1. Ronald Cris P. Rezarre
BS-ECE 5
Operational Amplifier
An operational amplifier ("op-amp") is a DC-coupled high-gain electronic voltage amplifier with a
differential input and, usually, a single-ended output.[1] An op-amp produces an output voltage that is
typically hundreds of thousands times larger than the voltage difference between its input terminals.
Operational amplifiers had their origins in analog computers where they were used in many linear, non-
linear and frequency-dependent circuits. Characteristics of a circuit using an op-amp are set by external
components with little dependence on temperature changes or manufacturing variations in the op-amp
itself, which makes op-amps popular building blocks for circuit design. Op-amps are among the most
widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific
devices. Many standard IC op-amps cost only a few cents in moderate production volume; however
some integrated or hybrid operational amplifiers with special performance specifications may cost over
$100 US in small quantities.[citation needed] Op-amps may be packaged as components, or used as
elements of more complex integrated circuits.The op-amp is one type of differential amplifier. Other
types of differential amplifier include the fully differential amplifier (similar to the op-amp, but with two
outputs), the instrumentation amplifier (usually built from three op-amps), the isolation amplifier
(similar to the instrumentation amplifier, but with tolerance to common-mode voltages that would
destroy an ordinary op-amp), and negative feedback amplifier (usually built from one or more op-amps
and a resistive feedback network).The power supply pins (VS+ and VS−) can be labeled in different ways
(See IC power supply pins). Despite different labeling, the function remains the same — to provide
additional power for amplification of the signal. Often these pins are left out of the diagram for clarity,
and the power configuration is described or assumed from the circuit.
Basic Internal Circuit of a Simple Operational Amplifier (Op-Amp)

2. An op amp basically has 4 circuit stages: 1) an input stage; 2) an intermediate stage; 3) a level-
shifting stage; and 4) an output stage. The input stage of an op-amp is usually a pair of matched
transistors configured as a dual-input differential amplifier. The output of this input stage is taken from
across the outputs (collectors in the above example) of the paired transistors. This balanced output is
fed into another dual-input differential amplifier that serves as the intermediate stage. The output of
this intermediate stage is taken from just one of the transistors, i.e., it is single-ended and therefore not
balanced. The dc level at the output of the intermediate stage is high with respect to ground, so a level-
shifting circuit such as an emitter follower is used to shift it down closer to ground. The output stage of
an op amp usually consists of a push/pull pair of complementary transistors which increases the swing
of the output voltage and enhances the load current capacity of the op amp. The gains of the input and
intermediate stages of an op amp are high, while those of the emitter follower and output stage are
generally close to 1.
The Ideal Op-amp
The IC Op-amp comes so close to ideal performance that it is useful to state the characteristics
of an ideal amplifier without regard to what is inside the package.
Infinite voltage gain
Infinite input impedance
Zero output impedance
Infinite bandwidth
Zero input offset voltage (i.e., exactly zero out if zero in).
These characteristics lead to the golden rules for op-amps. They allow you to logically deduce
the operation of any op-amp circuit.
The Op-amp Golden Rule
For an op-amp with external feedback
I. The output attempts to do whatever is necessary to make the voltage difference between the
inputs zero.
The Voltage Rule
II. The inputs draw no current.
The Current Rule