The document provides information about operational amplifiers (op amps), including their basic applications and circuit configurations. It discusses op amps being used as buffers, inverting and non-inverting amplifiers, comparators, and regulators. Feedback is described as crucial for controlling an op amp's gain. The document also mentions op amp applications like integration, differentiation, instrumentation, and as basic math operation components in analog computers.

1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite
ASSIGNMENT # 1
OPERATIONAL AMPLIFIER
Reyes, Ron Henreb July 26, 2011
Electronics 3/BSECE 41A1 Score:
Engr. Grace Ramones
Instructor

2. OPERATIONAL AMPLIFIERS:
Operational amplifiers (op amps) were originally used for mathematical operations
in 'analog' computers. They typically have 2 inputs, a positive (non_inverting) input and a
negative (inverting) input. A signal fed into the positive (non_inverting) input will produce
an output signal which is in phase with the input. If the signal is fed into the negative
(inverting) input, the output will be 180 degrees out of phase when compared to the input.
They are designed to be used with other circuit components to perform either
computing functions (addition, subtraction) or some type of transfer operation, such as
filtering. Operational amplifiers are usually high-gain amplifiers with the amount of gain
determined by feedback.
There are a bazillion (technical term) applications for op amps. The following
section is an attempt to give you a basic understanding of just a few applications. None of
the power supply connections are shown. Most op amp circuits used in audio use a ±15 volt
power supply (especially when the audio equipment has a switching power supply). They
can also be used with a single ended supply (no negative voltage) in head units and other
such equipment that have no switching power supply.
The diagram below shows the schematic symbol for an op amp.

3. OP AMP OPERATION:
The circuit below shows a simple buffer circuit. The input impedance of an op amp
is extremely high (on the order of 1012 ohms). It might be used if the input signal to the op
amp was coming from a source which could supply almost no current. The output of the op
amp can easily drive 1000 ohms or more. The output, when used as a buffer, will
theoretically be identical to the input signal. I can't say it is identical because there is a
small amount of distortion in all amplifier circuits. The distortion in this circuit would be
EXTREMELY low and would most likely be inaudible.
OP AMPs as Amplifiers:
An op amp can also easily amplify a signal such as audio. The diagram below shows
the circuit for an op amp that would give an output signal twice as large as the input. Op
amps don't like errors. To get amplification, you induce an error in the signal going back to
the negative input of the op amp. An op amp will do everything in it's power to get the
signal on the negative input to match the signal on its positive input. To get an output that's
twice as large as the input, you use 2 equal value resistors as a voltage divider to reduce the
return (feedback) signal at the negative input by half. If the return signal doesn't match the
input signal, the op amp will increase the output until the signal returned to the negative
input is the same as the input to the positive input. Since the voltage divider cuts the signal
in half, the signal at the output must be doubled. You can create any amount of gain needed
by changing the value of ONE of the resistors in the 'feedback' path. The actual limit of gain
will be determined by the op amp design. When using an op amp as a non-inverting
amplifier, the gain will always be greater than or equal to 1. To get a gain of less than 1, you
need to use a voltage divider on the input signal.

4. OP AMP Inverters:
An op amp can produce a signal which is 180 degrees out of phase (inverted) with
respect to the input signal. The diagram below shows an op amp used as an inverter. To use
an op amp as an inverting amplifier, you must send the signal into the negative input
instead of the positive input. As I said before, the op amp will do everything it possibly can
to make the voltage (signal) on the the negative input match the positive input. In the
following diagram, you can see that the positive input is connected to ground. It's shown as
being connected through a resistor but the resistance to ground in unimportant. What is
important is that the positive input has no signal (it's connected to the reference, ground).
This means that the op amp's negative input will have no visible (voltage) signal on it.
When you're driving the negative input it will act as a virtual ground. The input is
converted from a voltage drive to a current drive. The change in current is what drives the
op amp. This is important to know because when you look at the negative input with an
oscilloscope, you will see no signal (when the circuit is an inverting amplifier). I said earlier
that the op amp inputs had a very high impedance. While this is true, when using the
inverting input with feedback (which is necessary for audio reproduction), the input
impedance becomes the value of the input resistor.
OP AMP Error Correction:
An op amp is commonly used in a circuit where error correction is required. Op
amps can't (generally) supply a large amount of current at its output. If a signal is fed to the
positive input of an op amp and the op amp is driving a circuit which CAN supply a large
amount of current (like the regulator that we will work with later on this page), the output
of the whole system can be fed back into the negative input of the op amp. This will allow
the op amp to compare the output (of the whole system) to the input signal and correct as
needed. If the op amp is used in a circuit which needs little current at its output, the op amp
can still monitor the output and correct as needed.
If you read the servo page before this page, you'll remember that there was a
reference point indicated by a green arrow. The positive input on the op amp is analogous
to the green arrow. The height of the green arrow would be analogous to the voltage on the
positive input. The sensor would be analogous to the valves which would also be analogous
to the negative input of the op amp. The error correction would come at the output of the
op amp (instead of the hydraulic actuator).

