Astable multivibrators I
• A switching oscillator known as Astable Multivibrator can be formed
by adding an RC feedback network to a Schmitt Trigger circuit.
They are useful to generate low frequency square waves.
• The comparator and
feedback resistor
form an inverting
Schmitt Trigger
having threshold
levels of A/2 and
–A/2 assuming A
is the output level
of the comparator.
Graphs from Prentice Hall
Astable multivibrators II
• The operation of the Astable Multivibrators can be described as
follows: at time 0, the initial voltage on the capacitor is 0, assuming
the initial output voltage is +A (A is the level of the comparator
output). Thus, initially the capacitor is charged through the resistor
R toward +A. However, when the capacitor voltage reaches A/2,
the output voltage rapidly switches to –A.
• Then the capacitor starts to discharge, once the voltage drops below –
A/2, the output again
switches back to A. Thus,
the capacitor voltage
cycles back and forth
between A/2 and –A/2.
• Voltage across
capacitor resembles
Triangular wave and
comparator output
voltage is symmetrical
square wave.
Astable multivibrators III
• The period and frequency of the output square waveform can be
determined by analyzing the transient response of the RC feedback
network.
• The frequency of oscillation for the Astable Multivibrator shown before
is
1
f 
2 RC ln 3
• In real circuit design, several non-idealities related to the comparator
can affect the frequency, such as the propagation delay of the
comparator and bias current effects.
• To minimize the bias current effects, we usually need to make sure
that the smallest current charging to the capacitor should be much
larger than the bias current, for example, a few hundred times.
The 555 Timer I
• The 555 timer analog IC (Integrated Circuit) is very
economical and convenient for use in mutivibrator circuits
since few external components are required.
The 555 Timer II
• The 555 timer IC has two comparator, one SR flip-flop and
one switching BJT transistor.
• Power supply voltage Vcc is applied to a series of three
resistors so that almost 1/3Vcc and 2/3Vcc is established.
• The reset, threshold and trigger control the state of the flipflop. If reset is low, Q output is low regardless of the input
applied, and transistor is saturated in this case. So, reset
has the highest priority in setting Q. When reset is high
(connected to Vcc) it does not affect the output Q.
• If the trigger input is lower than 1/3Vcc, then the
comparator output is high, setting Q to high and transistor
is off.
• If the threshold input becomes higher than 2/3Vcc, then
the output of the comparator is high, resetting Q to low and
transistor is in saturation.
The 555 Monostable Multivibrator
• A Monostable Multivibrator is a circuit that produces an output pulse of
fixed duration each time the input of the circuit is triggered. This is
useful in producing timing signals.
The 555 Astable Multivibrator
• An Astable Multivibrator can be formed by adding two resistors and a
capacitor to the 555 timer.
Half-wave Precision Rectifiers I
• Precision half-wave and full-wave rectifiers are very useful in signal
processing applications, such as converting AC signal to DC signal.
• Consider
positive
input, the output
voltage
of
X1
becomes
positive,
and the diode is
forward biased. Due
to
feedback
connection, the input
voltage of X1 is
forced to 0. Then,
from the voltage
follower X2, input
voltage appears at
the output.
• Consider negative input, output voltage of X1 becomes negative, and
diode is reverse biased. So, no current flow on R (feedback path of
X1 is not in effect). Voltage at inverting terminal of X1 is set to 0,
which appears at the output voltage of X2 as 0.
Half-wave Precision Rectifier II
• Error
due
to
offset
voltage
and bias current
in
Half-wave
precision rectifier
circuit usually are
on the order of 12% of the peak
voltage (e.g, the
input voltage is a
few
hundred
milli-volts)
• Typical problems comes from non-zero reverse-biased diode current,
input bias current of OpAmp, offset voltage of the OpAmp, slew-rate
limiting when the output needs to change from 0 to negative extreme
and negative extreme to positive value.
Improved Half-wave Precision Rectifier
• A second diode D2 is
connected to keep
the output voltage of
X1 from being driven
to
its
negative
extreme.
• X2 is still a voltage
follower with a low
output impedance.
• The output signal is
an inverted version of
the half-wave rectified
input signal
• The circuit amplifiers the signal by the gain factor of R2/R1.
Precision Full-wave Rectifier
• The circuit can be considered to consist of two parts, a half-wave
rectifier and a summer circuit.
• At the positive cycle of Vin, the output voltage at point A is a inverted
version of Vin.
• At the negative cycle, the voltage at A is 0.
Precision peak detector
• An ideal peak detector should produces a DC output waveform that is
equal to the preceding peak value of the input signal
• If the difference is positive, OpAmp produces current to charge
capacitor.
• If the difference is negative, feedback path is broken and the voltage
on the capacitor remains.
Sample and hold circuit
• A clock MOS acts as a switch to control the state of operation,
sampling state and hold state
• Due to slew rate limitation of OpAmp, output takes some time to settle
to the value close to the input voltage in the sampling state
Precision clamp circuit
• Clamp circuit is to add a DC voltage to the input so that the sum is
never negative.
• OpAmp X2 is used as a voltage follower so that current could be
delivered to the load without affecting the charge on the capacitor
• If inverting input of X1 is positive, output of X1 is in negative extreme
and diode is off. But voltage capacitor remains steady.
• If inverting input of X1 is negative, diode is on and capacitor take the
voltage of vin