Principles of Analog Electronics

319

Diode Circuits

In this chapter, we will analyze the rst electronic circuits, in particular (but not

exclusively) based on diodes as semiconductor devices. Their function is to pro-

cess a signal, that is, a time-varying electronic quantity (voltage, current, etc.) to

which information is associated.

Diode applications include rectication (Sections 9.1 and 9.2), voltage multipli-

cation (Section9.3), and wave shaping (Sections 9.4 and 9.5) of the signal.

9.1 RECTIFIERS

Rectiers are circuits that convert an alternating (including both signs) input signal

to an unidirectional output signal (i.e., of a unique sign). So having an AC sinusoi-

dal iso-frequency input signal, the rectiers supply a DC pulsating multi-frequency

output signal, whose harmonic content has more frequencies, including the zero

frequency. Rectication is a nonlinear process implemented by the rectiers.

Most electronic devices require a plate DC voltage supply to work correctly, but

in most cases we have single-phase AC mains. So, rectiers are the basic circuit

to convert AC to DC.

9.1.1 Half-Wave Rectifier

The half-wave rectier is a circuit that, given a pure sinusoidal input voltage, sup-

plies an output positive (or negative) half-sine wave voltage. So, the signal is cut

into its negative (or positive) part. Let’s consider the sinusoidal voltage signal

v

s

= V

sM

sin(ωt)

9

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Principles of Analog Electronics

320

(V

sM

is its amplitude) as input to the following circuit utilizing

asingle diode in series with a resistor (Figure 9.1).

The output voltage is measured across the resistor.

In such a circuit, we can only have current when the diode

is forward biased beyond its threshold voltage V

γ

, so only when

v

s

> V

γ

. This is also the condition for the current to ow in the

resistor and for the output voltage v

R

, across the resistor, to

be non-zero, as in Figure 9.2.

The different maximum value reached by the two voltages,

V

sM

and V

RM

, depends on the non-null value of the voltage V

γ

that “triggers” the diode.

If (and only if) the amplitude V

sM

of the input signal is much

greater than V

γ

, then the delay in the time with which the positive part of v

R

starts

and how early it ends with respect to v

s

can be neglected.

9.1.2 Filtered Half-Wave Rectifier

To the half-wave rectier one can add a capacitor lter, so as to obtain a circuit that

converts an input AC signal to an output DC one. The resulting network is named

a peak rectier or ltered half-wave rectier.

The output waveform v

R

of the half-wave rectier was made of several compo-

nents in frequency, including the zero frequency. A low-pass lter can then be

added to “isolate” this zero-frequency component, resulting in a DC output.

Passive ltering can be realized with a shunt capacitor lter, or a series induc-

tor lter, or a choke input LC lter, or a Π CLC (see Figure 4.34b in Chapter 4, but

with two capacitances and one inductance replacing the resistors) lter. Here, we

will adopt the most common and easiest one, because of its low cost and small

size and weight, which is the shunt capacitor lter, that is, a (generally large value)

parallel capacitor.

The overall circuit is schematized in Figure 9.3.

During the positive half-wave of the input signal when v

s

> V

γ

, the diode is for-

ward biased, so the output signal replicates the input one, and the capacitor is

charged up to the voltage V

sM

− V

γ

. During the negative half-wave of the input signal,

the voltage remains below V

γ

and the diode is reverse biased, so no current ows

from the source, and the charged capacitor supplies current to the resistor. The

output voltage is now established by the capacitor discharge.

R

D

v

s

v

R

+

+

FIGURE 9.1 Half-

wave rectier.

t

v

s

V

sM

v

R

V

RM

FIGURE 9.2 Input (in blue) and output

(inred) waveforms for the half-wave rectier.

C

R

D

v

s

v

C

+

+

FIGURE 9.3 Half-wave

rectier with lter capacitor.

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