Class notes
Operational amplifier
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This consists of inverting and non-inverting amplifers, voltage follower, differential amplifier, comparator, instrumentation amplifier, schmitt trigger.
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Unit 4Operational Amplifier
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 Operational
Amplifier, or op-amp for short, is fundamentally a voltage amplifying device designed to be used
with external feedback components such as resistors and capacitors between its output and input
terminals. These feedback components determine the resulting function or “operation” of the
amplifier and by virtue of the different feedback configurations whether resistive, capacitive or
both, the amplifier can perform a variety of different operations, giving rise to its name of
“Operational Amplifier”.
An Operational Amplifier is basically a three-terminal device which consists of two high
impedance inputs. One of the inputs is called the Inverting Input, marked with a negative or
“minus” sign, ( – ). The other input is called the Non-inverting Input, marked with a positive or
“plus” sign ( + ).
A third terminal represents the operational amplifiers output port which can both sink and source
either a voltage or a current. In a linear operational amplifier, the output signal is the amplification
factor, known as the amplifiers gain ( A ) multiplied by the value of the input signal.
,Operational Amplifier General Conditions
• The Operational Amplifier, or Op-amp as it is most commonly called, can be an ideal
amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical
DC gains of well over 100,000 or 100dB.
• The basic Op-amp construction is of a 3-terminal device, with 2-inputs and 1-output,
(excluding power connections).
• An Operational Amplifier operates from either a dual positive ( +V ) and an corresponding
negative ( -V ) supply, or they can operate from a single DC supply voltage.
• The two main laws associated with the operational amplifier are that it has an infinite input
impedance, ( Z = ∞ ) resulting in “No current flowing into either of its two inputs” and zero
input offset voltage V1 = V2.
• An operational amplifier also has zero output impedance, ( Z = 0 ).
• Op-amps sense the difference between the voltage signals applied to their two input
terminals and then multiply it by some pre-determined Gain, ( A ).
• This Gain, ( A ) is often referred to as the amplifiers “Open-loop Gain”.
• Closing the open loop by connecting a resistive or reactive component between the output
and one input terminal of the op-amp greatly reduces and controls this open-loop gain.
• Op-amps can be connected into two basic configurations, Inverting and Non-inverting.
The Two Basic Operational Amplifier Circuits
, • For negative feedback, were the fed-back voltage is in “anti-phase” to the input the overall
gain of the amplifier is reduced.
• For positive feedback, were the fed-back voltage is in “Phase” with the input the overall
gain of the amplifier is increased.
• By connecting the output directly back to the negative input terminal, 100% feedback is
achieved resulting in a Voltage Follower (buffer) circuit with a constant gain of 1 (Unity).
• Changing the fixed feedback resistor ( Rƒ ) for a Potentiometer, the circuit will have
Adjustable Gain.
Inverting Operational Amplifier Configuration
In this Inverting Amplifier circuit the operational amplifier is connected with feedback to produce
a closed loop operation. When dealing with operational amplifiers there are two very important
rules to remember about inverting amplifiers, these are: “No current flows into the input terminal”
and that “V1 always equals V2”. However, in real world op-amp circuits both of these rules are
slightly broken.
This is because the junction of the input and feedback signal ( X ) is at the same potential as the
positive ( + ) input which is at zero volts or ground then, the junction is a “Virtual Earth”. Because
of this virtual earth node the input resistance of the amplifier is equal to the value of the input
resistor, Rin and the closed loop gain of the inverting amplifier can be set by the ratio of the two
external resistors.
We said above that there are two very important rules to remember about Inverting Amplifiers or
any operational amplifier for that matter and these are.
• No Current Flows into the Input Terminals
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