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 V_A becomes larger than V_BIAS+0.7.

– When diode is forward biased, V_A cannot become larger than V_BIAS + 0.7 V!

– Thus, the voltage across the load, R_L, will also be equal to V_BIAS + 0.7.

– When diode is reverse biased, it appears as an open, so the output voltage is the voltage of R_L alone.

– Desired voltage levels can be attained with a voltage divider.

– We replace the voltage source with a resistive voltage divider.

V_BIAS = R₃/(R₂ + R₃) V_SUPPLY

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 (V_p(in) – 0.7).

– Right after the negative peak, diode is reverse biased (because cathode is held near V_p(in) – 0.7 by charge on capacitor).

– Capacitor can only discharge through the R_L.

– Since R_L 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.

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