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2/18/2011 The NonInverting Configuration lecture 1/4 The Non-Inverting Configuration R1 R2 i1 i2 i- =0 vin i+ =0 v- - ideal v+ vout + Good heavens! The inverting input (v+) of this configuration is not at virtual ground (i.e., v + ≠ 0 )! Recall that v- =v+ (the virtual short) ALWAYS for feedback amplifiers. Jim Stiles The Univ. of Kansas Dept. of EECS 2/18/2011 The NonInverting Configuration lecture 2/4 No virtual ground here! Notice also that for the circuit above, the voltage at the non-inverting terminal is the input voltage vin : ∴ v − = vin ≠ 0 We use this fact to analyze this non-inverting configuration. First, we use KCL to determine that: i1 = i− + i2 R1 R2 i1 i2 i- =0 and since i− = 0 , we again find that: v- i1 = i2 and from Ohm’s Law: i1 = Jim Stiles 0 −v − R1 = −v − R1 vin i2 = i+ =0 - ideal v+ vout + v − −vout R1 The Univ. of Kansas Dept. of EECS 2/18/2011 The NonInverting Configuration lecture 3/4 i- = 0 is the key These results are of course very similar to the expressions we derived when analyzing the inverting configuration. The main difference is of course that v- is not equal to zero. Instead, we know that v − = vin . Thus: i1 = −vin and since i1 = i2 , we determine a relationship involving vin and vout only: −vin R1 = vin −vout R2 R1 i2 = vin −vout R1 R1 R2 i1 i2 i- =0 vin Jim Stiles The Univ. of Kansas i+ =0 v- - ideal v+ vout + Dept. of EECS 2/18/2011 The NonInverting Configuration lecture 4/4 Note the gain is a positive number Performing some simple algebra, we rearrange this expression and find the open-circuit voltage gain of the non-inverting configuration: oc vout R =1+ 2 Avo = vin R1 Note that the open-circuit voltage gain for this configuration is a positive number. We conclude then that the input and output voltage will have the same sign (i.e., ± ). This is why we call the configuration noninverting. Jim Stiles The Univ. of Kansas Dept. of EECS