Basic OP-AMP circuits :
This section concentrates on the principles involved with basic OP-AMP circuit viz, (i) inverting and (ii) non-inverting amplifiers.
(i) Inverting amplifier
(ii) Non-inverting amplifier
(iii) Summing amplifier
(iv)Difference amplifier

*Basic OP-AMP circuits*

This section concentrates on the
principles involved with basic OP-AMP circuit viz, (i) inverting and (ii)
non-inverting amplifiers.

*(i) Inverting amplifier*

The basic OP-AMP inverting
amplifier is shown in Fig. The input voltage V_{in} is applied to the
inverting input through the input resistor R_{in}. The non inverting
input is grounded. The feedback resistor R_{f} is connected between the
output and the inverting input.

Since the input impedance of an
op-amp is considered very high, no current can flow into or out of the input
terminals. Therefore I_{in} must flow through R_{f} and is
indicated by I_{f} (the feedback current). Since R_{in} and R_{f}
are in series, then I_{in} = I_{f}. The voltage between
inverting and non-inverting inputs

is essentially equal to zero
volt. Therefore, the inverting input terminal is also at 0 volt. For this
reason the inverting input is said to be at virtual ground. The output voltage
(Vout) is taken across Rf.

It can be proved that

I_{f}=V_{out}/R_{f}

Since I_{in} =I_{f },
then

V_{in}/R_{in} =
-V_{out}/R_{f}

Rearranging the equation, we
obtain

-V_{out}/V_{in} =
R_{f}/R_{in}

∴ The voltage gain of an inverting amplifier can be expressed as

A_{v} = -R_{f}/R_{in}

The amplifier gain is the ratio
of R_{f} to R_{in}

Finally, the output voltage can
be found by

V_{out} = -R_{f}/R_{in}
x V_{in}

The output voltage is out of
phase with the input voltage.

*(ii) Non-inverting amplifier*

The basic OP-AMP non-inverting
amplifier is shown in Fig. The input signal V_{in} is applied to the
non-inverting input terminal. The resistor R_{in} is connected from the
inverting input to ground. The feedback resistor R_{f} is connected between
the output and the inverting input.

Resistors R_{f} and R_{in}
form a resistive ratio network to produce the feedback voltage (V_{A})
needed at the inverting input. Feedback voltage (V_{A}) is developed
across R_{in}. Since the potential at the inverting input tends to be
the same as the non-inverting input (as pointed out with the description of
virtual ground), V_{in} = V_{A}.

Since V_{A} = V_{in},
the gain of the amplifer can be expressed as

_{Av}_{= }^{V}*ou t*

*V _{A}*

However, V_{A} is
determined by the resistance ratio of R_{in} and R_{f} ; thus,

V_{A}=[R_{in}/(R_{f}/R_{in})]
V_{out}

A_{v}=1+(R_{f}/R_{in})

Finally, the output voltage can
be found by, Vout = (1+R_{f}/R_{in})V_{in}

It is seen that the input and
output voltages are in phase

**(iii) Summing amplifier**

The summing amplifier provides an
output voltage equal to the algebraic sum of the input voltages.

Fig shows an inverting amplifier,
used to sum two input voltages. The input voltages v_{1} and v_{2}
are applied through the resistors R_{1} and R_{2} to the
summing junction (P) and Rf is the feedback resistor. At the point P,

i_{1} + i_{2} =i_{f}

Since the voltage at the point P
is ideally 0

(V_{1}/R_{1}) +
(V_{2}/R_{2}) =(V_{out}/R_{f})

Hence the output voltage,

V_{out}=-[(R_{f}V_{1}/R_{1})
+(R_{f}V_{2}/R_{2}) ]

If R_{1} = R_{2}
= R_{f} = R, then vout = - (v_{1}
+ v_{2})

Hence the output voltage is equal
to the sum of the input voltages and the circuit acts as a summing amplifier.
The negative sign indicates that OP-AMP is used in the inverting mode.

**(iv)Difference amplifier**

The difference amplifier is shown
in Fig. The output voltage can be obtained by using superposition principle. To
find the output voltage v_{01} due to v_{1} alone, assume that
v_{2} is shorted to ground. Then

V^{+} = R_{2}V_{1
}/ R_{1}+R_{2}

Therefore, with both inputs
present, the output is

V_{0} = V_{01} +
V_{02}

= (R_{3}+R_{4 / }R_{3})
(R_{2}/R_{1}+R_{2}) V_{1
} - (R_{4}/R_{3})V_{2}

If R_{1} = R_{2}
= R_{3} = R_{4} = R

then vo = v_{1} - v_{2}

If all the external resistors are
equal, the voltage difference amplifier functions as a voltage subtractor.

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