Diodes

Ideal diodes pass current easily (low resistance) in one direction, and block current (very high resistance) in the opposite direction. To pass current, a diode is forward biased, and to block current a diode is reverse biased.

A practical junction diode made of silicon will typically require about 500mV to 600mV of forward bias voltage to conduct. This means that there is a bit of a "dead band" from near 0V to 600mV where the diode doesn't conduct even if you are trying to apply current in the right direction.

Real diodes conduct in reverse too, but typically at a much higher voltage than the forward voltage. This is called the Zener voltage, and it is very useful for creating low impedance reference voltages that can work as a simple voltage regulator for relatively low current circuits. A pair of +/-15V Zener diodes can provide a simple op amp supply for preamplifier sections.

Rectification

Fig 1

Figure 1 shows diode rectification. The ac voltage which swings about ground only appears across the resistor during positive swings. Note the slight loss of voltage from the ≈600mV inherent voltage drop across the diode.

Fig 2

Figure 2 shows diode rectification again, but this time with the diode's orientation reversed. Now only the negative swings appear across the resistor. Again there is a slight loss due to the voltage drop across the diode.

Fig 3

In figure 3 there are 2 identical ac voltage sources run in series and grounded in the middle. This creates 2 identical signals with opposing polarity (180º out-of-phase). This simulates the output of a center tapped transformer, or a simple active splitter type circuit. Only the top source is shown as a yellow trace in the scope window. The bottom voltage source provides the positive swings when the yellow trace is negative, and these simply sum together where the diodes and resistor meet.