Essay On The Practical OP-AMP

The Practical Op-Amp chapter provides information on the practical operational amplifier (op-amp) and an ideal op-amp. The op-amps are single-ended and have a differential input in which information is transmitted electrically on differential pairs. The op-amps can produce dc output offset voltage and this result is caused by the limited values of the input offset voltage, input offset current and input bias current in the op-amps. With the passing of time the values of input offset voltage and input bias current keeps changing. Due to this drawback it is recommended to calibrate the op-amp regularly and use a smaller op-amp that changes the value of the parameter without affecting any external changes. It is a proven fact that the low pass filter at the input bias current improves the stability of the op-amps and balances the drift. It is possible to nullify these values with the use of offset minimizing resistors that can be used to reduce the output offset voltage that is caused by the input bias current.
The op-amp includes a few parameters like the temperature-sensitive parameters and supply voltage-dependant parameters. The input resistance, and the output short-circuit current are the temperature sensitive parameters; whereas the common mode voltage range, output voltage swing, open loop voltage gain are the supply voltage-dependant parameters. The op-amp also contains the common parameter, the “power consumption” that is considered a temperature sensitive parameter as well as the voltage dependant parameter. These parameters can be specified for the input bias current and input offset voltage.
The values of the input offset voltage and input bias currents keep varying with the change in temperature and this variation causes the predetermined dc error voltage in the output voltage. If the temperature is maintained at 25° then the dc error voltage can be eliminated. Depending on the quantity of the errors, the efficiency and accuracy of the dc amplifiers is affected. When there is a change in the power supply the input offset voltage and the output offset voltage which is dependant on the input offset voltage will change and it is necessary to keep the changes in the output offset voltage to a minimum. This can be achieved by two ways, either by using an op-amp that supports high supply voltage rejection ratio or by using a regulated power supply.
Some noises are also associated with the op-amps and a few well known noises are the Schottky noise, 1/f noise and the thermal noise. The noise is caused when an unwanted signal gets associated with the op-amps required output signal. The usage of the differential pairs in the op-amps is an advantage as it helps and acts as a shield to protect the op-amps from the different noises. Using the differential op-amps can act as a prevention to the op-amps. The error voltage not only affects the accuracy of the dc amplifiers, but also restricts the amplitude of the signal for the alternate current that could have been amplified without any distortion. The ratio of the differential gain to the common mode gain is known as the common mode rejection ratio.
This chapter has provided extensive information on the different types of op-amps, the parameters and the values that are used and how they can be modified to use the op-amps effectively. The various terms like the input offset voltage, thermal drift, common mode rejection ratio are well explained and by gathering this information it will be easier to design the offset-voltage compensating network. It is also clearly understood that the offset minimizing resistor reduces the values of the input bias currents.