PowerPoint PresentationMidtermTopicsSlide 4Slide 5Slide 6OP-AMPS AND COMPARATORSSlide 8I-V CHARACTERISTICS OF AN ACTUAL HIGH-GAIN DIFFERENTIAL AMPLIFIER (cont.)Slide 10EXAMPLE OF A HIGH-GAIN DIFFERENTIAL AMPLIFIER OPERATING IN COMPARATOR (A/D) MODEConversion from signals to digital dataOP-AMPSOP-AMPS – “TAMING” THE WILD HIGH-GAIN AMPLIFIEROP-AMP very high gain →predictable resultsOP-AMP very high gain →predictable resultsOP-AMPS – Another Basic CircuitInverting amplifier analysisSolving Op-Amp circuitsCapacitor in the feedbackIntegrator10/13/2004 EE 42 fall 2004 lecture 19 1Lecture #19 amplifier examples: comparators, op amps.Reminder:MIDTERM coming up one week from today (Monday October 18th)This week: Review and examples10/13/2004 EE 42 fall 2004 lecture 19 2Midterm•Monday, October 18,•In class•One page, one side of notes10/13/2004 EE 42 fall 2004 lecture 19 3TopicsToday:•Amplifier examples–Comparator–Op-Amp10/13/2004 EE 42 fall 2004 lecture 19 4Amplifier+V0+VIN•V0=AVIN•Output is referenced to “signal ground” •V0 cannot rise above some physical voltage related to the positive power supply VCC (“ upper rail”) V0 < V+RAIL• V0 cannot go below most negative power supply, VEE i.e., limited by lower “rail” V0 > V-RAIL V+railV+rail10/13/2004 EE 42 fall 2004 lecture 19 5WHAT ARE I-V CHARACTERISTICS OF AN ACTUAL HIGH-GAIN DIFFERENTIAL AMPLIFIER ?+V0+VIN•Circuit model gives the essential linear part•The gain may be 100 to 100,000 or more•But V0 cannot rise above some physical voltage V0 < V+RAIL• And V0 cannot go below the lower “rail” V0 > V-RAIL •CMOS based amplifiers can often go all the way to their power supplies, perhaps ± 5 volts10/13/2004 EE 42 fall 2004 lecture 19 6High gain Amplifier++VIN•We can make very high gain amplifiers by cascading lower gain amplifiers.•For example, if we have two amplifiers, each with a gain of 100, then when the output of the first is feed into the input of the second, the total gain is 10,000. •With a very high gain amplifier, a very small change in the input causes a large change in the output voltage, so the range of voltages over which the input results in a linear output is very narrow.V0VIN10/13/2004 EE 42 fall 2004 lecture 19 7OP-AMPS AND COMPARATORSA very high-gain differential amplifier can function either in extremely linear fashion as an operational amplifier (by using negative feedback) or as a very nonlinear device – a comparator.++V0AV1+V1RiCircuit Model in linear region)VV(AV0 +AV+VV0Differential Amplifier“Differential” V0 depends only on difference (V+ V-) “Very high gain” ABut if A ~ , is the output infinite?10/13/2004 EE 42 fall 2004 lecture 19 8I-V Characteristics of a real high-gain amplifierExample: Amplifier with gain of 105, with max V0 of 3V and min V0 of 3V.VIN(V)1 23V0 (V)0.10.23 21.2 (a)V-V near origin3(b)V-V over wider rangeVIN(V)10 2030V0 (V)130 20102123upper “rail”lower “rail”10/13/2004 EE 42 fall 2004 lecture 19 9I-V CHARACTERISTICS OF AN ACTUAL HIGH-GAIN DIFFERENTIAL AMPLIFIER (cont.)VIN(V)1 23V0 (V)123 212313(c)Same V0 vs VIN over even wider range3(b)V-V over wide rangeVIN(V)10 2030V0 (V)130 20102123upper “rail”lower “rail”Example: Amplifier with gain of 105, with upper rail of 3V and lower rail of 3V. We plot the V0 vs VIN characteristics on two different scales10/13/2004 EE 42 fall 2004 lecture 19 10I-V CHARACTERISTICS OF AN ACTUAL HIGH-GAIN DIFFERENTIAL AMPLIFIER (cont.)VIN(V)1 23V0 (V)123 212313(c)V-V with equal X and Y axesNote:•(a) displays linear amplifier behavior•(b) shows limit of linear region – (|VIN| < 30 V)•(c) shows comparator function (1 bit A/D converter centered at VIN = 0) where lower rail = logic “0” and upper rail = logic “1”Now plot same thing but with equal horizontal and vertical scales (volts versus volts)10/13/2004 EE 42 fall 2004 lecture 19 11EXAMPLE OF A HIGH-GAIN DIFFERENTIAL AMPLIFIER OPERATING IN COMPARATOR (A/D) MODESimple comparator with threshold at 1V. Design lower rail at 0V and upper rail at 2V (logic “1”). A = large (e.g. 102 to105 )NOTE: The actual diagram of a comparator would not show an amplifier with “offset” power supply as above. It would be a simple triangle, perhaps with the threshold level (here 1V) specified.If VIN > 1.010 V,V0 = 2V = Logic “1”If VIN < 0.99 V,V0 = 0V = Logic “0”V0VIN1 2012+V0VIN+1VV0VINComparator10/13/2004 EE 42 fall 2004 lecture 19 12Conversion from signals to digital datapulses intransmissioncomparatorregenerated pulsespulses outWe set comparator threshold at a suitable value (e.g., halfway between the logic levels) and comparator output goes to:+rail if VIN > VTHRESHOLD and to rail if VIN < VTHRESHOLD.Signals are conveyed as voltages, but signal levels must be converted into digital data. ( 1 bit A/D)The rails of the comparator are the logic levels, for example +rail = “1” or “true” and -rail→”0” or “false”10/13/2004 EE 42 fall 2004 lecture 19 13OP-AMPSA very high-gain differential amplifier can function in extremely linear fashion as an operational amplifier by using negative feedback.Negative feedback Stabilizes the output R2R1+V0VINEXAMPLEWe will show that that for A (and Ri 0 for simplicity)121IN0RRRVV++V0AV1-+V1RiR2Circuit ModelR1VINStable, finite, and independent of the properties of the OP AMP !10/13/2004 EE 42 fall 2004 lecture 19 14OP-AMPS – “TAMING” THE WILD HIGH-GAIN AMPLIFIERKEY CONCEPT: Negative feedbackCircuit (assume)RINV0(+)()1KVIN9KR2R1+)VV(AV0-+1KVIN9KR2R1Example:First of all, notice that if the input resistance of the amplifier is so large that the current into it is negligible, then R1 and R2 form a voltage divider to give the input to the negative terminal10/13/2004 EE 42 fall 2004 lecture 19 15OP-AMP very high gain →predictable resultsAnalysis:INoVVVVARRRVVRRRV)(211)(211)(IN211)(211IN211)(VRR)1A(ARVRRRAVRRAR1VLets solve for V- then find Vo from Vo = A (V+ - V-)10/13/2004 EE 42 fall 2004 lecture 19 16OP-AMP very high gain →predictable results 10VV RRRVVA if
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