Philadelphia University Faculty of Engineering Communication and Electronics Engineering. Amplifier Circuits-III

Transcription

1 Module: Electronics II Module Number: 6503 Philadelphia University Faculty o Engineering Communication and Electronics Engineering Ampliier Circuits-III Operational Ampliiers (Op-Amps): An operational ampliier, or op-amp, is a very high gain dierential ampliier with high input impedance and low output impedance. Typical uses o the operational ampliier are to provide voltage amplitude changes (amplitude and polarity), oscillators, ilter circuits, and many types o instrumentation circuits. An op-amp contains a number o dierential ampliier stages to achieve a very high voltage gain. - Symbol and Terminals: The ollowing Figure shows a basic op-amp with two inputs and one output as would result using a dierential ampliier input stage. Each input results in either the same or an opposite polarity (or phase) output, depending on whether the signal is applied to the plus (+) or the minus (-) input. To illustrate what an op-amp is, let's consider its ideal characteristics. A practical opamp, o course, alls short o these ideal standards, but it is much easier to understand and analyze the device rom an ideal point o view. These two considerations are clearly shown in the Figure Below. Firstly, the ideal op-amp has - ininite voltage gain, - ininite bandwidth, - it has an ininite input impedance (open) so that it does not load the driving source, - a zero output impedance. Lecturer: Dr. Omar Daoud Part III

2 Module: Electronics II Module Number: 6503 The input voltage, Vin appears between the two input terminals, and the output voltage is AvVin as indicated by the internal voltage source symbol. Secondly, characteristics o a practical op-amp are: - very high voltage gain, - very high input impedance, - low output impedance, and - wide bandwidth. - Internal Block Diagram: A typical op-amp is made up o three types o ampliier circuit: a dierential ampliier, a voltage ampliier, and a push-pull ampliier, as shown in the Figure below. A dierential ampliier is the input stage or the op-amp; it provides ampliication o the dierence voltage between the two inputs. The second stage is usually a class A ampliier that provides additional gain. Some op-amps may have more than one voltage ampliier stage. A push-pull class B ampliier is typically used or the output stage. Lecturer: Dr. Omar Daoud Part III

3 will is will will and Module: Electronics II Module Number: 6503 The Dierential Ampliier Input Stage The term dierential comes rom the ampliier's ability to ampliy the dierence o two input signals applied to its inputs. Only the dierence in the two signals is ampliied; i there is no dierence, the output is zero. A basic dierential ampliier circuit and its symbol are shown in the Figure. The transistors (Q and Q ) and the collector resistors ( c and c ) are careully matched to have identical characteristics. Notice that the two transistors share a single emitter resistor, E. UThe Dierential Ampliier Operation: - Assume both bases are connected to ground, - The emitter voltage will be -0.7 V because the voltage drops across both baseemitter junctions are equa. - The emitter currents are equal (IE IE) and each is one-hal o the current through E. - The collector currents are both equal and are approximately equal to the emitter currents. Because the collector currents are the same, the collector voltages are also the same, which relects the zero dierence in the input voltages (both bases are at 0V. - I the base o Q disconnected rom ground and connected to a small positive voltage, - Ql conduct more current because the positive voltage on its base causes the emitter voltage to increase slightly. - Although the emitter voltage is a little higher, the total current through E is nearly the same as beore. - The emitter current is now divided so that more o it is in Ql less in Q. - As a result, the collector voltage o Q decrease and the collector voltage o Q increase, relecting the dierence in the input voltages (one is 0 V and the other at a small positive value). Lecturer: Dr. Omar Daoud Part III 3

