Week 1. Ohms law, power, Kirchhoff`s law V,I.

Ohm`s law V=IR. So what is a Volt ? Potential energy per unit charge or work done to move a charge against electric field, form one potential to a higher potential.

Dynamic resistance R_dynamic= ΔV/ΔI it is the local resistance – the tangent to the slope of the device V-I curve.

Power P=I×V or derived: P=I²R or P=V²/R

Kirchhoff`s law: sum of voltages around circuit is zero, sum of currents in and out of node is zero.

Parallel resistance: R_total = (R1×R2)/(R1+R2)

Rule of thumb: if two parallel resistors differ by a factor of ten or more, then we can ignore the larger of two.

In other words: in a parallel circuit, resistor much smaller than other dominates. Ina series circuits, the larger resistor dominates.

Voltage divider. Rely on fact the I is the same on top and bottom.

V_out = V_in × R2 / (R1+R2)

Tip: since the current at the top and bottom are equal, the voltage drops are proportional to the resistances.

So if the lower leg is ten times the upper leg, it will show 90% of the input voltage ( or 10/11 if exactly).

Or if lower leg is 3 times than upper leg, output voltage should be 75% of input voltage ( or 3/4 if exactly ), lets check that:

Thevenin`s model. How to calculate circuit when it gets loaded? Model the actual circuit with the simpler circuit – Thevenin model – which is idealized voltage source with resistor in series.

Then we can rapidly see how that circuit behaves under various loads.

V_thevenin is just open circuit, the voltage when nothing is attached ( no load ).

R_thevenin is V_thevenin/I_{short-circuit} 

The fastest way to calculate R_thevenin is to see it as the parallel resistance of several resistances viewed from the output.

Any non ideal voltage source droops when loaded. How much it droop depends on it output impedance. The Thevenin equivalent model, with its R_thevenin describes this property neatly in a single number.

Design rule of thumb: when circuit A drives circuit B. Let R_out for A be <= 1/10 R_in for B.

It means we will pass strong signal to B without big droop.

If R_outA is much smaller then R_inB then the divider delivers nearly the all of the original signal.