Reading: Schwarz and Oldham (light on non-ideal) and comparator viewgraphs. Lecture 14: October 17, 2001

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1 Lecture 4: October 7, 00 Op-Amp Circuits and Comprators A)Cascade Op-Amps B)Integration/Differentiation Op-Amps C)I vs. V of Op-Amps Source Limits D)Comparator Circuits E)D to A Converters Reading: The following slides were derived from those prepared by Professor Oldham For EE 40 in Fall 0 Schwarz and Oldham (light on non-ideal) and comparator viewgraphs

2 CASCADE OP-AMP CIRCUITS R V V R R F I IN V 3 R 3 K 9K How do you get started on finding V O? Hint: Identify Stages Hint: I IN does not affect V O See the further examples of op-amp circuits in the reader

3 INTEGRATING OP-AMP R C i IN 0 and V( ) V = 0 How do you get started on finding V O? Hint: i IN 0 and V( ) V = 0 Hint: KCL at V - node with I IN- = 0

4 OP-AMP I-V CHARACTERISTICS WITH RAILS Circuit model (ideal op-amp) gives the essential linear part But cannot rise above some physical voltage related to the positive power supply V CC ( upper rail ) < V RAIL And cannot go below most negative power supply, V EE i.e., limited by lower rail > V -RAIL Example: Amplifier with gain of 0 5, with max of 3V and min of 3V. (a) (b) I-V near (V) V I-V over wider 0 (V) upper rail origin range (µv) lower rail (µv)

5 OP-AMP I-V CHARACTERISTICS WITH RAILS (cont.) Example: Amplifier with gain of 0 5, with upper rail of 3V and lower rail of 3V. We plot the vs characteristics on two different scales (c) Same vs over even wider range lower rail (b) I-V over wide range (V) upper rail (µv) (V) 3 (V)

6 Note: SIMPLE A/D CONVERTER I-V with equal X and Y axes (V) 3 (V) (a) displays linear amplifier behavior ( < 30 µv) and stops at rails (b) shows comparator decision function ( bit A/D converter centered at = 0) where lower rail = logic 0 and upper rail = logic

7 OP-AMP USE AS COMPARATOR (A/D) MODE Simple comparator with threshold at V. Design lower rail at 0V and upper rail at V (logic ). A = large (e.g. 0 to0 5 ) V If >.00 V, = V = Logic 0 If < 0.99 V, = 0V = Logic 0 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 V) specified. Comparator

8 ONE-BIT A/D CONVERSION REQUIRED IN DIGITAL SYSTEMS pulses in transmission line pulses out comparator regenerated pulses As we saw, we set comparator threshold at a suitable value (e.g., halfway between rails) and comparator output goes to rail if > V THRESHOLD and to rail if < V THRESHOLD. What would this circuit do? V The inverse pulse shaped function is generated by applying the input voltage to V- and setting V to the threshold voltage.

9 D/A CONVERSION Example: Digital representation of sound to analog (so you can hear it!) D/A conversion The summing junction op-amp provides a simple means of D/A conversion via weighted-adder D/A converter S closed if LSB = S4 0K S " if next bit = 0K S3 " if " " = S3 S4 " if MSB = 40K 5K 8V S 80K - S 4-Bit D/A Another way (not shown) is to sum charges instead of current with capacitor networks Binary number (volts) Analog output MSB LSB

10 Analog Output (V) CHARACTERISTIC OF A 4-BIT DAC Digital Input