Alright let's do something a little more interesting. Let's make a bandpass
filter circuit.
Start with a resistor, Ctrl + R to rotate.
Grab a capacitor.
Let's put down another capacitor. Ctrl + R. Finally another resistor.
Escape. F3 to draw the wires.
Escape.
We will need a voltage source so hit F2.
Let's put that down here.
F3, Escape. And never ever forget
the ground. Excellent.
Now let's add some component values.
Let's make this Resistor 10 kilohms. This capacitor... let's make it
1nF.
Let's say this capacitor is 100pF.
This resistor...
let's say it's going to be 20 kilohms. As before we have to set up our voltage
source. Go to "Advanced", "AC amplitude"
and let's do 10 volts this time. Now we have to set up our simulation.
Edit simulation command. AC analysis.
I like to do the decade sweeps.
Points per decade... let's say 100 points per decade.
(Lots of resolution). Start at 1Hz
and let's finish at a 100 MHz just in case there's something interesting
happening at a really high frequency.
Okay and it gives you the box and you can put that anywhere you want.
Alright, ready to run the simulation.
Simulate... Run... let's see what happens
on the output node. Exactly as expected!
We get a bandpass filter response on the magnitude curve.
So you can see that around 36 kHz the signal is about 3.8 decibels.
Frequencies higher than that get attenuated and frequencies lower than
that also get attenuated.
And again if you want to look at things in terms of volts,
right click, "manual limits", decimals to logarithmic.
Let's make the top 20 volts because our input is 10 volts.
Leave the bottom as a tiny number. Okay you can see
the peak above the curve is about
1.5 volts. Okay
let's see what happens when we tweak some of the values.
let's make this
1 kilohms instead and do the same simulation.
Simulate, Run, Output node.
Now the bandpass filter's center frequency
has shifted up to 100Khz. Okay let's add a heavy load resistance and
see what happens.
What would happen if we
added a 4 ohm speaker? Right click...
4 ohms... simulate again...
So it looks like we've almost completely lost the original band pass
characteristic.
And that doesn't surprise me. Take a look at the load resistance compared to the other
resistors in the circuit.
We've got 4 ohms in parallel with 20 kilohms.
That will completely change the RC time constant of
this high pass section over here. We have the series resistor
of 1 kilohm over here compared to 4 ohms over there. That's obviously going to
change
the entire characteristics of the circuit.
So you can see that it's a really good idea to simulate your filters before you go out
and build them.
Otherwise you'll put different loads on them and you'll be wondering
why aren't they working the way you thought they should.
Okay I'm tired of filters so I'm going to show you
how to simulate a little amplifier circuit that you can build out of a
transistor.