V is sensitive only to the difference between the input currents,

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1 PHYSICS 56 Experiment : IC OP-Amp and Negative Feedback In thi experiment you will meaure the propertie of an IC op-amp, compare the open-loop and cloed-loop gain, oberve deterioration of performance when the loop gain i not large, and oberve the baic amplifier (inverting, non-inverting, and follower). Pleae remember to compare your oberved and expected value.. DC Characteritic. The DC output voltage i determined by the input current ( I + and I ), the input off-et voltage ( V io ), and the feedback reitor (,,and ). + V = [ I+ + I (, ) ± Vio] (.88) When = (, ), V i enitive only to the difference between the input current, called the input off-et current, I = I I. o + V + = ± I ± V ] [ o io. Cloed-oop Gain. NON - INVET: G G + G = = with G = + / AB + j( f / f) INVETING: G G G = = with G = + / AB + j( f / f ) (.89) (.6,.) (.7) where f = BfT = ft / G ( non inv) (.7) The cloed-loop gain i controlled by the feedback component when the loop-gain (AB) i large. arge loop-gain mean that A G or f f. The quality of the amplifier i determined by the loop-gain, which become approximately one a the ignal frequency (f) approache the cloed-loop break frequency ( f ). AB = A/ G ( non inv) = f / f (.7). Effect of Quality atio on Input and Output Characteritic. When A G, the inverting input act like a virtual ground, hence the ignal (v ) at that point i very mall. vg v = A + ( G + ) G = with / (.7)

2 A i not much greater than G unle f f, hence the virtual ground concept ( v v ) i not valid when the ignal frequency (f) i cloe to f. The cloed loop output reitance ( r ) alo i very mall when oc f f. roo roo roc = = (.8) + AB jf / f r become approximately equal to the open-loop output reitance ( ) when oc roo f f. Since the gain and output reitance are both frequency dependent, it i convenient to combine the two effect. (not ued on tet) (.8,.84) G G = f + j ( f )/( + r / ) oo Thi equation how that adding a mall load ( ) at the output of a feedback-amplifier ha the ame effect a reducing the break frequency ( f ). ha little effect when, but it can caue a ubtantial decreae in gain when f f f f. 4. The Slew ate determine the maximum rate at which output voltage can change. The maximum amplitude that a ine wave can have without ditortion i, dv / ( / ec) dt V u vo ( p p) = π f( MHz) (.57) The 74 op-amp i a general purpoe device with f MHz. It i fully compenated i.e., uitable for G = T ). Set up the following circuit on the plug-in chai. The op-amp hould traddle the center pace of the plug board. The two power pin (4 and 7) mut be connected at the beginning, but the null circuit (pin and 5) i not connected until tep- 5. The adjutable reitor i decribed on the lat page of the general lab intruction. Adjut the power upplie for 5 volt before they are connected to the circuit. The opamp will be detroyed if the power voltage exceed 8V. Turn off the power upply before you change component. The pin on the op-amp can be damaged when you pull it out of the circuit board.

3 B K +5V - K.µf V A +.µf 4-5V Hold it at the end or inert a mall crewdriver under the op-amp to lift it from the board. In ome experiment it i ufficient to put by-pa capacitor at one end of the green and red power ocket. Op-amp may ocillate unle the by-pa capacitor are connected cloe to the DC voltage pin on the op-amp. Connect one end of a by-pa capacitor to the ame row of ocket ued for the op-amp pin (4 and 7). Connect the other end of the capacitor to ground. A. DC imitation,, and will be adjuted in thi part of the experiment to oberve eparately the effect of each of the input parameter. Table 4. in the text give only the maximum value for and. The following typical value provided by the manufacturer hould I b I o be ued in calculation: V, io = ma I 8, and Io = na. b = na ) Calculate the expected value of V for the reitor combination given on the component heet for tep,, and 4. The term meter in the column i explained in tep. You hould check that they op-amp i working before beginning DC meaurement. Apply a mall, Hz inuoidal ignal at the input (A) and oberve that an amplified ine wave appear at the output. Diconnect the ignal generator before tep. ) Meaurement. Meaure and calculate. Why can Vio V V io I be neglected in thi b meaurement? Do not connect the null circuit until tep 5. ) I + Meaurement. i replaced by the digital meter, which i et to record voltage. Connect a.µf capacitor from pin to ground to uppre noie generated by the digital meter at the input of the amplifier. The current I + goe into the +

