Operational Amplifiers
Operational Amplifier Stages
Inverting Amplifier
Non-Inverting Amplifier
Summing Amplifier
Introduction
The name operational amplifier was originally adopted for a series of high performance
DC amplifiers used in analog computers. These amplifiers were used to perform
mathematical operations applicable to analog computation such as
summation ,
scaling,
subtraction,
integrating, etc .
Equivalent Circuit for an Ideal Operational Amplifier
(1) The voltage gain is infinity,
Avo = ∞ .
(2) The input resistance is infinity,
rin = ∞ .
(3) The output resistance is zero,
ro = 0 .
(4) The bandwidth is infinity,
BW = ∞ .
(5) There is zero input offset voltage,
Eo = 0 if Ein = 0 .
Input Stage
The input stage is a
dual input,
balanced output differential amplifier.
It has 2 inputs V
in1 and V
in2 which are applied at the bases B1 and B2
of transistors Q
1 and Q
2 .The output V
0 is measured between the two collectors
C
1 and C
2 which are at the same dc potential.Because of the equal dc
potential at the two collectors with respect to ground, the output is
referred to as balanced output.
Intermediate Stage
The next stage is
dual input, unbalanced output difference amplifier.
Here two input signals are used however the output is measured at only
one of the two collectors with reference to ground. The output is
referred to as an
unbalanced output because the collector at which
the output voltage is measured is at some finite dc potential with
reference to ground. In other words, there is some
dc voltage at the
output terminal without any input signal applied
One of the most common applications is the simple inverting amplifier.
The output is inverted, and the gain is determined by the
ratio of the
feedback resistor
(Rf) to the
input resistor
(Rin).
Another common configuration is the non-inverting amplifier, where the output
signal is
not inverted . In this circuit, the input voltage is applied to the
positive input of the op amp, and a fraction of the output signal is applied
to the negative input from the
(Rf) -
(Rin) voltage divider .
This is a special case of the inverting amplifier, as it gives an inverted output
which is equal to the weighted
algebraic sum of all inputs. If the input resistors,
and the feedback resistor are chosen to be
equal , the output is simply the
negative
sum of the inputs. Since there is no interaction between inputs, the operations
of summing and weighting is very easily done.
R5 = R1 || R2 || R3 || R4
