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WHAT IS AN OP-AMP? |
Before we describe anything technical, let's see one in action.
An op-amp can be connected to a single voltage rail (called UNIPOLAR SUPPLY - 0v to Vcc) or a dual voltage rail (called BIPOLAR SUPPLY +/-Vcc).
When connected to a single voltage rail, the output can go from 0v to approx full rail voltage.
The OP-AMP has two inputs. A "+" inputfor non-inverting and "–" for inverting. When the "+" input is a few millivolts higher than the "–" input, the output goes HIGH. It's that SIMPLE.
Study the animation below:
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 theanimation below:
From the animations above we have shown two things:
1. The "+" input must be higher that the "–" input for the output to be HIGH.
2. A small increase in voltage on the "+" input (above the "–" input) will change the output from 0v to approx full rail voltage. This represents HIGH GAIN or AMPLIFICATION.
VOLTAGE FOLLOWER |
The OP-AMP can be used as a VOLTAGE FOLLOWER. Theoutput voltage follows the input.
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.
Note: the output follows the input
The OP-AMP inthe arrangement above has UNITY GAIN (gain = 1).
We will now show how to obtain GAIN or AMPLIFICATION from an OP-AMP.
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 midpoint 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 AMPLIFIER.
The gain of an OP-AMP is determined by the ratio of resistors R1 and R2.
Here is an OP-AMP with a gain of 5:
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 anOP-AMP.
We will now cover some technical details.
OP-AMPs contain a number of transistors (25 or so) but the internal workings do not concern us. The only thing we need to know is how to get it to operate.
An OP-AMP is represented as a "block" in a circuit diagram with two inputs and an output:
An increasing signal (voltage) on the Non-Inverting Input "+" will create an increasingsignal on the output.
An increasing signal (voltage) on the Inverting Input "–" will create an decreasing signal on the output.
An OP-AMP can be connected to a single voltage rail (called UNIPOLAR SUPPLY) or a dual voltage rail (called BIPOLAR SUPPLY) as shown in the diagrams below:
An OP-AMP connected to a single voltage rail will produce an output from 0v to approx rail voltage.
An OP-AMPconnected to dual rails will produce an output from –V to +V as show below:
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 producing.
SPLIT RAILS or DUAL RAILS - also called BIPOLAR SUPPLY can be produced as follows:
This will allow the output of the OP-AMP to change from negative to positive as shown...