WHAT IS AN OP-AMP?
WHAT IS AN OP-AMP?
If the "–" 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 can control the output as shown in the animation below:
From the animations above we have shown two things:
In this arrangement the OPerational AMPlifier is called a BUFFER and has unity gain.
The OP-AMP works like this:
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.
The OP-AMP in the arrangement above has UNITY GAIN (gain = 1).
In the following animation, the OP-AMP has a gain of 2.
For a gain of 2, the two resistors on the inverting input are EQUAL VALUE. The actual value of resistance is not important. It can be 10k to 100k, for example.
The point to note is this: The voltage at the mid point of two equal-value resistors is half the delivered voltage. 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 TWICE the voltage on the "+" input.
This is shown in the animation below:
From the animation above, you can see how to turn an OP-AMP into an
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 OP-AMP connected to a single voltage rail will produce an output
from 0v to approx rail voltage.
You need to know if an OP-AMP is connected to a single rail or dual
rails as this will determine the type of signal it is capable of
This will allow the output of the OP-AMP to change from negative to positive as shown in the animation below:
The positive and negative rail is normally equal in magnitude
however if they are not equal, the OP-AMP will produce waveforms equal
to size of each rail.
The basic parameters for a 741 are:
Rail voltages : +/- 15v DC (+/- 5v min, +/- 18v max)
From the discussion above we can see how the circuit above sets its
Suppose the voltage on the input rises 1mV.
This rise will pass through the 100n capacitor and appear on the "–" input as a 1mV increase.
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. Thus the 1mV produced by the signal will be negated by the effect of the output dropping.
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.
The following animation shows (in slow-motion) how the voltages flow though the OP-AMP:
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