4 will will is will will and Module: Electronics II Module Number: I the base o Ql placed back at ground and a small positive voltage is connected to the base o Q, - Q conduct more current, - Ql conduct less, - The emitter current is now divided so that more o it is in Q less in Q, - As a result, the collector voltage o Q increase and the collector voltage o Q decrease. - The dierential ampliier exhibits three modes o operation based on the type o input signals. These modes are single-ended, dierential, and common. Since the dierential ampliier is the input stage o the op-amp, the op-amp exhibits the same modes. Op-Amp Input Modes and Parameters ) UInput Signal ModesU USingle-Ended ModeU: one input is grounded and a signal voltage is applied only to the other input. - In the case where the signal voltage is applied to the inverting input, an inverted, ampliied signal voltage appears at the output. - In the case where the signal is applied to the noninverting input with the inverting input grounded, a non inverted, ampliied signal voltage appears at the output. UDierential Input ModeU: Two opposite-polarity (out-o-phase) signals are applied to the inputs. This type o operation is also reerred to as doubleended. The ampliied dierence between the two inputs appears on the output. Lecturer: Dr. Omar Daoud Part III 4

5 Module: Electronics II Module Number: 6503 UCommon Mode InputU: Two signal voltages o the same phase, requency, and amplitude are applied to the two inputs. When equal input signals are applied to both inputs, they cancel, resulting in a zero output voltage. This action is called Ucommon-mode rejectionu. Its importance lies in the situation where an unwanted signal appears commonly on both op-amp inputs. U) Input Parameters UCommon Mode ejection atiou: Common-mode rejection means that this unwanted signal will not appear on the output and distort the desired signal. Common-mode signals (noise) generally are the result o the pick-up o radiated energy on the input lines, rom adjacent lines, the 60 Hz power line, or other sources. The measure o an ampliier's ability to reject common-mode signals is a parameter called the UCMU (Ucommon-mode rejection ratiou). where Ad dierential gain o the ampliier Ac common-mode gain o the ampliier Vd dierence voltage Vc common voltage Ex. Calculate the CM or the circuit measurements shown in Fig. Lecturer: Dr. Omar Daoud Part III 5

6 Module: Electronics II Module Number: 6503 Ex. Determine the output voltage o an op-amp or input voltages o Vi 50 µv, Vi 40 µv. The ampliier has a dierential gain o Ad 4000 and the value o CM is: (a) 00. (b) 05. Lecturer: Dr. Omar Daoud Part III 6

7 Module: Electronics II Module Number: 6503 UCommon mode Input voltage rangeu: It is the range o input voltages which, when applied to both inputs, will not cause clipping or other output distortion. Many op-amps have common-mode input voltage ranges o ±0 V with dc supply voltages o ±5 V. UInput ImpedanceU: Two basic ways o speciying the input impedance o an op-amp are the dierential and the common mode. The dierential input impedance is the total resistance between the inverting and the noninverting inputs (Umeasured by determining the change in bias current or a given change in dierential input voltageu). The common-mode input impedance is the resistance between each input and ground (Umeasured by determining the change in bias current or a given change in common-mode input voltageu). UInput Oset VoltageU: The ideal op-amp produces zero volts out or zero volts in. In a practical op-amp, however, a small de voltage. VOUT(error), appears at the output when no dierential input voltage is applied (Ucauses a slight mismatch o the base-emitter voltages o the dierential ampliier input stage o an op-ampu). The input oset voltage, Vos, is the dierential dc voltage required between the inputs to orce the output to zero volts (Typical values o in the range o mv or less, while it is 0V in ideal cases). UInput Oset CurrentU: Ideally, the two input bias currents are equal, and thus their dierence is zero. In a practical op-amp, however, the bias currents are not exactly equal. The input oset current, Ios, is the dierence o the input bias currents, expressed as an absolute value (Actual magnitudes o oset current are usually at least an order o magnitude (ten times) less than the bias current). V V V os os Iin I in I os in A I OUT (error) v os in ( I I ) in Lecturer: Dr. Omar Daoud Part III 7

