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Op Amps: Comparators

Op Amps: Comparators. Are things In the “-” column Bigger or Smaller than those in the “+” column?. +. -. Rules For comparing. If something in the “+” column is bigger than the “-” column then we produce a logic ‘1’

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Op Amps: Comparators

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  1. Op Amps: Comparators

  2. Are things In the “-” column Bigger or Smaller than those in the “+” column? + -

  3. Rules For comparing • If something in the “+” column is bigger than the “-” column then we produce a logic ‘1’ • If something in the “+” column is smaller than the “-” column then we produce a logic ‘0’

  4. Task: Table Creation

  5. Comparing two Waveforms • What waveform would I expect at the output ?

  6. A circuit To Do the comparing for us • The guts that does the work is an Op amp (the triangle) • R2 and VR1 form a _________________

  7. How It Works:

  8. What could this circuit be useful for?

  9. Op amp application 2:

  10. Keywords So Far • Op amp (operational amplifier) • Inverting • Non-inverting • comparator

  11. +V - Inverting input Vout + Non-inverting input -V Op Amp Connections • An op amp has 5 terminals • 2 inputs • 1 output • 2 power supply

  12. Op Amp Power supply • Electronic circuits we have seen so far 0V and supply voltage • Op amps require a ±supply • Why ? • Because the signal going into them can be positive or negative ? +9V -9V 0V

  13. Op Amp Input Signal Voltage Time • The input signal can be both positive & negative • The size of the input waveform changes over time • Alternating current (AC) • It could come from an MP3 player or a microphone

  14. Why do we have 2 inputs ? Vout • Reason 1: to get rid of noise • We will put our good signal onto 1 input • Any noise in the circuit will be seen by both inputs • We only amplify the difference in voltage between the two inputs (V1-V2) • Example: • V1 V2 Vout

  15. The Output • Without any other components, the gain of the input signal seen at the output is massive ! • However, this is not good because: • We want the shape of the input waveform at the output – just made bigger (higher => more voltage) • We cant have an output voltage bigger than supply • Anything bigger than supply becomes a flat line • This is known as clipping or distortion Input (mV) +2 +9 Gain = 10,000 Supply = ±9V -2 -9

  16. So How Do We Control the Output ? • Feedback • Negative feedback takes some of the output and takes it away from the input RF • We place a feedback resistor (RF) between: • The output • The inverting input

  17. The Inverting Amplifier (continued) • Gain with feedback: • We can choose the gain that we want with the ratio of 2 resistors Gain = -RF R1 • Open loop gain: • Is enourmous and we have no control over its value

  18. Keywords • Gain • clipping • Feedback • Amplify • Inverting • Non-inverting • comparator

  19. Quiz • What is open-loop gain ? • What are the 5 inputs and outputs on an op-amp called ? • What is gain ? • How can we reduce open-loop gain ? • What is the equation for gain ? • Besides being bigger, what is the relationship between the input and output waveforms on an inverting amplifier ? • Describe two applications for a comparator circuit ? • In an inverting amplifier, what is it that we are amplifying ?

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