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.

L

Greatly Expanded Scale Circuit Using LM3914

This is the circuit for bar mode using LM3914 IC for great expanded scale. This circuit is only consists of some resistors circuit. This is the figure of the circuit.


Placing the LM3914 internal resistor divider in parallel with a section (230Ω) of a stable, low resistance divider greatly reduces voltage changes due to IC resistor value changes with temperature. Voltage V1 should be trimmed to 1.1V first by use of R2. Then the voltage V2 across the IC divider string can be adjusted to 200mV, using R5 without affecting V1. LED current will be approximately 10mA. [Schematic source: National Semiconductor, Inc].