We can hear so many sounds in nature. These sounds may have different frequencies. How we can separate a particular type of sound(waves) from others?..We can do it by using a filter circuit. A filter circuit helps us to 'filter out' that waves having particular frequencies. Many circuits are used as filters. Here, explain the circuit diagram and working of a high pass RC circuit(high pass filter). Frequency response is also given. This is only a stepping stone to differentiator.
Passive High pass filter
A first order high pass RC circuit contains one pair of Resistor(R) and Capacitor(C). It act as a simple high pass filter - it allows high frequency (input)signals to appear at the output side while attenuating low frequency signals.
Circuit diagram:
Input is an AC source, Output is taken across resistor.Contains one pair of R & C.
Working:
Voltage across R is taken as output voltage. Let’s see what happened to this voltage at low and high frequencies.For refreshments of basic concept see Refreshments.
For high frequency
input:
Xc is very low(Xc inversely proportional to frequency). So Vc is also low. For very high frequencies, magnitude of Vc is very less and so we can say that Vc << VR,
where VR is the output voltage. In conclusion, for high frequency input, output is higher.
For low frequency input:
Xc is very high.Vc is also high. Therefore for lower frequencies VR<< Vc. So output voltage VR is much less for low frequency input.
Xc is very high.Vc is also high. Therefore for lower frequencies VR<< Vc. So output voltage VR is much less for low frequency input.
If you are comfortable with 'working', directly go to 'Summary'.
Feel something odd.. just look below.
Feel something odd.. just look below.
When working with filter circuit , we are keeping the
amplitude of input AC constant(here 6v) and varying its frequency.
First case: we give a high frequency input(fig a), say 10MHz. For this suppose Xc = 10. Then the 6v input is divided as Vc : VR = Xc : R = 1:2. VR = 4v. Then Vc= ?? (2v).Vc <VR .
Second case: increase input frequency as 50MHz. Suppose in this case Xc =5. But R doesn’t depend on frequency, so fixed value R=20 . Then the 6v input is divided as Vc : VR = Xc : R = 5:20 = 1:4 . VR = 4.8v. Then Vc = 1.2v.
This example shows that as frequency increases, Vc is also increases. So at higher frequencies it is taken as Vc>>VR .
First case: we give a high frequency input(fig a), say 10MHz. For this suppose Xc = 10. Then the 6v input is divided as Vc : VR = Xc : R = 1:2. VR = 4v. Then Vc= ?? (2v).Vc <VR .
Second case: increase input frequency as 50MHz. Suppose in this case Xc =5. But R doesn’t depend on frequency, so fixed value R=20 . Then the 6v input is divided as Vc : VR = Xc : R = 5:20 = 1:4 . VR = 4.8v. Then Vc = 1.2v.
This example shows that as frequency increases, Vc is also increases. So at higher frequencies it is taken as Vc>>VR .
Then check for a low
frequency input(fig b), say 10KHz. Suppose Xc = 40. Find VR. Then you will get, why VR << Vc for low frequencies?.
Summarising :
for high frequency input signal ----- output voltage is larger
for low frequency input signal ----- output voltage is lesser
for high frequency input signal ----- output voltage is larger
for low frequency input signal ----- output voltage is lesser
Then we can call it as a “high
pass”.
Equation for
output voltage
Vout = VR =
where Vin indicates amplitude of input signal
where Vin indicates amplitude of input signalFrequency response of first order High pass filter:
A frequency response gives you an idea about 'the amplitude of wave at different frequencies'. So in X-axis, we plot 'frequency' and in Y-axis, Gain(Gain= amplitude of output wave/amplitude of input wave). |
| Frequency response of High pass filter |