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Unformatted text preview: 520.345 Electrical and Computer Engineering Laboratory Experiment No. 8 Schmitt Triggers and Voltage Controlled Oscillators Lab report due at the beginning of your lab session on November 11 th or 12 th 2010 Introduction Although operational amplifiers are most frequently used with negative feedback, they can also be used with positive feedback to form electrically bistable devices. One example is a circuit known as a Schmitt trigger which has the hysteresis property shown in the diagram below. The transition voltages, V 1 , V 2 are determined by the positive feedback network. The combination of a Schmitt trigger with a current source and capacitive charging circuit can be used to construct a voltage-controlled oscillator. This is a circuit that produces a periodic output waveform (sinusoidal or rectangular in shape) with a period proportional to an input control voltage. This type of circuit is sometimes called a voltage-to-frequency converter. Voltage-controlled oscillators are used in measurement circuits and in phase-lock-loops which are the basic building blocks used in electronic tuning circuits found in FM radio and television sets, and in other types of communication receivers. Schmitt Trigger Circuits The basic form of the Schmitt trigger is shown below. The feedback network connects the positive terminal of the op-amp to the output through a feedback resistor, R f , and to a reference voltage V REF through resistor R 1 . An input voltage is connected to the (-) terminal of the op-amp. This arrangement is bistable in that the op-amp output voltage will be at its most negative V-Rail or its most positive, V +Rail . These “Rail” voltages are usually within a volt or two of the power supply voltages. The transition voltages are obtained as follows: When the op-amp output voltage is at V +Rail , the current flow is as shown if the input voltage, V in , is less than V 1 . Since no current can enter the op-amp, we have ( ¡¢£¤¥ ¦ § ) ¨ © ª ( § ¦ ¢«¬ ) ¨ § ⁄ ⁄ and clearly V 1 must exceed V REF . This can be rewritten in terms of the ration R f /R 1 as ¨ © ¨ § ª ( ¡¢£¤¥ ¦ § ) ( § ¦ ¢«¬ ) ⁄ ⁄ Similarly, when the op-amp output is at V-Rail , the current flow is as shown if the input voltage V in is greater than V...
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