Circuit Theory/IV curves

The current–voltage characteristics of four devices: 100K ohm, 10 ohm, a P–N junction diode, and a battery with real world internal resistance. The horizontal axis is voltage drop, the vertical axis is current.

These curves reflect that most circuits are designed with voltage as the independent variable; with voltage sources and currents being dependent variables. What this means is that the slopes of the IV curves are 1/R. So normally whereas normally horizontal is a slope of 0, it is now a slope of infinity.

Ideally, a diode would turn on at 0 volts and go from horizontal to vertical instantly.

Ideally, a voltage source would be a vertical line.

Ideally, resistance does not change due to voltage or current and are thus straight lines.

transistor curves

A transistor is understood through the above type of graph. The horizontal lines represent current source behavior. Notice how the current stays constant while the voltage changes.

The angled straight lines coming out of the origin represent "linear" behavior where the transistor is operating like an amplifier. The horizontal lines are where the "linear" behavior stops and the current source behavior begins.

Understanding what is controlling this and why the many blue lines starts with understanding what the three wires into a transmitter do:

Three wires into a transistor
  • S = source (of power .. connect to a voltage source)
  • G = gate (like the switch of a light switch .. more like a dimmer knob)
  • D = drain (current leaves .. connected to ground)
  • B = Body ... (often connected to the source)

Think of S and D as the wires going into a light switch and G as the dimmer knob. Each blue line represents a position of the dimmer switch. The gate or dimmer switch determines the amount of amplification and the value of the current source behavior.

So what are the dependent and independent variables above? There are two independent variables: gate Vg and source voltage Vs. Right now it is not important to understand how Vg is physically measured. The important question is "What is the nature of the current source behavior? It seems to be dependent on two things." An ideal, independent current source manipulates the voltage to keep the current constant. The current source behavior above is dependent upon the Gate.

The symbols for a transistor are: