Transcript Lecture 6 Diode Circuits` Applications

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
Vr Peak-to-peak
riple voltage
Large
Capacitance
Start
i=dq/dt or Q = IL T
Q = Vr C
then
C ~ (ILT) / Vr
typically :VL ~V m- (Vr /2)
Forward bias
charge cycle
Reverse bias
discharge cycle
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
+ 600
mV
I flow
below
600
mV
I flow
Above
600
mV
Voltage
divider
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.
- 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:
(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
 di D
i D  
 dv D

 v D
Q
iD –the small change in diode current from the Q-point
vD –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
 di D
i D  
 dv D

 v D
Q
Dynamic resistance of the diode
 di
rD   D
 dv D
 
 
 Q 
1
v D
i D 
rD
iD –the small change in diode current from the Q-point
vD –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 (3)
From small signal diode analysis
  vd
i D  I s exp 
  nVT
kT
VT 
q
 
  1
 
Differentiating
the Shockley eq.
 vD 
di D
1

 IS
exp 
dv D
nVT
 nVT 
… and following the math on p.452 we can write that dynamic
resistance of the diode is
nVT
rD 
I DQ
where
I DQ
 vDQ
~ I s exp 
 nVT



Example - Voltage-Controlled Attenuator
Find the operating point and perform the small signal analysis
to obtain the small signal voltage gain
DC control signal
1
ZC 
jC
C1, C2 – small or large ?
C in dc circuit – open circuit
C in ac circuit –short circuit
Example - Voltage-Controlled Attenuator
DC control signal
Dc circuit for Q
point (IDQ, VDQ)
nVT
rD 
I DQ
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
Rp
v0
Av 

v in R p  R