Electrical Engineering ⇒ Topic : 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 Vs 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:
Since, the current into each input terminal is zero, the current passing through R1 also passes through R2. Again as Vi is zero, it can be written as:
Vo/Vs is called the closed loop voltage gain Av. In other words, it is the gain with feedback
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,
From Eq. (3), it can be said that the input resistance Ri is equal to the external resistance R1 for an ideal OP Amp.
Output resistance Ro is defined as the resistance seen at the output of the amplifier when the input terminal is set equal to zero
For ideal inverting OP Amp, Vi = 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 Vi to zero, there is no current from terminal 1 to terminal 2. Therefore, the word virtual is present
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