2/13/2011 Ri and Ro of the Inverting Amplifier lecture 1/11 Rin and Rout of the Inverting Amplifier Recall that the input resistance of an amplifier is: Rin = vin iin For the inverting amplifier, it is evident that the input current iin is equal to i1 : R2 i2 R1 vin v- oc vout iin = i1 v+ Jim Stiles + The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 2/11 Its input resistance From Ohm’s Law, we know that this current is: iin = i1 = The non-inverting terminal is “connected” to virtual ground: vin − v1 R1 R2 v− = 0 i2 and thus the input current is: iin = i1 = vin R1 R1 vin vin = vin iin oc vout iin = i1 v+ We now can determine the input resistance: Rin = v- + ⎛ R1 ⎞ ⎜ ⎟ = R1 ⎝ vin ⎠ The input resistance of this inverting amplifier is therefore Rin = R1 ! Jim Stiles The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 3/11 Output resistance is harder Now, let’s attempt to determine the output resistance Rout. Recall that we need to determine two values: the short-circuit output current sc oc and the open-circuit output voltage vout . iout ( ) ( ) To accomplish this, we must replace the op-amp in the circuit with its linear circuit model: R2 i2 vin R1 - v- op Rout ii = i1 + v + Jim Stiles + - vout = 0 op iout Aop (v + − v − ) The Univ. of Kansas sc iout Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 4/11 First, the short circuit output current R2 From KCL, we find that: i2 op sc iout = i2 + iout where: i op out and: vin = oc −Aop v − − vout R op o = −Aop v − R1 - v- op Rout ii = i1 + v + R op o + - vout = 0 op iout Aop (v + − v − ) sc iout oc v − − vout v i2 = = − R2 R2 Therefore, the short-circuit output current is: i sc out Jim Stiles op v − Aop v − ⎛ Rout − R2 Aop = − op = ⎜ op ⎜ R2 Rout R2 Rout ⎝ ⎞ Aop ⎟v − ≅ − op v − ⎟ Rout ⎠ The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 5/11 Now, the open circuit output voltage R2 The open-circuit output voltage can likewise be determined in terms of Aop and v- . i2 R1 vin - v- ii = i1 + v + iout =0 op Rout + - op iout Aop (v + − v − ) + oc vout - Here, it is evident that since iout = 0: op i2 = −iout where we find from Ohm’s Law: i2 = Jim Stiles v − − ( − Aopv − ) op R2 + Rout ⎛ 1 + Aop =⎜ op ⎝ R2 + Rout The Univ. of Kansas ⎞ ⎟v − ⎠ Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 6/11 The open-circuit output voltage R2 Now from KVL: i2 oc vout = v − − R2 i2 R1 vin Inserting the expression for i2 : ⎛ 1 + Aop ⎞ v op ⎟ − ⎝ R2 + Rout ⎠ ( ≅− Jim Stiles R2 Aop R2 + R op out v- + v + ) ⎞⎟ v ⎟ ⎠ iout =0 op Rout ii = i1 oc = v − − R2 ⎜ vout ⎛ R + R op R2 1 + Aop out =⎜ 2 − op op ⎜ R2 + Rout R2 + Rout ⎝ ⎛ Roop − R2 Aop ⎞ =⎜ ⎟ v− op ⎜ R2 + Rout ⎟ ⎝ ⎠ - + - op iout Aop (v + − v − ) + oc vout - − v− The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 7/11 Now we find the output resistance Now, we can find the output resistance of this amplifier: Rout oc vout = sc iout ⎛ −R2 Aop ⎞ ⎛ − Aop ⎞ =⎜ op ⎟ ⎜ op ⎟ R R R + 2 o o ⎝ ⎠⎝ ⎠ −1 R2 Roop = R2 + Roop = R2 Roop In other words, the inverting amplifier output resistance is simply equal to the op . value of the feedback resistor R2 in parallel with op-amp output resistance Rout Jim Stiles The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 8/11 This is zero if the op-amp is ideal Ideally, of course, the op-amp output resistance is zero, so that the output resistance of the inverting amplifier is likewise zero: op Rout = R2 Rout = R2 0 =0 Note for this case—where the output resistance is zero—the output voltage will be the same, regardless of what load is attached at the output (e.g., regardless of iout )! R2 i2 vin R1 - v- iout ≠ 0 ii = i1 + v + + - op iout Aop (v + − v − ) + R vout = − R vin 2 1 - Jim Stiles The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 9/11 For real op-amps the output resistance is small Thus, if Rout = 0 , then the output voltage is equal to the open-circuit output voltage—even when the output is not open circuited: vout = − R2 v R1 in for all iout !! Recall that it is this property that made Rout = 0 an “ideal” amplifier characteristic. We will find that real (i.e., non-ideal!) op-amps typically have an output op R2 ), so that the inverting amplifier output resistance that is very small ( Rout resistance is approximately equal to the op-amp output resistance: op Rout = R2 Rout op ≈ Rout Jim Stiles The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 10/11 A summary Summarizing, we have found that for the inverting amplifier: Rin = R1 op Rout ≈ Rout (ideally zero) Thus, this inverting amplifier… iin (t ) R2 R1 + iout (t ) - vin (t ) + + − vout (t ) − Jim Stiles The Univ. of Kansas Dept. of EECS 2/13/2011 Ri and Ro of the Inverting Amplifier lecture 11/11 The inverting amp equivalent circuit …has the equivalent circuit: iout (t ) iin (t ) op Rout + vin (t ) − R1 + − − ( ) v (t ) R2 R1 + vout (t ) − in Note the input resistance and open-circuit voltage gain of the inverting amplifier is VERY different from that of the op-amp itself! Jim Stiles The Univ. of Kansas Dept. of EECS