Diode Circuits:Applications

Applications – Rectifier Circuits

Half-Wave Rectifier Circuits

Applications – Rectifier Circuits

Battery-Charging Circuit

Half-Wave Rectifier with Smoothing Capacitor

Large Capacitance

i=dq/dt or Q = IL T Q = Vr Cthen C ~ (ILT) / Vr

Half-Wave Rectifier with Smoothing Capacitor

Large Capacitance

Forward biascharge cycle

Reverse biasdischarge cycle

Start

Vr Peak-to-peak riple voltage

i=dq/dt or Q = IL T Q = Vr Cthen C ~ (ILT) / Vr

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

Full-Wave rectifier Circuits

The sources are out of phase

Wave Shaping Circuits Clipper Circuits

Batteries replaced by Zener diodes

Review examples: 10.14

10.15 10.16 10.17 10.18

+ 600 600 mVmV

II flow belo

w 600 600 mVmV

II flow Above 600 600 mVmV

- 600 600 mVmV

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

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: (a)analyze the dc circuit to find the operating

point(b)consider the small ac signal

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.

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

D

QD

DD v

dv

dii

iD –the small change in diode current from the Q-pointvD –the small change in diode voltage from the Q-point(diD/dvD) – the slope of the diode ch-tic evaluated at the point Q

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

D

QD

DD v

dv

dii

iD –the small change in diode current from the Q-pointvD –the small change in diode voltage from the Q-point(diD/dvD) – the slope of the diode ch-tic evaluated at the point Q

1

QD

DD dv

dir

Dynamic resistance of the diode

D

DD r

vi

From small signal diode analysis

q

kTV

nV

vIi

T

T

dsD

1exp Differentiating

the Shockley eq.

T

D

TS

D

D

nV

v

nVI

dv

diexp

1

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

DQ

TD I

nVr

Linear Small Signal Equivalent Circuits (3)

T

DQsDQ nV

vII exp~

where

Example - Voltage-Controlled Attenuator

DC control signal

C1, C2 – small or large ?C in dc circuit – open circuitC in ac circuit –short circuit

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

CjZC

1

Example - Voltage-Controlled Attenuator

DC control signal

Dc circuit for Q point (IDQ, VDQ)

DQ

TD I

nVr Compute at the

Q point (IDQ, VDQ)

Example - Voltage-Controlled Attenuator

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

Voltage gainRR

R

v

vA

p

p

inv

0