Analog Electronics Basic Op-Amp Applications - LIGO

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5.6. ANALOG COMPARATOR 117 D 0 v C v i A D 1 R − + − + A v o C 0 RESET Figure 5.11: Peak detector circuit. 5.6 Analog Comparator An analog comparator or simply comparator is a circuit with two inputs v i , v re f and one output v o which fulfills the following characteristic: { V1 , v v o = i > v re f V 2 , v i ≤ v re f An Op-Amp with no feedback behaves like a comparator. In fact, if we apply a voltage v i > v re f , then V + − V − = v i − v re f > 0. Because of the high gain, the Op-Amp will set v o to its maximum value +V sat which is a value close to the positive voltage of the power supply. If v i < v re f , then v o = −V sat . The magnitude of the saturation voltage are typically about 1V less than the supplies voltages. Depending on which input we use as voltage reference v re f , the Opamp can be an inverting or a non inverting analog comparator. It is worthwhile to notice that the analog comparator circuit is also a 1 bit analog digital converter , which converts voltages to the two levels V 1 and V 2 . DRAFT
118 CHAPTER 5. BASIC OP-AMP APPLICATIONS V i V 1 − + V o R + R f (utp) V + t (ltp) V + +V sat t −V sat Figure 5.12: Schmitt Trigger and its qualitative response to a signal that swings up and down between and through the saturation voltages±V sat . 5.7 Regenerative Comparator (The Schmitt Trigger) The Regenerative comparator or Schmitt Trigger 1 shown in Figure 5.12 is a comparator circuit with hysteresis. It is important to notice that the circuit has a positive feedback. With positive feedback, the gain becomes larger than the open loop gain making the comparator to swing faster to one of the saturation levels. 1 Otto Herbert Schmitt (1913-1998) American scientist considered the inventor of this device, that appeared in an article in 1938 with the name of "thermionic trigger"[3]. DRAFT
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