Puls And switching digital circuits ppt.pptx

PULSE AND DIGITAL CIRCUITS
A. ANAND KUMAR
Advisor to
Dean Academics and Dean Quality
K.L. University
Green Fields, Vaddeswaram
Guntur District
Andhra Pradesh

Chapter 1
Linear Wave Shaping

LOW-PASS RC CIRCUIT
 Transmits only low-frequency signals and attenuates or stops high-
frequency signals
• Sinusoidal Input
 This curve is obtained by keeping the amplitude of the input
sinusoidal signal constant and varying its frequency and noting
the output at each frequency.

 The upper cut-off frequency is 70.7%, of its maximum value.
 the magnitude of the steady-state gain A is given by
 Step-Voltage Input
 The expression for the voltage across the capacitor of an RC
circuit excited by a step input is given by
vo(t) = Vfinal – (Vfinal – Vinitial)e–t/RC
 Rise time tr is proportional to the time constant RC of the
circuit.
 Rise time is inversely proportional to the upper 3-dB frequency
LOW-PASS RC CIRCUIT (Contd.)

 Pulse Input
 A pulse shape will be preserved if the 3-dB frequency is
approximately equal to the reciprocal of the pulse width.
 Square-Wave Input
 Ramp Input
 When a low-pass RC circuit is excited by a ramp input, we have

 Exponential Input
Figure: Exponential input Figure: The low-pass RC circuit.

LOW-PASS RC CIRCUIT
 LOW-PASS RC CIRCUIT AS AN INTEGRATOR
 If the time constant of an RC low-pass circuit is very large in
comparison with the time required for the input signal to
make an appreciable change, the circuit acts as an
integrator.
 An RC integrator converts a square wave into a triangular
wave.

HIGH-PASS RC CIRCUIT
 This circuit, also called the capacitive coupling circuit, is used to provide
dc isolation between the input and the output.
 As the frequency increases, the reactance of the capacitor decreases and
hence the output and gain increase.
Figure: The high-pass RC circuit.

HIGH-PASS RC CIRCUIT (Contd.)
 Sinusoidal Input
 The magnitude of the steady-state gain A, and the angle q by
which the output leads the input are given by
Fig: Gain versus frequency plot.
 Step Input
 The expression for the output voltage for t > 0 is given by

 Pulse Input
 Square-Wave Input
 A square wave may be treated as a series of positive and
negative steps.

Figure: A square wave input.
 Ramp Input
 A waveform which is zero for t < 0 and which increases linearly
with time for t > 0 is called a ramp or sweep voltage.

Figure: Response of the high-pass circuit for a ramp input when (a) RC >> T
and (b) RC << T.
 Exponential Input

Figure: Response of high-pass circuit for exponential input.
 HIGH-PASS RC CIRCUIT AS A DIFFERENTIATOR
 The high-pass RC circuit acts as a differentiator provided the RC time
constant of the circuit is very small in comparison with the time
required for the input signal to make an appreciable change.
 An RC differentiator converts a triangular wave into a square wave.

ATTENUATORS
 Attenuators are used to reduce the amplitude of the input signal.
• The decay or rise of the output (when the attenuator is not perfectly
compensated) from the initial to the final value takes place
exponentially with a time constant t = RC.
Figure: Cont.

ATTENUATORS
Figure: Response of compensated attenuator: (a) perfect compensation,
(b) over compensation, and (c) under compensation.

RL CIRCUITS
 RL low-pass circuit
 At very low frequencies the reactance of the inductor is small, so the
output across the resistor R¢ is almost equal to the input.
 As the frequency increases, the reactance of the inductor increases
and so the signal is attenuated.
 At very high frequencies the output is almost equal to zero.
 RL high-pass circuit
 At low frequencies, since the reactance of the inductor is small, the
output across the inductor is small
 At high frequencies the output is almost equal to the input.

RL CIRCUITS (Contd.)
Figure: (a) RL low-pass circuit and (b) RL high-pass circuit.

RLC CIRCUITS
 RLC Series Circuit
Figure: A series RLC circuit.
 The transfer function of the circuit is

RLC CIRCUITS (Contd.)

Figure: (a) Current response and (b) voltage response of series RLC circuit to
a step voltage.

 RLC Parallel Circuit
 The transfer function of the network is
 The circuit is overdamped if R < 1/2 L/C, critically damped if R =
1/2 L/C and underdamped if R > 1/2 L/C.
Fig 1.77

RINGING CIRCUIT
 A circuit, which can provide as nearly undamped oscillations as
possible is called a ringing circuit.
 If the damping is very small the circuit will ring for many cycles.
 A ringing circuit may be used to generate a sequence of pulses
regularly spaced in time.

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