Diode Limiting and Ciruits
Diode limiting and clamping circuits
a) Limiters:
– Diodes can be used to clip off portions of signal voltages (above or below certain levels).
– Diode will become forward biased as soon as VA becomes larger than VBIAS+0.7.
– When diode is forward biased, VA cannot become larger than VBIAS + 0.7 V!
– Thus, the voltage across the load, RL, will also be equal to VBIAS + 0.7.
– When diode is reverse biased, it appears as an open, so the output voltage is the voltage of RL alone.
– Desired voltage levels can be attained with a voltage divider.
– We replace the voltage source with a resistive voltage divider.
VBIAS = R3/(R2 + R3) VSUPPLY
Example:
b) Diode Clampers
– A clamper adds a dc level to an ac voltage.
– Also called dc restorers.
– When input voltage goes initially negative, diode is forward biased.
– Capacitor charges to near peak of inpt (Vp(in) – 0.7).
– Right after the negative peak, diode is reverse biased (because cathode is held near Vp(in) – 0.7 by charge on capacitor).
– Capacitor can only discharge through the RL.
– Since RL has high resistance, the capacitor discharges very little each period.
– Note that time constant should be large (at least 10 times the period of the input voltage).
– Since capacitor retains charge, it acts like a battery in series with the input voltage.
