Inverting Mode Amplifier

Inverting Operational Amplifier

The most widely used of all the op-amp circuits, the circuit is shown in figure. The output voltage V₀ is fed back to the inverting input terminal through the R_f - R₁ Network where R_f is the feedback resistor. Input signal V_i is applied to the inverting input terminal through R₁ and non- inverting input terminal of op-amp is grounded.

Consider as V_d = 0, node a is at ground potential and the current i₁ through R₁ is

also since op-amp draws no current, all the current flowing through R₁  must flow through R_f. The output voltage

Hence, the gain o f the inverting amplifier (also referred as closed loop(CL) gain) is,

Alternatively, the nodal equation at the node ‘a’ in following figure is

Where v_a  is the voltage at node ‘a’. Since node ‘a’ is at virtual ground v_a = 0, Therefore, we get

Physical diagrammatical phenomena of Inverting Amplifier: The basic circuit for the inverting op amp circuit is shown below. It consists of a resistor from the input terminal to the inverting amplifier input of the circuit, and another resistor connected from the output to the inverting input of the op-amp. The non-inverting input is connected to ground.

The negative sign indicates a phase shift of 180 degrees between v_i and v₀ .  In this inverting amplifier circuit the non-inverting input of the operational amplifier is connected to ground. As the gain of the op amp itself is very high and the output from the amplifier is a matter of only a few volts, this means that the difference between the two input terminals is exceedingly small and can be ignored. As the non-inverting input of the operational amplifier is held at ground potential this means that the inverting input must be virtually at earth potential. As a result, this form of amplifier is often known as a virtual earth amplifier.

Input impedance:   With the value of R₁, the input resistor being lower than the feedback resistor R_f, care has to be taken when designing he circuit to ensure the input resistance is not too low for any given application. Often values of 10kΩ are used as this provides a reasonable load resistance for many circuits. However the exact requirements need to be determined for each application.

Range of values for R1 and R2:   R_f must be higher. Even though the input impedance of the integrated circuit itself circuit may be high, using a high value of feedback resistor is not advisable as results may become a little unpredictable. As a very broad rule of thumb values for R_f of up to 100kΩ or a little more should be fine.

Summing amplifier or virtual earth mixer

The fact that the inverting input to the amplifier is virtually at earth potential provides some significant possibilities for audio mixer applications.

This form of virtual earth mixer or summing amplifier adds several different signals in a linear or additive fashion. It is not the form of multiplier mixer used in RF circuits where a non-linear action is used.