Inverting Amplifier Gain Analysis

This is a constant gain amplifier with negative feedback where the input signal is applied to the inverting terminal of the opamp.  The resultant output is inverted.

As per Kirchoff’s Current Law,

I + If = Ii

Opamp input current is Zero i.e Ii = 0

I + If = 0

If Vi is the voltage at the inverting input terminal,

(Vs - Vi)/Ri + (Vo - Vi)/Rf = 0

Opamp input voltages are always equal, ∴ Vi = 0, because non inverting input terminal is connected to ground.

Vs/Ri + Vo/Rf = 0

Vo/Vi = -Rf/Ri

∴ The gain of inverting amplifier is A = -Rf/Ri

OPAMP Basics

OPAMP (Operational Amplifier) is an Integrated Circuit (IC) designed for Linear Amplification.

Ideal OPAMP Characteristics:

1. Infinite Input Impedance

2. Zero Output Impedance

3. Differential Mode Gain is infinite

4. Common Mode Gain is zero

5. Bandwidth is infinite

6. Slew rate is infinite

Understand the benefits of Ideal OPAMP Characteristics:

• Infinite Gain because of Infinite input impedance and Zero output impedance
• No noise amplification as the Common mode gain is zero
• Amplification of all the frequency ranges from DC to infinite frequency of AC signal as the Bandwidth is infinite

OPAMP Golden Rules:

1. Inputs draw no current

2. Output does whatever is necessary to maintain the two inputs at equal voltages

In every Opamp analysis the above rules are applicable.

OPAMP Circuits:

1. Inverting Amplifier

2. Non-Inverting Amplifier

3. Voltage Follower

4. Comparator

5. Differential Amplifier

6. Integrator

7. Differentiator

8. Summation Amplifier

9. Instrumentation Amplifier

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