Operational Amplifiers (Op-Amps)

Transcription

1 Phladelpha Unversty Faculty o Engneerng Communcaton and Electroncs Engneerng Operatonal Amplers (Op-Amps) Bascs Operatonal ampler or op-amp s a very hgh gan derental ampler wth a hgh nput mpedance (typcally a ew meg-ohms) and low output mpedance (less than 100 W). Note the op-amp has two nputs and one output. Typcal uses o the operatonal ampler are to provde voltage ampltude changes (ampltude and polarty), oscllators, lter crcuts, and many types o nstrumentaton crcuts. An op-amp contans a number o derental ampler stages to acheve a very hgh voltage gan. The gven gure shows a basc op-amp wth two nputs and one output as would result usng a derental ampler nput stage. ecall rom Parts II and III that each nput results n ether the same or an opposte polarty (or phase) output, dependng on the ampler type tsel. Accordng to ths, the sgnal could be appled to the plus (+) or the mnus (-) nput to gve a phase sht or not, smultaneously. Op-amp gan Op-Amps have a very hgh gan. They can be connected open- or closed-loop. a. Open-loop reers to a conguraton where there s no eedback rom output back to the nput. In the open-loop conguraton the gan can exceed 10,000. b. Closed-loop conguraton reduces the gan. In order to control the gan o an op-amp t must have eedback. Ths eedback s a negatve eedback. A negatve eedback reduces the gan and mproves many characterstcs o the op-amp. Lecturer: Dr. Omar Daoud ١

2 Basc Operaton The basc crcut connecton usng an op-amp s shown n Fg The crcut shown provdes operaton as a constant-gan multpler. An nput sgnal, 1, s appled through resstor 1 to the mnus nput. The output s then connected back to the same mnus nput through resstor. The plus nput s connected to ground. Snce the sgnal 1 s essentally appled to the mnus nput, the resultng output s opposte n phase to the nput sgnal. Operaton o op-amp as constant-gan multpler: (a) op-amp ac equvalent crcut; (b) deal op-amp equvalent crcut; (c) redrawn equvalent crcut. The Op-Amps AC equvalent crcut Where: d s the derental voltage (equals 1-2 ) s the nput resstance A d s the derental gan o s the output resstance A c s common mode gan Lecturer: Dr. Omar Daoud ٢

3 Derental Inputs When separate nputs are appled to the op-amp, the resultng derence sgnal s the derence between the two nputs ( d = 1-2 ) Common Inputs When both nput sgnals are the same, a common sgnal element due to the two nputs can be dened as the average o the sum o the two sgnals (c= 0.5( 1-2 )) Output oltage Snce any sgnals appled to an op-amp n general have both n-phase and out-o phase components, the resultng output can be expressed as o =A d d -A c c. The common-mode rejecton rato (CM) could be dened as Example: Calculate the CM or the crcut measurements shown n the gven Fg. below Lecturer: Dr. Omar Daoud ٣

4 Invertng Op-Amp The sgnal nput s appled to the nvertng ( ) nput. The non-nvertng nput (+) s grounded. The resstor s the eedback resstor. It s connected rom the output to the negatve (nvertng) nput. Ths s negatve eedback. Apply kvl to the nput loop 1 o o I (1) But, I I I 1 (2) Apply kvl to the output loop (3) substtute1and 2 n 3 A v Nonnvertng Op-Amps Summng Op-Amps Because the op-amp has hgh nput mpedance, the multple nputs are treated as separate nputs. Lecturer: Dr. Omar Daoud ٤

5 Integrator Op-Amps The output s the ntegral o the nput. Integraton s the operaton o summng the area under a waveorm or curve over a perod o tme. Ths crcut s useul n low-pass lter crcuts and sensor condtonng crcuts. Derentator Op-Amps The derentator takes the dervatve o the nput. Ths crcut s useul n hgh-pass lter crcuts. Lecturer: Dr. Omar Daoud ٥

6 Example 1: Calculate the output voltages 2 and 3 n the gven crcut below. Soluton: Lecturer: Dr. Omar Daoud ٦

7 Example 2: What range o output voltage s developed n the gven crcut below. Soluton: Lecturer: Dr. Omar Daoud ٧