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An operational amplifier (aka Op-Amp) is a differential amplifier with one output coupled to a very high gain DC amplifier. Input stage has a very high input impedance, typically around one megohm or more. The output stage is a push-pull circuit that is capable of providing a relatively high output current, typically 20 mA to 40 mA. Therefore, the circuit has a relatively low output impedance.
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| An operational amplifier schematic. Do these subcircuits look familiar? |
The input stage, being a differential amplifier, has two inputs that tend to counteract each other; increasing the voltage on the non-inverting input tends to drive the output to a higher voltage while increasing the voltage on the inverting input tends to drive the output to a lower voltage.
Op-amps can be made with discrete components but are virtually always acquired as integrated circuits.
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An LM324 with an LED and a transistor in a TO-92 package for size comparison. The LM324 contains four op-amps. |
The following diagram is the typical schematic symbol for an op-amp.
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| An Operational Amplifier schematic symbol. |
The ideal op-amp would have an infinite gain, an infinite input impedance and an output impedance of zero. Of course, a real op-amp has something less than these ideal parameters. With its high input impedance, an op-amp takes very little current, and thus has a small loading effect, on the circuit connected to its inputs. Its low output impedance means that it can deliver a relatively high current to its load without a voltage drop. Relatively high current meaning 20 to 40 mA. However, if more current is needed, the output of the op-amp can be connected to a DC amplifier made with substantial power transistors (typically almost identical to the transformerless push-pull amplifier in Power Amplifiers).
Like
differential amplifiers, op-amps are usually powered by dual power supplies. For
example, the circuit may be powered by two 10 volt power supplies. These would
provide a positive supply voltage of +10 volts, a negative supply voltage of -10 volts and ground (see
the concept of
ground in
DC circuits). The output of an op-amp would ideally be able to present
any voltage between the positive and negative supply voltages. However, typical
op-amps cannot make the output voltage as high or as low as the supply voltages.
For example, if the positive supply voltage is +10 volts, the output voltage
will only be able to reach about +9.5 volts. Likewise, if the negative supply
voltage is
The two inputs of an operational amplifier are labeled with plus and minus symbols. The plus input is called the non-inverting input and the minus input is called the inverting input. An operational amplifier follows a few simple rules:
If the voltage at the non-inverting input (+) is higher than the voltage at the inverting input, the output voltage will go higher until one of two conditions are met: either the two inputs become equal or the output cannot go higher.
If the voltage at the inverting input (-) is higher than the voltage at the non-inverting input, the output voltage will go lower until one of two conditions are met: either the two inputs become equal or the output cannot go lower.
If the inputs cannot be made equal, the output voltage will sit at its maximum or minimum level, depending on which input is more positive.
The output voltage will not change as long as the two input voltages are equal.
These rules can be boiled down to this: an operational amplifier will change its output voltage to whatever it takes to make the input voltages equal. If this is impossible, the output voltage will go to its upper limit or lower limit trying to make the inputs equal.
There are several circuits that use operational amplifiers. All of these circuits take advantage of the rules listed above.
Keep in mind while analyzing op-amp circuits that the "output" of a circuit doesn't necessarily mean that current flows out of the output. The output of a circuit merely provides a useful signal for the next circuit in the chain. Take another look at the transformerless push-pull amplifier in Power Amplifiers. Notice that during the negative half of the input cycle conventional current is flowing from ground, through the speaker and into the output of the amplifier.
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A push-pull
amplifier during the positive half of the input cycle. Conventional
current is flowing out of the output, through the speaker to ground. The
amplifier is sourcing current. |
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| During the negative half of the input cycle, conventional current is flowing from ground, through the speaker and into the amplifier output. The amplifier is sinking current. |
When conventional current flows out of the output of a circuit, that circuit is said to be sourcing current. When conventional current flows into the output a circuit, that circuit is said to be sinking current. The amount of current that an op-amp can source is typically slightly more or less than the current it can sink. For example, the LM324 can source 20 mA but can only sink 8 mA.
Don't forget while analyzing op-amp circuits that negative voltage is not the opposite of positive voltage. A negative voltage is simply a voltage that is lower than some arbitrary voltage that has been labeled as 0 volts. If you are driving into Death Valley, nothing notable happens when you drop below the sea-level line. The negative altitude on your GPS receiver simply tells you that you are below the altitude that has been arbitrarily labeled as zero. Voltage works the same way. When you find an input with a positive voltage and an output with a negative voltage, there is not some reversal of voltage, current or anything of the sort. The output voltage is simply something lower than the input voltage and 0 volts is somewhere in between. Even though the term "opposite polarity" is used below, this is used for convenience. Opposite polarity simply means different sides of zero.
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