SP.764, Practical Electronics Lecture 6: Op Amps Topics: 1) Op Amps as Amplifiers - Example - “Golden Rules” of their Use - Analyze example - More Examples 2) Op Amp as a Comparator Op Amps: - Op amps are semiconductor devices. - Op amps are mostly optimized to have high-speed or low-noise features. - Even though they are remarkably sophisticated circuits, they are really easy to work with. - If you connect the power supply incorrectly, the op amp might blow up. Comparator: - Remarkably useful: Feed two voltages, and comparator compares them. - The comparator is heart of digital electronics because of its ability to decide whether a voltage is higher or lower than a threshold value. Question 1: What is the output going to be? 1mA 10kΩ 1mA 1kΩ +1V A - VOUT + ∆V = IR = 1mA ×10kΩ = 10V VA = 0 ⇒ VOUT = −10V Question 2: If instead of 1V, the input is half a volt. - You would get half the output. Question 3: What about double? - Tricky! The op amp saturates at -15V, so output cannot go beyond -15V. Op amps are usually run off bipolar supplies (+/-15V). Op Amps: +V (V+ ,V− ) - 2 Inputs: - Non-inverting Inverting Input Input 1 Output: ( −V < VOUT < +V ) - 2 Voltage Supplies: - ( +V , −V ) + -V Golden Rules: 1) The Op-Amp Inputs draw no current. 2) With Negative Feedback, the inputs are at the same voltage (i.e., V+=V-) Now, one can generalize the first circuit: IF I IN RF VIN - VOUT RI I IN + V −V = IN − RI Golden Rule #2 tells us Golden Rule #1 requires V− = 0 ⇒ I IN = I F = I IN VIN RI ⇒ (V− − VOUT ) = I F RF 0 ⇒ −VOUT = VIN RF RI ⎛R ⎞ ⇒ VOUT = − ⎜ F ⎟VIN ⎝ RI ⎠ “Negative Feedback” means there is a current path from VOUT to the “-” input. More examples: VOUT = f (VIN ) Unity Gain Buffer +V - VOUT VIN + -V VIN = VOUT R4 +V - R3 VOUT R1 VIN + -V R2 ⎛ R2 ⎞ V− = V+ = VIN ⎜ ⎟ ⎝ R1 + R2 ⎠ VOUT − V− V− = R4 R3 R4 V− R3 ⎛ R ⎞ R3 VOUT = V− ⎜ 1+ 3 ⎟ R4 R4 ⎠ ⎝ ⎛ R2 ⎞ ⎛ R4 + R3 ⎞ R3 VOUT = VIN ⎜ ⎟⎜ ⎟ R4 ⎝ R1 + R2 ⎠ ⎝ R4 ⎠ VOUT = R2 R3 ⎛ R3 + R4 ⎞ ⎜ ⎟ VIN ⎝ R1 + R2 ⎠

Lecture 6: Op Amps

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