8 Module: Electronics II Module Number: 6503 Slew ate: The maximum rate o change o the output voltage in response to a step input voltage is the slew rate o an op-amp. The slew rate is dependent upon the high-requency response o the ampliier stages within the op-amp. A pulse is applied to the input and the resulting ideal output voltage is indicated as below. The width o the input pulse must be suicient to allow the output to "slew" rom its lower limit to its upper limit. A certain time interval Δt is required or the output voltage to go rom its lower limit V max to its upper limit +V max once the input step is applied. Slew ate t where ΔV out +V max - (- V max ), The unit o slew rate is volts per microsecond (V/µs). V out Ex. The output voltage o a certain op-amp appears as shown in the given Figure in response to a step input. Determine the slew rate. Slew ate 9 ( 9) 0 6 8V/μs UInput Bias Current:U The input terminals o a bipolar dierential ampliier are the transistor bases and, thereore, the input currents are the base currents. The input bias current is Uthe dc current required by the inputs o the ampliier to properly operate the irst stageu (It is the average o both input currents). Lecturer: Dr. Omar Daoud Part III 8

9 Module: Electronics II Module Number: 6503 Comparison between Op-amps with and without negative eedback: Practical Op-Amps Circuits: ) Inverting Op-Amp The signal input is applied to the inverting ( ) input. The non-inverting input (+) is grounded. The resistor is the eedback resistor. It is connected rom the output to the negative (inverting) input. This is negative eedback. Apply kvl to the input loop V I i () But, I I () o i i Apply kvl to the output loop V I (3) substituteand in 3 Vo Vi Av i ) Noninverting Op-Amps Lecturer: Dr. Omar Daoud Part III 9

10 Module: Electronics II Module Number: ) Summing Op-Amps Because the op-amp has high input impedance, the multiple inputs are treated as separate inputs. 4) Integrator Op-Amps The output is the integral o the input. Integration is the operation o summing the area under a waveorm or curve over a period o time. This circuit is useul in low-pass ilter circuits and sensor conditioning circuits. Lecturer: Dr. Omar Daoud Part III 0

11 Module: Electronics II Module Number: ) Dierentiator Op-Amps The dierentiator takes the derivative o the input. This circuit is useul in high-pass ilter circuits. Ex. Calculate the output voltages V and V3 in the given circuit below. Lecturer: Dr. Omar Daoud Part III

12 Module: Electronics II Module Number: 6503 Ex. What range o output voltage is developed in the given circuit below. Lecturer: Dr. Omar Daoud Part III

13 Module: Electronics II Module Number: 6503 Ex. The dierential ampliier shown below has a dierential gain o 500 and a CM o In part (a), a single ended input o 500µV rms is applied and at the same time a 00mV, 60Hz common-mode intererence signal appears on both inputs as a result o radiated pick-up rom ac power system. In part (b), dierential input signals o 500µVrms each are applied to the inputs. The common-mode intererence is the same as in part (a). ) Determine the common-mode gain, ) Express the CM in db, 3) Determine the rms output signal or both parts a and b, 4) Determine the rms intererence voltage on the output. Ad ) CM Ad A 083 CM 0. c Ac ) CM db 0log(30000)89.5dB 3) (a) v A v µ.5V o d id rms o Ad vid µ 500µ (b) v ( ( )).5Vrms 4) voc Acvic ( 00m) 8.3mVrms Ex. In the given op-amp, i 0kΩ, 0kΩ, and rom the datasheet Z in MΩ, Z out 75Ω, and A d (A ol ) a) Determine the input and output impedances, b) A c (A cl ). i The attenuation B + Z Z A in( NI ) out( NI ) c B Z in Z ( + A 3 ( + A out d d B) 7.4GΩ 8.6Ω B) i Lecturer: Dr. Omar Daoud Part III 3

14 Module: Electronics II Module Number: 6503 Ex. In the given op-amp, i kω, 00kΩ, and rom the datasheet Z in 4MΩ, Z out 50Ω, and A d (A ol ) c) Determine the input and output impedances, d) A c (A cl ). The attenuation B Z Z A in( NI ) out c.0kω i + Z out ( NI ) 0mΩ ( + Ad B) 00 i i i Ex. Determine the output or the circuit o the given Fig. with components MΩ, 00kΩ, 50kΩ, and 3 500kΩ. Lecturer: Dr. Omar Daoud Part III 4

