Diode Circuits:
Applications

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Applications Rectifier Circuits

Half-Wave Rectifier Circuits

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Applications Rectifier Circuits

Battery-Charging Circuit

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Half-Wave Rectifier with Smoothing Capacitor

Large Capacitance

i=dq/dt or Q = IL T

Q = Vr C

then

C ~ (ILT) / Vr

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Half-Wave Rectifier with Smoothing Capacitor

Large Capacitance

Forward bias

charge cycle

Reverse bias

discharge cycle

Start

Vr Peak-to-peak riple voltage

i=dq/dt or Q = IL T

Q = Vr C

then

C ~ (ILT) / Vr

typically :VL ~V m- (Vr /2)

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Full-Wave rectifier Circuits

The sources are out of phase

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Wave Shaping Circuits Clipper Circuits

Batteries replaced by Zener diodes

Review examples:

10.14

10.15

10.16

10.17

10.18

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Half-Wave Limiter Circuits

Current flows thru the resistor until +600 mV is reached, then flows thru the Diode.

The plateau is representative of the voltage drop of the diode while it is conducting.

Voltage

divider

+ 600 mV

I flow below 600 mV

I flow Above 600 mV

- 600 mV

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Linear Small Signal Equivalent Circuits (1)

When considering electronic circuits in which dc supply voltages are used to bias a nonlinear devices at their operating points and a small ac signal is injected into the circuit to find circuit response:

Split the analysis of the circuit into two parts:

analyze the dc circuit to find the operating pointconsider the small ac signal

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Linear Small Signal Equivalent Circuits (1)

Since virtually any nonlinear ch-tic is approximately linear (straight) if we consider a sufficiently small segment

THEN

We can find a linear small-signal equivalent circuit for the nonlinear device to use in the ac analysis

The small signal diode circuit can be substituted by a single equivalent resistor.

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Linear Small Signal Equivalent Circuits (2)

dc supply voltage results in operation at Q

An ac signal is injected into the circuit and

swings the instantaneous point of operation

slightly above and below the Q point

For small changes

DiD the small change in diode current from the Q-point

DvD the small change in diode voltage from the Q-point

(diD/dvD) the slope of the diode ch-tic evaluated at the point Q

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Linear Small Signal Equivalent Circuits (2)

dc supply voltage results in operation at Q

An ac signal is injected into the circuit and

swings the instantaneous point of operation

slightly above and below the Q point

For small changes

DiD the small change in diode current from the Q-point

DvD the small change in diode voltage from the Q-point

(diD/dvD) the slope of the diode ch-tic evaluated at the point Q

Dynamic resistance of the diode

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From small signal diode analysis

Differentiating

the Shockley eq.

and following the math on p.452 we can write that dynamic resistance of the diode is

Linear Small Signal Equivalent Circuits (3)

where

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Example - Voltage-Controlled Attenuator

DC control signal

C1, C2 small or large ?

C in dc circuit open circuit

C in ac circuit short circuit

Find the operating point and perform the small signal analysis to obtain the small signal voltage gain

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Example - Voltage-Controlled Attenuator

DC control signal

Dc circuit for Q point (IDQ, VDQ)

Compute at the Q point (IDQ, VDQ)

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Example - Voltage-Controlled Attenuator

The dc voltage source is equivalent to a short circuit for ac signals.

Voltage gain

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D

Q

D

D

D

v

dv

di

i

D

=

D

1

-

=

Q

D

D

D

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di

r

D

D

D

r

v

i

D

=

D

q

kT

V

nV

v

I

i

T

T

d

s

D

=

-

=

1

exp

=

T

D

T

S

D

D

nV

v

nV

I

dv

di

exp

1

DQ

T

D

I

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r

=

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s

DQ

nV

v

I

I

exp

~

C

j

Z

C

w

1

=

R

R

R

v

v

A

p

p

in

v

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=

=

0