An operational amplifier is actually a high-gain differential amplifier, and as such has both an inverting and a non-inverting input. Both inputs must be connected in order for the circuit to work. This is the figure of the circuit;
The input and feedback connections are both made to the inverting (-) input. The non-inverting input (+) is grounded through a resistor. This is what forces the inverting input to be a virtual ground: the amplifier output voltage depends on the voltage difference between the two inputs, rather than the absolute voltage at either input. Because the input transistors inside the op amp do actually require a very slight input current, there is a very slight corresponding voltage drop across the resistors connected to those inputs. However, if the resistor connected to the non-inverting input is approximately the same as the parallel combination of Rin and Rf, the resulting voltage drops will be the same and will tend to cancel each other out. If Rin and Rf are each 10K, then a standard 4.7K resistor to the non-inverting input will be satisfactory in this capacity. We could also use a 5.1K resistor here.
Because the ratio of Rf/Rin completely controls the effective gain of the amplifier, these resistors should be as precise as possible. Radio Shack sells a package of 50 ¼-watt metal film resistors, having a tolerance of 1%. You don't have to use them in these experiments, but your results will be more accurate if you do. All results discussed in these pages will be based on the use of 1% resistors in critical locations. We'll identify these in the assembly diagrams, and show the bodies of the 1% resistors in light blue rather than the more usual light brown color.
The input and feedback connections are both made to the inverting (-) input. The non-inverting input (+) is grounded through a resistor. This is what forces the inverting input to be a virtual ground: the amplifier output voltage depends on the voltage difference between the two inputs, rather than the absolute voltage at either input. Because the input transistors inside the op amp do actually require a very slight input current, there is a very slight corresponding voltage drop across the resistors connected to those inputs. However, if the resistor connected to the non-inverting input is approximately the same as the parallel combination of Rin and Rf, the resulting voltage drops will be the same and will tend to cancel each other out. If Rin and Rf are each 10K, then a standard 4.7K resistor to the non-inverting input will be satisfactory in this capacity. We could also use a 5.1K resistor here.
Because the ratio of Rf/Rin completely controls the effective gain of the amplifier, these resistors should be as precise as possible. Radio Shack sells a package of 50 ¼-watt metal film resistors, having a tolerance of 1%. You don't have to use them in these experiments, but your results will be more accurate if you do. All results discussed in these pages will be based on the use of 1% resistors in critical locations. We'll identify these in the assembly diagrams, and show the bodies of the 1% resistors in light blue rather than the more usual light brown color.
0 comments:
Post a Comment