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lm1458n datasheet

When connected to a single voltage rail, the output can go from 0v to approx full rail voltage. When the + input is a few millivolts higher than the – input, the output goes HIGH. – input sits at half rail voltage via two equal-value resistors, the + input must go above ½V for the output to go HIGH, as shown in the animation below: . The + input must be higher that the – input for the output to be . A small increase in voltage on the + input above the – input will change the output from 0v to approx full rail voltage. The output voltage follows the input. As the + input rises, the output rises. Normally the output would rise to rail voltage, but since it is connected to the – input, it will always be a few millivolts below the + input. We have already seen from the animation above that an OP-AMP needs a voltage on the inverting input that is almost equal to the non-inverting input to produce the following effect. Thus, to get this voltage on the – input, the output of the OP-AMP must be the voltage on the + input. If the + and – inputs are reversed, the OP-AMP will not work or produce a valuable output as shown in the following two animations: The above animations show how to amplify a signal with an OP-AMP. An increasing signal voltage on the Non-Inverting Input + will create an increasing signal on the output. An increasing signal voltage on the Inverting Input – will create an decreasing signal on the output. An OP-AMP connected to a single voltage rail will produce an output from 0v to approx rail voltage. The + input sits at half-rail voltage via the two 47k voltage-divider resistors. This makes the output go HIGH and the voltage on the – input increases until it is just below the + input. The – input cannot rise above the + input as this will make the output of the OP-AMP go LOW . The end result is the OP-AMP is half-turned-on and any increase or decrease in voltage on the – input will make the output go LOW or HIGH. Don't forget: the output will move in the opposite direction to the voltage applied to the – input. The OP-AMP will amplify this signal 100,000 times and the output will try to FALL as much as 100v - but the voltage-divider resistors come into operation as follows: The output will fall and this will be passed to the – input via the 100k resistor. As soon as the output falls 100mV, the voltage seen by the – input will be 1/100th of 100mV or 1mV. The effect is slightly less than 1mV being fed back to the – input and the output drops 100mV. The – input sees about 100th of 1mV and the output drops 100mV. When both inputs are connected to the same voltage, the output should be zero. The circuit shows an OP-AMP connected as a NON-INVERTING AMPLIFIER: The circuit shows an OP-AMP connected as an INVERTING AMPLIFIER: The circuit shows an OP-AMP connected as a VOLTAGE FOLLOWER: The OP-AMP can compare two signals voltages . The diagram below shows this arrangement: When the input of the Schmitt Trigger is LOW, the output is HIGH. As the input rises, nothing happens to the output until the input is 3v3. This is the voltage on the + input due to the effect of the three 10k resistors. When the – input is 3v3, the output of the OP-AMP goes LOW and it remains LOW until the input falls to less than 1v6. The 1v6 voltage on the + input is produced by the three 10k resistors. When the voltage at – input, the output at the output is LOW. When voltage at – input, output is HIGH. It is usual to hold the voltage at – input at a particular voltage, known as the reference voltage, and vary the voltage at + input to obtain a particular function. The input impedance of an OP-AMP is very high and probing either input with a multimeter or CRO will change the voltage on the input and alter the state of the output. It is also impossible to measure the difference in potential between the inverting input and non-inverting input. We have covered these in the previous pages of this topic and the most important point to remember is the voltage on the + input must be slightly higher than the – input for the output to be HIGH. The – input rises normally due to the voltage from the output until it is just lower than the + input and this makes the output nearly equal to the voltage on the input. In the first circuit, the output must fall by 100mV if the + input falls 1mV, to maintain the bias conditions. ifficult for the OP-AMP to produce a lower voltage on the inverting input. For each millivolt lower than the + input, the output must be 100millivolts lower than 5v. You cannot work on a OP-AMP stage if you don't know how it is being driven as the input line is very sensitive to the slightest change in voltage. It will charge via the two 100k resistors and after about 5 seconds the + input will be higher than the – input and the output will go HIGH. The actual voltage on the output will be lower than 5v so that the – input is a fraction of a millivolt below the + input. Any slight voltage on the 2u2 will be passed to the non-inverting input of the OP-AMP and cause the output to rise. It must be a low leakage type to allow the voltage on non-inverting input to rise above the inverting input. To see if the OP-AMP is sitting correctly, place the 47k on test leads between the non-inverting input and the 2v rail, while monitoring the output. Placing the resistor between the + input and 0v rail, will make the output go LOW. The output rises slowly to the same level as that on the + input due to the effect of the 10u electrolytic charging slowing via the 1M resistor. If the output is not 5v, place the 47k on jumper leads between the + input and 12v and see if the output rises. You will not be able to accurately measure the voltage on the non-inverting input and that's why the 47k on jumper leads is needed to check the operation of the stage.
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