Non Inverter Operational Amplifier theory and study

In this chapter we will study the other topology that the operational amplifiers can assume, that is, the non-inverting configuration.

2. The Non-Inverting Operational Amplifier

In this configuration the input signal is connected directly to the non-inverting input and the entire feedback circuit is connected to the inverter input. This configuration is show in Figura 43-01

Let's calculate the gain of the circuit in closed loop. Remember that we are considering the opamp as ideal. So we know that V₁ = V₂ and also that V_s = V₂.With these data, we conclude that V_s it's on the resistor R_i. With these data, and using a resistive voltage divider, we can write that:

V_s = V_o (R_i / (R_i + R_f ))

How we want the relationship V_o / V_s, so working algebrically the equation, we arrived at:

eq. 43-01

Once again we realize that the gain of the closed loop circuit is independent of the device used, being exclusively dependent on the values of the elements that make up the feedback circuit.

Note that this configuration does not allow to be used as attenuator, because analyzing the equation, we realize that the minimum gain is unity (= 1), and for this we must have R_f = 0, that is, a short circuit between the output and the inverting input.

This artifice to use R_f = 0 and eliminate R_i, characterizes the configuration called buffer. This configuration is used when we want to "isolate" a circuit that has a reasonably high output impedance from another circuit that has a low input impedance. If we do not use a buffer, we run the risk of overloading the output of the first circuit.

When we use a buffer, we know that the input impedance of the amplifier operating system tends to infinity, thus not causing an overload to the predecessor circuit. And because the output impedance tends to zero, we guarantee the possibility of feeding a low-impedance successor circuit. This configuration was already studied in chapter 41, and you can access Here!!

3. Amplifier with Variable Gain + K to - K

Through a special configuration it is possible to adjust the feedback of an amplifier from the feedback positive to negative feedback. With this, we can vary the amplifier gain between + K and - K. Look at Figure 43-02 below.

When the P_aj potentiometer cursor is connected to ground (lower part of P_aj in the figure), the amplifier it will behave like an inverting amplifier. In this case, using eq. 42-01 we can easily deduce that the amplifier gain, which we will represent by A_v, will be:

On the other hand, when the P_aj potentiometer cursor is connected to the upper part of P_aj, the amplifier will behave like a non-inverting amplifier, as the input voltage V_i will be connected directly to the input positive from OP. And as we know, virtually the voltage at the negative input is the same as that at the positive input. In this case, the resistor R will be under a null potential difference. In other words, it is as if the points a and b were connected to each other. Then the circuit gain will be given by eq. 43-01. Thus, the gain will be:

Therefore, this circuit allows the amplifier gain to be varied from a positive value to a negative value. Assuming a voltage gain from the order of 8, when we connect a voltage of 1 V to the input, the output may vary from + 8 V to - 8 V .

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