This tutorial covers the very basics of operational amplifiers. However, if you wish to get more technical with things like slew rate calculations, op-amp integrators and differentiators etc then please see the link provided at the bottom of the page (P.S. for those studying op-amps this is advised)
Operational Amplifiers, or Op-amps as they are more commonly called, are one of the basic building blocks of Analogue Electronic Circuits. Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation. An ideal Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs, one called the Inverting Input, marked with a negative sign, ("-") and the other one called the Non-inverting Input, marked with a positive plus sign ("+"). The third terminal being the output as seen below:
Operational Amplifiers, or Op-amps as they are more commonly called, are one of the basic building blocks of Analogue Electronic Circuits. Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation. An ideal Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs, one called the Inverting Input, marked with a negative sign, ("-") and the other one called the Non-inverting Input, marked with a positive plus sign ("+"). The third terminal being the output as seen below:
The internals of a basic op-amp
The amplified output signal of an Operational Amplifier is the difference between the two signals being applied to the two inputs. In other words the output signal is a differential signal between the two inputs and the input stage of an Operational Amplifier is in fact a differential amplifier as shown below:
Differential Amplifiers
The above circuit operates from a dual supply+Vcc and -Vee which ensures a constant supply. The voltage that appears at the output, Vout of the amplifier is the difference between the two input signals as the two base inputs are in anti-phase with each other. So as the forward bias of transistor,TR1 is increased, the forward bias of transistor TR2 is reduced and vice versa. Then if the two transistors are perfectly matched, the current flowing through the common emitter resistor, Re will remain constant.
Like the input signal, the output signal is also balanced and since the collector voltages either swing in opposite directions (anti-phase) or in the same direction (in-phase) the output voltage signal, taken from between the two collectors is, assuming a perfectly balanced circuit the zero difference between the two collector voltages. This is known as the Common Mode of Operation with the common mode gain of the amplifier being the output gain when the input is zero.
Operational Amplifiers on their own have a very high open loop DC gain and by applying some form of Negative Feedback we can produce an operational amplifier circuit that has a very precise gain characteristic that is dependent only on the feedback used. An operational amplifier only responds to the difference between the voltages on its two input terminals, known commonly as the "Differential Input Voltage" and not to their common potential. Then if the same voltage potential is applied to both terminals the resultant output will be zero. An Operational Amplifiers gain is commonly known as theOpen Loop Differential Gain, and is given the symbol (Ao).
The table below describes a few important concepts needed when dealing with op-amps:
The table below describes a few important concepts needed when dealing with op-amps:
PARAMETER | IDEALIZED CHARACTERISTIC |
Open Loop Gain, (Avo) Infinite | The main function of an operational amplifier is to amplify the input signal and the more open loop gain it has the better. Open-loop gain is the gain of the op-amp without positive or negative feedback and for an ideal amplifier the gain will be infinite but typical real values range from about 20,000 to 200,000. |
Input impedance, (Zin) Infinite | Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry (Iin = 0). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps. |
Output impedance, (Zout) Zero | The output impedance of the ideal operational amplifier is assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it can supply as much current as necessary to the load. This internal resistance is effectively in series with the load thereby reducing the output voltage available to the load. Real op-amps have output-impedance in the 100-20Ω range. |
Bandwidth, (BW) Infinite | An ideal operational amplifier has an infinite frequency response and can amplify any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have an infinite bandwidth. With real op-amps, however, the bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the frequency where the amplifiers becomes unity. |
So now that we have a basic understanding of how the internals of an op-amp actually work, we can dive into the more "fun" stuff.. applications of op-amps! However, before we describe anything (more) 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.
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 the animation 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. The output 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 in the arrangement above has UNITY GAIN (gain = 1).
GAIN:
We will now show how to obtain GAIN or AMPLIFICATION from an OP-AMP. The Voltage Gain (A) of the amplifier can be found using the following formula:
Voltage gain (A) = Vout / Vin
and in Decibels or (dB) is given as:
dB = 20log(A) or 20log(Vout / Vin)
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 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 an OP-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 increasing signal 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-AMP connected 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 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.
One of the cheapest and most-popular OP-AMP is the 741 (although it is rather outdated by now with it being 30 odd years old) and it is perfect for explanation and demonstration purposes.
