zalzon said:
No matter how much effort I put into understanding these low and high
pass circuits I always come up clueless. I can't seem to visualise
what the electrons are doing at the resistor and capacitor. Its been
slowly driving me insane.
Why is low frequency AC allowed to pass but not high? Can someone
explain this to me in easy to understand terms referring to the
electrons in particular as they move around the circuit.
Please don't give me equations like Xc = 1/2pifC..etc cos that is just
crap. I know that already and its not helping me visualise anything.
I want to know what the electrons do when they reach the resistor in a
low pass circuit with low frequency AC. What happens then when they
encounter the capacitor. Why is low freq allowed to pass but not high
frequency?
Thank you
Do you understand how a series RR voltage divider can reduce the
voltage applied across it to something less across one of the
resistors?
There is a resistor, say R1 between input and output, and a second
resistor, say R2, between output and ground. Lets also say the source
of the input signal has zero impedance (current from this source does
not affect its voltage) and the output signal feeds an infinite
impedance load (the output voltage causes no current to pass through
the load). So the only currents are those through R1 and R2 from
source to ground. The way voltage across a resistor is related to the
current through a resistor is proportionality. The voltage is always
proportional to the current. Ohms are just a short hand way of saying
"volts across per ampere through". Since there are no other paths for
current in this circuit except through R1 and R2, those currents must
be equal. So the voltages across those resistors must be proportional
to their resistances by the same factor (the common current). So the
input voltage is used up partly by R1 and partly by R2. If R1 is 4
times the resistance of R2, then 4/5 of the input signal voltage must
be dropped across R1 and 1/5 must be dropped across R2. The frequency
(or wave shape, which can be thought of as a combination of
frequencies) of the signal voltage does not matter, since this voltage
division is instantaneous. All frequencies are treated equally.
RC high and low pass filters are similar voltage dividers, except that
one of the elements (the capacitor) has different impedances (AC
voltage across per ampere through) at different frequencies.
For example a resistor R is between the input signal and the output
signal, and capacitor C is between the output signal and ground. The
source and load impedances are as in the first example.
The voltage drop across the resistor is instantaneously proportional
to the current through it (its resistance is the proportionality
factor), but knowing the voltage across a capacitor does not tell you
anything about the instantaneous current through it, and vice versa.
The relation between voltage and current for a capacitor is that the
current through the capacitor is proportional to the time rate of
change of the voltage across the capacitor, regardless of what the
voltage happens to be. Capacitor voltage rises as current passes one
way, and falls as current goes the other way.
You might picture a capacitor with a fluid analogy as a rigid tank
that has two pipe connections and an elastic membrane stretched across
the tank between the two pipes, separating the tank into two volumes.
The voltage across the capacitor is equivalent to the difference of
pressures in the two halves of the tank. You might imagine that this
difference has nothing directly to do with how much fluid is passing
into one pipe and out of the other, but has a lot to do with how much
the membrane is stretched to one side or the other at the moment.
flow changes the pressure difference by changing how much the membrane
is stretched. If you think of the resistor as a capillary tube you
have all that you need to visualize the RC low pass filter. The input
end of the capillary is driven by a pressure that varies sinusoidally
at some frequency. The electrical ground (zero volts) is replaced
with a vent to zero (atmospheric) pressure. Now, as the input
pressure swings positive and negative, fluid flows into and out of the
side of the tank that is connected to the capillary tube, and the
membrane is pushed and pulled by various amounts and the pressure in
the half of the tank connected to the capillary varies positive and
negative, but never quite catches up with the input pressure wave.
The pressure in that side of the tank is also the output signal.
The faster the input wave changes, the further the output signal falls
behind and the smaller the relative pressure swing it achieves.
At very low frequencies, the tank pressure almost reaches the applied
pressure before the direction reverses, and there is little difference
in output amplitude compared to input amplitude. Small changes in
either the resistance of the capillary or tank membrane elasticity
(resistance or capacitance) have little effect on the output signal.
This shows frequencies that are in the pass band. But at some
characteristic frequency, the pressure swing in the tank reaches only
70.7 % or the applied pressure in each direction and falls behind the
input wave by 45 degrees. This is the filter corner frequency and
represents the transition between pass band ands stop band. Above
that frequency, the tank pressure swing drops rapidly with increasing
frequency. At much higher frequencies the ratio of the pressure
swings in the capillary side of the tank to the applied pressure
swings fall almost in proportion to the source frequency. Double the
frequency and the output swing falls by almost half.
The characteristic or corner frequency where the the filter response
changes is related to the product of the resistance and capacitance.
When 2*pi*f=R*C, f is at the corner frequency.