Of course, there are still some inaccuracies. When the load is 1 ohm, the loss is greater than in the other cases, although the pass band is much flatter. Look at this plot (I used the annotation feature to add some labels and arrows): However, if the load gets close to the value of R1, it makes a big difference. Suppose this were feeding an amplifier and the input resistance is 2K ohms. This is probably more accurate but still isn’t entirely correct because there is no load connected across C1. You can see the roll off starts earlier, and attenuates faster. The green trace is the original response, and the blue trace is the new curve. That’s an easy question to answer with Spice and the plot on the left makes it easy to see. How does that change the circuit response?
#GROUND LTSPICE FOR MAC SERIES#
Suppose the source had an 8 ohm series resistor. The resulting AC analysis showed a nice filter response that was flat to about 1 kHz and then started roll off as the frequency increased. Even an oscilloscope probe will load the circuit a little. There was no load resistance either, which is unlikely. Only in simulation do voltage sources and wires have zero resistance. Last time we looked at a low pass filter, but it wasn’t practical because the components were too perfect. In this final installment, I want to talk a bit more about real-world component performance and also look at modeling transformers. In the second, I examined two other parts of Spice: parameter sweeps and AC circuits. In the first installment of this three-part series, I looked at LTSpice and did a quick video walkthrough of a DC circuit. While a simulator can’t tell you everything, it will often give you valuable insight into the way your circuit behaves, before you’ve even built it.
Spice is a circuit simulator that you should have in your toolbox.
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