
What is meant by inverting amplifier?  
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Explanation of inverting amplifier working principle and circuit diagram using opamp.  
InvertingAmplifierOpampCircuit  
 
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 The basic OPAMP inverting amplifier is shown in below circuit.
The input voltage [$:]V[/:$] in is applied to the inverting input through the input resistor [$:]R_{in}[/:$]. The non inverting input is grounded. The feedback resistor [$:]R_f[/:$] is connected between the output and the inverting input. Since the input impedance of an opamp is considered very high, no current can flow into or out of the input terminals. Therefore [$:]I_{in}[/:$] must flow through [$:]R_f[/:$] and is indicated by [$:]I_f[/:$] (the feedback current). Since [$:]R_{in}[/:$] and [$:]R_f[/:$] are in series, then [$:]I_{in}[/:$] = [$:]I_f[/:$]. The voltage between inverting and noninverting inputs is essentially equal to zero volt. Therefore, the inverting input terminal is also at 0 volt. For this reason the inverting input is said to be at virtual ground. The output voltage ([$:]V_{out}[/:$] ) is taken across [$:]R_f[/:$]. So, [$$:]I_f =  \frac{V_{out}}{R_f}[/:$$] But [$:]I_{in}[/:$] = [$:]I_f[/:$]. So, [$$:]\frac{V_{in}}{R_{in}} =  \frac{V_{out}}{R_f}[/:$$] Rearranging the equation, [$$:]\frac{V_{in}}{V_{out}} =  \frac{R_{in}}{R_f}[/:$$] The voltage gain of an inverting amplifier can be expressed as, [$$:]A_v =  \frac{R_f}{R_{in}}[/:$$] The amplifier gain is the ratio of [$:]R_f[/:$] to [$:]R_{in}[/:$]. Finally, the output voltage can be found by, [$$:]V_{out} =  \frac{R_f}{R_{in}}V_{in}[/:$$] The output voltage is out of phase with the input voltage.  
 
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