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Subject Name: LINEAR IC’s AND APPLICATIONS
Subject Code:10EC46
Prepared By: Kumutha A
Department: Electronics and Communication
Date:30-3-2015
5/13/2017
UNIT 5
More Applications
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Clamping Circuits
Peak detectors
Sample and hold circuits
Log and antilog Amplifiers
Multiplier and divider
Triangular and rectangular wave generators
Phase shift Oscillator
Wein bridge Oscillator
Clampers circuits
In clamper circuits, a predetermined dc
level is added to the input voltage. In other words,
the output is clamped to a desired dc level. If the
clamped dc level is positive, the clamper is called a
positive clamper. On the other hand, if the clamped
dc level is negative, it is called a negative clamper.
The other equivalent terms for clamper are dc
inserter or dc restorer. A clamper circuit with a
variable dc level is shown in fig. Here the input
wave form is clamped at +Vref and hence the
circuit is called a positive clamper.
The output voltage of the clamper is a net result of
ac and dc input voltages applied to the inverting
and non-inverting input terminals respectively.
Therefore, to understand the circuit operation,
each input must be considered separately. First,
consider Vref at the non-inverting input.
The output voltage of the clamper is a net result of ac and dc input
voltages applied to the inverting and non-inverting input terminals respectively.
Therefore, to understand the circuit operation, each input must be considered
separately. First, consider Vref at the non-inverting input. Since this voltage is
positive, is +VO is positive, which forward biases diode D1. This closes the
feedback loop and the op-amp operates as a voltage follower. This is possible
because C1 is an open circuit for dc voltage. Therefore VO = Vref. As for as
voltage Vin at the inverting input is concerned during its negative half-cycle D1
conducts, charging C1 to the negative peak value of the VP. However, during the
positive half-cycle of Vin diode D1 is reverse biased and hence the voltage VP
across the capacitor acquired during the negative half-cycle is retained. Since this
voltage VP is in series with the positive peak voltage VP, the output peak voltage
VO=2VP. Thus the net output is Vref+VP, so the negative peak of 2VP is at Vref.
For precision clamping C1Rd<<T/2, where Rd is the forward resistance of the
diode D1 (100Ω typically) and T is the time period of Vin. The input and output
wave forms are shown in fig.
Peak Detector Circuit
Square, triangular, saw-tooth and pulse
waves are typical examples of non-sinusoidal
wave forms. A conventional ac voltmeter cannot
be used to measure the rms value of the pure sine
wave. One possible solution for this problem is
to measure the peak values of the non-sinusoidal
wave forms. Fig shows a peak detector that
measures the positive peak values of the input.
Sample and hold circuit
The voltage V’ would ideally stay constant in the hold
mode by having a value equal to Vin at the instance of
clock going low. But there are two error sources due to
switch:
1.The channel charge go to both junctions to causes
negative glitches. If source impedance of Vin is small,
then the glitch is small and last a short duration.
2.The channel charge go to V’ causes a negative voltage
that is long lasting.
An improved sample and hold circuit by using two OpAmps. Here the
hold capacitor is placed in the feedback path of a second OpAmp. The
advantage is that now both sides of Q1 is almost signal independent if OpAmp2
has a large gain. So, when Q1 turns off, there is still charge going to capacitor,
but the Vout will have only a fixed DC error voltage.
Log Amplifier
• Log Amplifier
• Logarithmic amplifier
gives the output
proportional to the
logarithm of input signal.
• The fig shows log
amplifier.
• Derivation gives output
voltage of log amplifier.
I E  I S e qVE / kT  1)
I E  IC
I c  I S (e qVE / kT  1)
Ic
 (e qVE / kT  1)
IS
Ic
 1
IS
Ic
 e qVE / kT
IS
Log Amplifier
taking natural log on both side
I
kT
VE 
ln( c )
q
IS
from circuit,
V
Ic  i
R1
VE  Vo
V0  
Vi
kT
ln(
)
q
R1 I S
V0  
V
kT
ln( i )
q
Vref
where,Vref  R1 I S
Log Amplifier
•The previous circuit has some
problem. The emitter saturation
current Is varies from transistor to
transistor and with temperature .
•Thus stable reference voltage cannot
be obtained. This Is eliminated by
modification in circuit as shown. The
two transistor are
•Integrated close together in same
silicon wafer, This provide a close
match of saturation currents and
ensure good thermal tracking.
IS 1  IS 2  IS
V1  
V
kT
ln( i )
q
R1 I S
V2  
Vref
kT
ln(
)
q
R1 I S
V0  V2  V1
V0 
Vref
V
kT
[ln( i )  ln(
)]
q
R1 I S
R1 I S
V0 
V
kT
ln( i )
q
Vref
V0 comp  (1 
V
R2 kT
) ln( i )
RTC q
Vref
Antilog Amplifier
• The output is
proportional to the
antilog of input
voltage.