5. The next diagram has a resistor in series with the output of the op amp and the load
which is to be driven by the op amp. The resistor represents anything that may be between
the op amp and the load. The resistor could actually be a long run of wire, resistance in the
copper of a printed circuit board or anything else that may cause the signal to be distorted.
If the op amp didn't monitor the signal at the load, the signal would be distorted (in this
case, the simple series resistance would only reduce the signal level). If the resistor would
instead be an external circuit designed to increase the output current (such as the
transistors, resistors, capacitors... of a power amplifier), the output of the op amp may not
even resemble the final output signal. The op amp would do everything possible to get the
final output to match the input signal. A look back at the amplifier page shows a circuit
where an op amp is used for error correction in a power amplifier circui
. The output resistance simulates any series resistance or anything else that may
distort the signal. The op amp tries to make theoutput voltage match the input voltage. If
you make the output resistance 0 ohms the opamp voltage and output voltage are equal
because there is no output resistance to cause a voltage drop (distortion).
Op Amps as Comparators:
An op amp can be used to compare 2 different voltages. If you apply a reference
voltage to one of the inputs and then use the other input to monitor a voltage from some
point in a circuit, the output of the op amp will go from high to low (or vice-versa) as the
monitored voltage crosses the reference voltage.

6. The voltage follows a curve because the capacitor is charging. The capacitor charges
faster at first then it slows as it approaches full charge. The blue line indicates the voltage
on the positive input. Since the 2 voltage divider resistors are of equal value, the voltage on
the positive input is exactly half of the power supply voltage. You can also see that the
voltage on the output of the op amp is high (close to the power supply voltage). If you push
the button, the capacitor will start to charge. The little lines in the window are voltage
indicators. Think of them as a portion of the trace on an oscilloscope. As the capacitor
charges (and the voltage starts to rise), the line goes up (it follows the voltage). The voltage
on the output does not change until the voltage on the negative input is higher than the
voltage on the positive input. Remember that the previous circuits had a feedback signal
return path between the output of the op amp to the negative input. Since there is no
feedback, the gain is essentially (ideally) infinite. This will make the output swing from its
maximum positive output voltage to its maximum negative output voltage. If there were a
feedback resistor, the output voltage would not swing as far. With a feedback resistor, you
could get the op amp's output voltage to be an inverted version of the voltage on the
negative input. Remember that the circuit is a comparator. It's comparing the voltage on
the 2 inputs. When the voltage on the negative input is below the reference voltage (on the
positive input), the output is high. As soon as the voltage on the negative input goes above
the voltage on the positive input, the output goes low. If you look at the white line sweeping
from left to right you can see that the green line instantly transitions from high to low at
the point where the blue and yellow lines intersect
Note:
This is the same basic principle that the fan controller on the cooling fans page
employs. The fan controller uses the negative input for the reference and the positive input
(who's voltage is controlled by the thermistor) as the input to the comparator.
Maximum and Minimum Input/Output Values:
Some op amps can not accept inputs that equal the power supply voltage (or ground
in the case of a single ended† supply). If the input is beyond the safe input values, the input
may lead to unexpected output values. For instance, if the negative input in the previous
circuit were grounded, some op amps would erroneously give a low output instead of a
high output. As soon as the input voltage moved slightly above ground, the op amp would
again operate as you'd expect. If you need an op amp to accept inputs close to ground, you
need to get an op amp suited for the task. The op amp on the cooling fans page is such a
device. It's an LM358 which can be found on the TI site.
A single ended supply is one that uses only ground and EITHER a positive OR
negative voltage. A 'split' supply has a positive voltage AND a negative voltage (below
ground). If you see a power supply described as a 12 volt supply, it likely means that it is
simply a single ended supply. If the supply voltage is expressed as ±15 volts, it's a split
supply. In audio, split supplies are most common. For digital equipment, the comparators
would likely be powered by a single ended 5 volt supply.

7. Op Amps as Regulators:
If you need a high quality linear regulator, an op amp can save a lot of effort. In the
following demo, you can see that there is a simple zener shunt regulator connected to the
positive input of the op amp. This becomes the reference voltage. If the zener is a 6.2 volt
device, the reference will be 6.2 volts. Actually the reference voltage will likely be a little
more or less than 6.2 volts (due to tolerances and the actual current flowing through the
diode). If the voltage is precisely 6.2 volts on the positive input, the output of the regulator
(the emitter of the current boost NPN bipolar transistor) will be precisely 6.2 volts. The
feedback line from the emitter to the negative input of the op amp allows the op amp to
monitor the output and compensate for changing load current. If the load resistor
decreases in resistance, the output current increases (because we have a regulated voltage
source). Without the feedback, the output from the regulator would likely drop a little. In
most cases that would be fine. In some circuits, however, the change in voltage would be
unacceptable.
When you push the button in the following demo, the resistance will decrease. You
will notice that the regulator output current through the resistor increases in proportion to
the fall in resistance. You will also notice that the output voltage is rock solid. If you look
carefully, you can see that the output voltage from the op amp increases slightly to increase
the current through the base of the transistor (which is needed to maintain the proper
output voltage)
Op-amps can be connected into two basic configurations, Inverting and Non-
inverting. The Open-loop gain called the Gain Bandwidth Product, or (GBP) can be very
high and is a measure of how good an amplifier is. Very high GBP makes an operational
amplifier circuit unstable as a micro volt input signal causes the output voltage to swing
into saturation. By the use of a suitable feedback resistor, (Rf) the overall gain of the
amplifier can be accurately controlled.

8. The Differential Amplifier produces an output that is proportional to the
difference between the 2 input voltages. Adding more input resistor to either the inverting
or non-inverting inputs Voltage Adders or Summers can be made. Voltage follower op-
amps can be added to the inputs of Differential amplifiers to produce high impedance
Instrumentation amplifiers.
The Integrator Amplifier produces an output that is the mathematical operation
of integration. The Differentiator Amplifier produces an output that is the mathematical
operation of differentiation. Both the Integrator and Differentiator Amplifiers have a
resistor and capacitor connected across the op-amp and are affected by its RC time
constant. In their basic form, Differentiator Amplifiers suffer from instability and noise but
additional components can be added to reduce the overall closed-loop gain.