Astable multivibrators I

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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
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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.

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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
• 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.

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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.

5
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 flip-flop. 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.

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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.

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The 555 Astable Multivibrator

• An Astable Multivibrator can be formed by adding
  two resistors and a capacitor to the 555 timer.

8
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.

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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 1-2 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.

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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.

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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.

12
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.

13
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

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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