Understanding op-amps -3

Prerequisites

• Remember the fundamentals you learnt in the second lesson.
• Keep the op-amp practical kit ready with you.
• We are using the LM 324 ic which can be used with two power supplies, +ve and -ve 9V.
• While making connections, always refer to the pin configuration of LM324 ic from the last lesson.
• Op-amp can be connected in two configurations. Each of them is useful in different ways. In this lesson you will perform small experiments to verify the theory about these configurations. For this you will need a duel polarity power supply (+ and – 9V). Connect the two 9V flat batteries in the fashion shown below.

In the op-amp based circuit diagrams, where you see ground, connect the center terminal of the above assembly.

Configurations

Salient features

• Inverting amplifier uses inverting input terminal of the op-amp to apply signal.
• Negative feed-back is used. i.e. Resistor is connected from output to inverted input.
• This configuration uses concept of virtual earth. (Notice that non-inverting terminal is connected to ground. We know that due to very high gain, voltage of inv and non-inv inputs is almost the same. This makes the inverting input to have zero potential but is not actually connected to ground. So it is called virtual earth)
• Output voltage in this configuration has opposite sign as that of input signal voltage. It also means that they are in phase opposition with each other.
• Relation between input and output voltage is given by:

$V_{out}=\frac{- R_f}{R_i}*V_{in}$

Where, $R_f$ is feed back resistance and $R_i$ means input resistance. In the circuit above the gain turns out to be: 10.

Experiment

After implementing the above circuit, note output voltages for various positions of the preset screw and confirm the above formula.

• We can add two (or more) voltages by using this circuit.
• Notice how two different voltages are connected to two different input resistors. These two resistors have a common point (inverting input terminal).
• Current flowing through each input resistor is added and the total current flows through the feedback resistance. The voltage drop, across the feed-back resistor is thus the sum of two input voltages and is the output voltage.
• This circuit can be used to implement digital to analog converter.
• The addition of current takes place because of the virtual earth at inverting terminal so adder circuit is possible only with inverting configuration and not non-inverting one.
• The formula of output voltage for the above circuit is:

$V_{out}=- [( V_1*4.55)+(V_2*10)]$ (Think, why?)

Salient features

• Notice NEGATIVE feed-back.
• Input signal to the non-inverting input.
• No change in sign of the output voltage.
• Relation between Vout and Vin is given by:

$V_{out}=V_{in}*[1+\frac{R_f}{R_i}]$

• Implement the above circuit.

Salient features

• A special variation of non-inverting amplifier is voltage follower.
• $R_f$ is zero.
• So for any value of $R_i$, gain is 1+0 = 1
• So $V_{in}=V_{out}$
• This circuit offers very high input impedance so can be used as unity gain buffer. (Where we want to keep the magnitude of the signal same but don’t want to “load” the signal source.)
• Implement the above circuit.
• Now if you don’t have any doubts regarding any of the above configurations, take the quiz below.
• This is the last article in this series to get introduced to the operational amplifiers.