Electrical Engineering ⇒ Topic : Inverting Amplifiers

Gaurav
 
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 R_{1}. In this circuit, feedback is applied through a resistor R_{2} 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 R_{1} also passes through R_{2}. 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 R_{1} and R_{2}. Input resistance R_{1} 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 R_{1} 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  
 
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