4 input and flow through a M reitor inide the meter to produce the meaured voltage. (efer to GI.) Calculate I + uing I=V (meter)/m. Meaure uing the analog meter or the cope with the input connected for DC. Why i the effect of V on minor in thi arrangement? io V 4) I o Meaurement. Continue uing the digital meter in place of. Inert equal M, oberve V, and calculate I. Notice that i much maller in tep 4 than in o V tep, although V ha not changed. Explain thi obervation. emove the meter + and.µf capacitor from pin. 5) Null Adjutment. Connect the null circuit to pin and 5. Make the connection pecified for tep 5A on the component heet. Since the output i connected to V and V i zero, i equal to. Adjut the variable reitor until i. + V Vio V ± mv Next, change the gain to by uing the 5B connection. Now a mall maladjutment of V will have much more effect on. Adjut the variable reitor io V to ee how cloe you can get to zero. It hould not be neceary to readjut the null becaue V =.V V i atifactory for the ret of thi experiment. B. AC imitation The non-inverting amplifier form will be ued for all meaurement except tep 7-, i.e. the input ignal ( V ) hould be applied to point A. 6) Include in your laboratory report the calculated value of. v / v 7) Meaure o at khz with v =V. Compare v and uing dual trace to ee that they are in phae. 8) Saturation limit. Increae v until the peak of are cut off by the amplitude limit. eport the oberved aturation limit. The maximum output voltage hould be approximately volt maller than the power upply voltage. Slew rate limit will be oberved for quare and ine wave. 9) Include in your laboratory report the following calculation. Given a poitive lewrate V / t =.5V/µec and a negative lew-rate of.7v/µec. Aume that the output ignal i a V, khz, quare wave. Sketch howing time and voltage cale. Calculate the maximum amplitude hat a khz ine wave can have without lew-rate ditortion. ) Initially ue a V, khz quare wave for v, the oberved lew rate may be ubtantially lower than the etimate give in tep 9. If neceary, reduce the quare wave frequency until ha a contant voltage period between the change. Meaure the poitive and negative lew rate from the cope trace. Draw a ketch of. ) Ue a khz ine wave to oberve lew-rate ditortion. Start with approximately V and increae the amplitude of the input ignal ( v ). You will notice that become ditorted, looking more like a triangle than a ine wave. G V

5 Compare the amplitude at which the ditortion begin to that expected for the lewrate oberved in tep. ) I O limit. When the external load reitor ( ) i mall, the output voltage i limited by the current available from the op-amp, (max) = o v I O (max). Ue a Khz, V input ignal. Connect a ohm reitor from the output to ground. Meaure the poitive and negative limit on the output ignal and calculate the output current limit. C. Open-oop and Cloed-oop Bode Plot Open-oop Bode Plot It i practically impoible to meaure the true open-loop gain of a high-gain op-amp. The low frequency gain i o high that the input noie caue large random variation at the output. The implet way to reduce the low-frequency gain i to ue negative feedback. Above the break frequency, the gain with feedback i approximately equal to the true open loop gain, a hown in the following ketch. log gain A true open-loop gain G gain with feedback G=A f b log frequency ) Include the following calculation in your laboratory report. Given f = MHz, T calculate the break frequency f b. 4) Inert and in the circuit. Meaure f b and the gain G at.,,, and time f b. Ue thee 4 point to draw G on log-log graph paper. Ue the two high frequency point to draw A. Your plot will be like the one hown above, except your axe hould be quantitative. Cloed-oop Bode Plot. Change. 5) Calculate the new cloed-loop gain G and break frequency f MHz. Include thi in your laboratory report. 6) Meaure G at mid frequency and meaure f exceed the lew limit. Calculate f from G and T, auming f = T. (eview GI-5.6A) Don t f and compare it to the typical value of MHz. Ue the meaured value of G and f to add thi cloed loop again

6 to the Bode plot tarted in tep 4. Show f on the Bode plot. The final Bode plot hould be imilar to the following ketch. D. Dependence of Quality on A/ G O. It i inconvenient to oberve the input reitance of the op-amp becaue it i rather high, even without feedback (typically Meg). We will oberve the output reitance and the virtual-ground ignal at the input of the op-amp ( v ) to ee that both are very mall when A >>, but they increae when A drop with increaing frequency. G O Virtual Ground. Ue the inverting amplifier arrangement. Diconnect point-b from common and connect the input ignal ( v ) to point B. 7) Aume the input ignal ( v ) i V and calculate the ignal ( v ) at the inverting output of the op-amp at Hz, khz, and khz. Include thi in your laboratory report. 8) Meaure the ignal ( ) at the inverting input of the op-amp at khz. Oberve that the ignal v v become very mall a the frequency i decreaed. 9) Calculate G O for the inverting form. Include thi in your laboratory report. ) Meaure G at mid-frequency. Compare the input ignal (at point B) to the output ignal uing dual trace to ee that the circuit invert. Output eitance. eturn to the non-inverting form (reconnect point B to common) and change the feedback to obtain a follower. ) Compare the input and output ignal and oberve that they are eentially identical. ) Include the following calculation in your laboratory report. Given r oo = 75 ohm, v =.V, and f T = MHz. Calculate the magnitude of r oc at khz, khz, and MHz uing equation.8. You hould ee that i mall until the ignal i near the break frequency. The mot convenient way to ee the effect that r oc r oc ha on the

7 output ignal v i to calculate the magnitude of v with and without o o uing equation.8 and.84. ) Ue an input ignal of V and meaure at khz, and MHz, with and without. COMPONENTS D = direct connection and X = no connection STEPS Ω Κ Ω X X D Ω X 4 X Μ Μ (Meter) X 5A X D Μ (Meter) X 5B Ω Κ Ω X 6- Κ Κ Ω X Κ Κ Ω Ω -4 Ω Κ Ω X 5-6 Κ Κ Ω X 7- Κ Κ Ω X -4 X D Ω Ω COMPONENTS 74 op-amp. µ f Ω Ω Κ Κ Κ Κ Μ Κ Pot

8 B K +5V - K.µf V A +.µf -5V