The pinout for an LM741 is shown below:
The basic parameters for a 741 are:
Rail voltages : +/- 15v DC (+/- 5v min, +/- 18v max)
Input impedance: approx 2M
Low Frequency voltage gain: approx 200,000
Input bias current: 80nA
Slew rate: 0.5v per microsecond
Maximum output current: 20mA
Recommended output load: not less than 2k
The following diagram shows the 741 in a typical audio circuit:
From the discussion above we can see how the circuit above sets its operating conditions.
1. The "+" input sits at half-rail voltage via the two 47k voltage-divider resistors.
2. 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).
3. 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.
HOW DOES THE OP-AMP AMPLIFY?
The circuit above is set to have a gain of 100 via the 100k/1k resistors. These two resistors form a voltage divider. We have seen the ratio of the two resistors produces the gain of the stage.
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:
OFFSET NULL:
Many OP-AMPs have two pins labeled OFFSET NULL. When both inputs are connected to the same voltage, the output should be zero. If the project requires a zero output under these conditions, the OFFSET NULL should be adjusted by adding a 10k pot between the Offset Null pins with the center of the pot connected to 0v.
By adjusting the pot, the output will produce 0v.
NON-INVERTING AMPLIFIER:
The circuit shows an OP-AMP connected as a NON-INVERTING AMPLIFIER:
INVERTING AMPLIFIER:
The circuit shows an OP-AMP connected as an INVERTING AMPLIFIER:
THE OP-AMP AS A VOLTAGE FOLLOWER:
The circuit shows an OP-AMP connected as a VOLTAGE FOLLOWER:
THE OP-AMP AS A COMPARATOR:
The OP-AMP can compare two signals (voltages). This is called a COMPARATOR or DIFFERENTIAL AMPLIFIER (amplifies the difference between two signals).
There are two arrangements - connection to a single rail or dual rails.
The animations below show the output for each configuration:
THE OP-AMP AS A SCHMITT TRIGGER:
The OP-AMP can be wired as a Schmitt trigger. 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. These 3 resistors form a voltage divider with two 10k resistors connected to the 5v supply and one 10k resistor connected to 0v. 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 output is LOW, one 10k resistor is connected to the 5v supply and two resistors are connected to the 0v rail. This produces 1v6 on the "+" input. The purpose of a Schmitt Trigger is to detect and respond to a signal that rises and falls a large amount - in other words it has "large excursions." There are also signals that rise and fall very slowly - such as a photo transistor detecting daylight. During the detection process, the output will rise and fall slightly during the morning light and the change from one level to the other will cause the project to turn on and off.
This is unwanted. The Schmitt trigger will produce an output when a definite condition is met and will not change until the daylight is reduced considerably.
PRACTICAL CIRCUITS:
Here are some practical circuits using OP-AMPS:
TIMER:
When the push-button is pressed and released, the LED illuminates after a period of time. The heart of the circuit is an OP-AMP configured as a comparator. The operation is as follows. When the voltage at "+" input is less than the voltage at "–" input, the output at the output is LOW. When voltage at "+" input is more than the 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 two 10k resistors connected in series form a voltage divider, the voltage at the mid-point being 4.5v
The 500k pot sets the time for the 2200u to charge above 4v5. The 1k stop-resistor prevents a short-circuit if the pot is set to minimum resistance and the button is pressed.
Pressing the switch resets the circuit.
SIMPLE INTERCOM:
A simple intercom can be built around an OP-AMP:
CRYSTAL RADIO:
A simple amplifier can be added to a crystal set with an LM1458 OP-AMP:
TRIANGLE AND SQUAREWAVE GENERATOR:
The following circuit shows a simple triangle/squarewave generator using a common 1458 dual op-amp to produce very low frequencies to about 10 KHz. The time interval for one half cycle is about R*C and the outputs will supply about 10mA. Triangle amplitude can be altered by adjusting the 47k resistor and waveform offset can be removed by adding a capacitor in series with the output.
I hope the following tutorial helps with the understanding of op-amps as they are key elements in the electronic world.
For more detailed information regarding op-amps go HERE
No comments:
Post a Comment