Resolved Problems about Special Circuits with Diodes

Problem 65-1 Source: Problem elaborated by the author of the site.

Given the circuit shown in the Figure 65-01.1, calculate the output voltage. V_o, assuming the input voltage is given by V_i = 10 sin 1000 t. Sketch the waveform of V_o. Consider the voltage on the conduction diode as V_D = 0.7 V.

Solution of the Problem 65-1

When the input voltage V_i starts its value growth towards the positive peak, we already know beforehand that the diode D₂ will remain cut across the positive portion of the input voltage. On the other hand, the D₁ diode will not conduct until the input voltage reaches the 0.7 V value. From that moment, the current that circulates through the diode and the two resistors is given by:

I = (V_i - 0.7 ) / (100 + 200 )

And as the output voltage V_o is given by V_o = 200 I, so we easily conclude that:

Therefore, the correct answer is:

If 0 ≤ V_i ≤ + 0.7 V ⇒ V_o = 0 V

If V_i > + 0.7 V ⇒ V_o = (2/3 ) (V_i - 0.7 ) V

0.7 V

zero

D₁

negative

D₂

V_i ≤ - 0.7 V

zero

rectifier

Figure 65-1.2

t = 0

zero

0.7 V

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