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555 Vco circuit with logarithmic characteristic

2015-02-05 09:09  
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555 Vco circuit with logarithmic characteristic
As shown in Figure 5-20, 555 and IC1, VT1, C etc. form VCO, the changing of input voltage VB1 causes the oscillation frequency of IC2 changing with logarithmic characteristics. VT1, VT2 are the same curve matching tube, because the base of VT2 is in zero potential, while VB1 changes, it can be equivalent to the VBE2 changing of VT2. Capacitor charging current is the ic2, the relation between the input voltage VB1 and f has exponential characteristics. KT / q is the junction voltage of PN junction, it is about 26mV under room temperature.