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A Level     >AC Circuits<     Active Filters     Passive Filters     Tuned Circuits    

AC Circuits


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RC Circuits

At high prequencies, capacitors pass currents as easliy as wire.

DC and low frequencies are blocked by capacitors.

This blocking effect is called REACTANCE ( XC ) and it's similar to resistance but it depends on frequency.

XC = 1 / ( 2 π f C )

RC Filter Loading

The circuit above shows an RC high pass filter connected to a choice of two loads. SIMULATION

  1. RL is a 10 K resistor.
    When the output is switched to this load, the two 10 K resistors are placed in parallel.
    This completely changes the performance of the filter.
    The circuit could be designed to take this into account or option 2 below could be used instead.
     
  2. With the switch in the top position, the op-amp voltage follower has a very high input impedance.
    In this case, the filter performance is not altered by connecting the load.
    Often this is the preferred option.
    Other non-inverting amplifiers also have a very high input impedance and are aslo suitable.
    This use of a voltage follower is called BUFFERING. The filter is ISOLATED from the next sub-system in the circuit.

RL Circuits

Inductors pass DC and low frequencies just like wire although they often have more resistance than a short wire.

At high frequencies, inductors block AC signals.

This blocking effect is called REACTANCE ( XL ) and it's similar to resistance but it depends on frequency.

Inductors have a couple of disadvantages. They can be bulky and waste heat can be generated due to the DC resistance of the wire making them less efficient.

XL = 2 π f L


Comparing L and C Current Lag and Lead

Capacitor - The current leads the voltage. To charge or fill a capacitor, a current has to flow. As it fills, the voltage across the capacitor builds up, like the water level in a bucket. Once it's nearly full, this voltage reaches its maximum and the current dies away to zero. Note that this is the voltage across the capacitor and not the external voltage causing it to charge in the first place.

Inductor - The current lags the voltage. Inductors only have a voltage across them if the current is changing. With a steady current, the voltage across an ideal inductor would be zero because its resistance is zero. Whenever the current changes, the altering magnetic field induces a back EMF that opposes the change in current.

Phase - This is the angle 90o or, in radians, π / 2 between the voltage and the current. This is a quarter of a cycle of the wave.

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