circuit The current to be measured would flow to mass (the reference) in the circuit given in Fig. 2.62. However, the current is redirected to flow over the resistor R towards output Out. Since Ini is directly connected to mass, In2 also has assumed mass potential (zero). For the current I the condition is as if it were connected to mass. Indeed it flows via R and generates there an IR drop of the magnitude I R. Consequently, the output voltage I/out = -I R. This value can be measured by any voltage meter. Overall, one can measure a current in a virtually short-closed circuit. 

---