Wednesday, October 10, 2018

Why be concerned with component layout - OR - Strays everywhere

Consider a wire in free space. Would it be a resistor? Would it be an inductor? Would it be a capacitor? Any or all?
I'm using an online calculator. This one is on the eeweb site. A search will find more. Anyway, a 3 cm wire has 17.9nH.
The same wire spaced 4 cm from another wire has 9 nH. So moving it close to another conductor decreases the inductance? How can that be? Mutual inductance is in play. If you connect an inductance meter to the primary of a transformer and leave the secondary open you will get a reading. Short the secondary and the reading goes down. A current produces an EMF in the primary and the secondary couples to it. The secondary's induced EMF is in opposition to the force producing it. So the primary feeds power to the load. The primary sees an open circuit with no load and sees the load when applied.
If we continue to bring the wires closer the load overcomes the source and we see a small inductance.
What about capacitance? Using another calculator we see the capacitance of a wire 1 cm from our ground plane.
As you can see moving it farther away will reduce the capacitance. Now put these 2 together. One wire will have inductance and capacitance. The question is will it help or hurt our circuit performance? How to use this knowledge?
 I just clipped this from a text book. Stray capacitance everywhere. Some things are beyond our control. The junction capacitance for our transistor have to be considered. Generally capacitance is the limiting factor for our circuit high end. But the previous data should be applied too.
What about that inductance in our wires? Each little red box represents a source for inductance.
What can we do?
Keep the leads short and close to the ground plane. Ground the input  leads close together. Separate the output lead for each stage. Remember the ground plane can be a source of reactance. In the MMIC circuits they etch the inductors into the circuit board.
Study this oscillator circuit. The foil is the tank. It is tuned by adjusting the foil length and width.
So keep the leads short, keep the ground plane as large as practical, isolate the input and output, and use components with a self resonance to match your circuit.

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