Electrical Engineering ⇒ Topic : Inverting Amplifiers

INVERTING AMPLIFIERS

The circuit symbol and equivalent circuit of an ideal inverting amplifier is shown in Figure (a).

                        Figure (1) Ideal OP Amp in inverting mode.

In inverting mode, the positive terminal is being grounded. The input signal V_s is applied to the negative terminal through a resistor R₁. In this circuit, feedback is applied through a resistor R₂ from the output to input. For ideal OP Amp, the following two conditions are always satisfied:

• The current into each input terminal is zero.
• Between the two input terminals, the potential difference V_i, is always zero.

Since, the current into each input terminal is zero, the current passing through R₁ also passes through R₂. Again as V_i is zero, it can be written as:

                                         ...........(1)

V_o/V_s is called the closed loop voltage gain A_v. In other words, it is the gain with feedback

                                         ............. (2)

From Eq. (2), it is clear that the voltage gain is dependent on R₁ and R₂.

Input resistance R₁

Since the input terminal of the ideal amplifier is held at virtual ground by the feedback. Thus,

                                            .............(3)

From Eq. (3), it can be said that the input resistance R_i is equal to the external resistance R₁ for an ideal OP Amp.

Output resistance

Output resistance R_o is defined as the resistance seen at the output of the amplifier when the input terminal is set equal to zero

Virtual ground

For ideal inverting OP Amp, V_i = O. This indicates that terminal 1 has the same potential as that of terminal 2. Since terminal 2 is grounded, its voltage is zero. Although terminal 1 is not grounded, its voltage is zero. So, it is called virtually grounded. Since the feedback serves to keep the voltage V_i to zero, there is no current from terminal 1 to terminal 2. Therefore, the word virtual is present