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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 C
then
C ~ (ILT) / Vr
*
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)
*
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
*
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
*
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*
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
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
*
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
*
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
*
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
*
Example - Voltage-Controlled Attenuator
DC control signal
Dc circuit for Q point (IDQ, VDQ)
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 gain
*
D
Q
D
D
D
v
dv
di
i
D
=
D
1
-
=
Q
D
D
D
dv
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
nV
r
=
T
DQ
s
DQ
nV
v
I
I
exp
~
C
j
Z
C
w
1
=
R
R
R
v
v
A
p
p
in
v
+
=
=
0