15 Module: Electronics II Module Number: 6503 Op-Amps Applications (Active Filters): A ilter circuit can be constructed using passive components: resistors and capacitors. An active ilter additionally uses an ampliier to provide voltage ampliication and signal isolation or buering (UThe high input impedance o the op-amp prevents excessive loading o the driving source. and the low output impedance o the op-amp prevents the ilter rom being aected by the load that it is driving. Active ilters are also easy to adjust over a wide requency range without altering the desired responseu). A ilter that provides a constant output rom dc up to a cuto requency OH and then passes no signal above that requency is called an ideal lowpass ilter. A ilter that provides or passes signals above a cuto requency OL is a high-pass ilter. When the ilter circuit passes signals that are above one ideal cuto requency and below a second cuto requency, it is called a bandpass ilter. Lecturer: Dr. Omar Daoud Part III 5

16 Module: Electronics II Module Number: 6503 Active Low Pass Filters (LPF): First Order LPF: A v OH + F G π C Second Order LPF: OH π C C Lecturer: Dr. Omar Daoud Part III 6

17 Module: Electronics II Module Number: 6503 Active High Pass Filters (HPF): Active Band Pass Filters (BPF): Lecturer: Dr. Omar Daoud Part III 7

18 Module: Electronics II Module Number: 6503 Ex. For the given Active BPF circuit, i 3 30kΩ, 4 5kΩ, A B 0kΩ, A 5kΩ, B 30kΩ, C C C 3 0.0µF and C 0.0 µf then ind a) The circuit BW, b) The geometric center requency, and c) The Q value (Quality actor is deined as the peak energy stored in the circuit divided by the average energy dissipated in it per cycle at resonance). c c o ( HPF) π ( LPF) π BW c c c c o Q.4 BW A A 380Hz 9Hz B B C C A C A B C B 750Hz 30Hz Lecturer: Dr. Omar Daoud Part III 8

19 and provide is and is and are is is Module: Electronics II Module Number: 6503 Multiple-Feedback Band-Pass Filter Another type o ilter coniguration, shown in the Figure below, it is a multipleeedback bandpass ilter. The two eedback paths are through C). Components and C the low-pass response, and C provide the high-pass response. The maximum gain Ao occurs at the center requency. Assume that C>C C<C ) C Open circuit as long as C open circuit Vout0V. ) C<in<C C short circuit and C still open circuit Thus, it acts as inverting ampliier Vout exists. 3) in C Both C C short circuit is shorted Vout 0V. o π πc A < Q ( // ) ( // ) Q, π ocao Ao o 3 3 C C πc Q, 3 π C o, making ( + ) C 3 π C o C 3 ( Q A ), This a limitation on the gain to give a positive value or Ex. Determine the center requency, maximum gain, and bandwidth or the ilter in the given Figure. Q C o 3 Lecturer: Dr. Omar Daoud Part III 9

20 Module: Electronics II Module Number: 6503 o A o BW πc Q π C o ( + 3 ) ( 68kΩ +.7kΩ) π (0.0µ F) ( 68kΩ)( 80kΩ)(.7kΩ) 80kΩ.3 (68kΩ) o Q π 3 ( 736Hz)( 0.0µ F )( 80kΩ) 736Hz Hz Hz Active Notch/Band-Stop Filters (NF): ) in < C C and C is open circuit V out 0V rom the inverting stage but there is a noninverting one. ) C< in < C C is short circuit V i is shown to be common mode signal. 3) in C Both C and C are short circuit C is short out the signal at inverting input V out comes rom the noninverting input (ampliy the dierence between the two inputs). o, Ao π C C Lecturer: Dr. Omar Daoud Part III 0