Audio Systems (continued) ET6

Frequency Dependant Resistors As we have seen previously capacitors and resistors are used together to create RC circuits. Capacitors by nature charge up to try and match the voltage of their power supply. If the power supply is alternating at a high frequency (the case of an AC) then the capacitor never has chance to charge or discharge properly. In an AC circuit the capacitor doesn’t act like a charging device but instead acts like a resistor. The resistance of the capacitor is dependant on the frequency of the alternating voltage signal. XCXC XFXF

Capacitors and alternating frequencies The combined resistance of the circuit shown is always less than the smallest resistor. Therefore at high frequencies the resistance is that of the resistor and at low frequencies is that of the capacitor. This means that the resistance of the circuit can be changed by altering the frequency of the signal – very useful for Audio Systems. NOTE: As the frequency of an Alternating Voltage goes up, the resistance of a capacitor also goes up.

Capacitors and Op Amps The relationship of R 1 and R 2 is such that as R 2 increases so does the gain factor of the Op-Amp. The circuit below however, is reliant on the frequency of the input signal for its gain factor. If the frequency increases then the capacitor’s resistance increases and the combined resistance of the parallel section becomes that of R 2. If the frequency decreases then the resistance of the capacitor also decreases and the combined resistance becomes that of the capacitor. If this happens then the Op Amp will amplify less than previously. R1R1 R2R2