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Variations in Feedback Circuits

Mixing Analog and Digital Technologies

Generating Waveforms

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A Square Wave Generator 

One requirement in a wide range of applications is a spontaneous source of some continuous signal, having a regular and definable wave shape. One of the most important of these is a squarewave.
The circuit to the right uses a comparator with both positive and negative feedback to control its output voltage. Because the negative feedback path uses a capacitor while the positive feedback path does not, however, there is a time delay before the comparator is triggered to change state. As a result, the circuit oscillates, or keeps changing state back and forth at a predictable rate.
Because no effort is made to limit the output voltage, it will switch from one extreme to the other. If we assume it starts at 10 volts, then the voltage at the "+" input will be set by R_{2} and R_{1} to a fixed voltage equal to 10R_{1}/(R_{1} + R_{2}) volts. This then becomes the reference voltage for the comparator, and the output will remain unchanged until the "" input becomes more negative than this value.
But the "" input is connected to a capacitor (C) which is gradually charging in a negative direction through resistor R_{f}. Since C is charging towards 10 volts, but the reference voltage at the "+" input is necessarily smaller than the 10 volt limit, eventually the capacitor will charge to a voltage that exceeds the reference voltage. When that happens, the circuit will immediately change state. The output will become +10 volts and the reference voltge will abruptly become positive rather than negative. Now the capacitor will charge towards +10 volts, and the other half of the cycle will take place. The output frequency is given by the approximate equation:
f_{out} = 

1  
2R_{f}C ln (  2R_{1}  + 1 ) 
R_{2} 
In practice, circuit values are chosen such that R_{1} is approximately R_{f}/3, and R_{2} is in the range of 2 to 10 times R_{1}.


 
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