VQ1B  E 
kT
Vo
ln(
)
q
R1 I S
VQ2 B  E  
Vref
kT
ln(
)
q
R1 I S
VA  

VQ1B  E
kT
Vo
ln(
)
q
R1 I S
Antilog Amplifier
VB  (
RTC
)Vi
RTC  R2
VQ2 E  VB  VQE  B
VQ 2 E  (
Vref
RTC
kT
)Vi 
ln(
)
RTC  R2
q
R1 I S
VA  VQ 2 E

Vref
Vo
RTC
kT
kT
ln(
)(
)Vi 
ln(
)
q
R1 I S
RTC  R2
q
R1 I S
RTC
V
q
Vi  ln( 0 )
kT RTC  R2
Vref
 0.4343
RTC
V
q
Vi  0.4343 ln( 0 )
kT RTC  R2
Vref
 k 'Vi  log 10 (
V0
)
Vref
V0
 10  k 'Vi
Vref
V0  Vref 10  k 'Vi
where, k '  0.4343
RTC
q
kT RTC  R2
Multiplier
•The output voltage can be written as
•If both inputs are positive, then the multiplier is called a one-quadrant multiplier.
• If one input is kept at a positive value and the other input is allowed to take
either a positive or negative value, then it is called a two-quadrant multiplier.
• If both the inputs are allowed to take either positive or negative values,
then it is called a four-quadrant multiplier.
•There are some commercially available multiplier ICs and multiplier circuits can
be constructed from op-amp ICs such as 741.
•The applications of multipliers include frequency doubling, frequency shifting,
phase angle detection, real power computation, and squaring signals.
Divider
• The divider circuit is
shown in fig.
• The output can be
written as
• The applications of dividers
include taking square root
and dividing one number by
another.
Triangular/ Rectangular wave generator
• The circuit consist of a triangular and
a inverting Schmitt trigger circuit.
• The Schmitt output is applied as a
input to the integrator and integrator
output is applied as Schmitt trigger
input.
• When the Schmitt trigger output is
positive the integrator output goes
negative until it reaches the Schmitt
LTP.
• The Schmitt Output switches to
negative and integrator Output then
moves in a positive direction to the
UTP of Schmitt trigger.(refer the
waveform).
RC phase Shift Oscillator
RC phase shift oscillator is a sinusoidal
oscillator used to produce sustained well
shaped sine wave oscillations. It is used
for different applications such as local
oscillator for synchronous receivers,
musical instruments, study purposes etc.
The main part of an RC phase shift
oscillator is an opamp inverting
amplifier with its output fed back into its
input using a regenerative feedback RC
filter network, hence the name RC phase
shift oscillator.
RC phase Shift Oscillator
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The feedback network offers 180 degrees phase shift at the oscillation frequency and the op
amp is configured as an Inverting amplifier, it also provide 180 degrees phase shift. Hence to
total phase shift around the loop is 360 degrees, it is essential for sustained oscillations.
At the oscillation frequency each of the resistor capacitor filter produces a phase shift of 60°
so the whole filter circuit produces a phase shift of 180°.
The energy storage capacity of capacitor in this circuit produces a noise voltage which is
similar to a small sine wave, it is then amplified using op amp inverting amplifier.
By taking feedback, the output sine wave also attenuates 1/29 times while passing through the
RC network, so the gain of inverting amplifier should be 29 in order to keep loop gain as
unity.
The unity loop gain and 360 degree phase shift are essential for the sustained oscillation.
RC Oscillators are stable and provide a well shaped sine wave output with the frequency
being proportional to 1/RC and therefore, a wider frequency range is possible when using a
variable capacitor.
However, RC Oscillators are restricted to frequency applications because at high frequency
the reactance offered by the capacitor is very low so it acts as a short circuit.
Wein bridge oscilltor
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The output of the operational amplifier is fed
back to both the inputs of the amplifier. One
part of the feedback signal is connected to the
inverting input terminal (negative feedback)
via the resistor divider network of R1 and R2
which allows the amplifiers voltage gain to be
adjusted within narrow limits. The other part
is fed back to the non-inverting input terminal
(positive feedback) via the RC Wien Bridge
network.
The RC network is connected in the positive
feedback path of the amplifier and has zero
phase shift a just one frequency. Then at the
selected resonant frequency, ( ƒr ) the
voltages applied to the inverting and noninverting inputs will be equal and “in-phase”
so the positive feedback will cancel out the
negative feedback signal causing the circuit to
oscillate.
Wein Bridge Oscillator
• The voltage gain of the amplifier circuit MUST be equal too or greater than
three “Gain = 3″ for oscillations to start because as we have seen above, the
input is 1/3 of the output. This value, ( Av ≥ 3 ) is set by the feedback
resistor network, R1 and R2 and for a non-inverting amplifier this is given
as the ratio 1+(R1/R2).
• Also, due to the open-loop gain limitations of operational amplifiers,
frequencies above 1MHz are unachievable without the use of special high
frequency op-amps.