The second edition of this well-received text continues to provide a coherent and comprehensive coverage of Pulse and Digital Circuits, suitable as a textbook for use by undergraduate students pursuing courses in Electrical and Electronics Engineering, Electronics and Communication Engineering, Electronics and Instrumentation Engineering, and Telecommunication Engineering. It presents clear explanations of the operation and analysis of semiconductor pulse circuits. Practical pulse circuit design methods are investigated in detail. The book provides numerous fully worked-out, laboratory-tested examples to give students a solid grounding in the related design concepts. It includes a number of classroom-tested problems to encourage students to apply theory in a logical fashion. Review questions, fill in the blanks, and multiple choice questions offer the students the opportunity to test their understanding of the text material. This text will be also appropriate for self-study by AMIE and IETE students.

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PULSE AND DIGITAL CIRCUITS
A. ANAND KUMAR
Advisor to
Dean Academics and Dean Quality
K.L. University
Green Fields, Vaddeswaram
Guntur District
Andhra Pradesh
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Chapter 1
Linear Wave Shaping

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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.

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 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.)

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 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
LOW-PASS RC CIRCUIT (Contd.)

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 Exponential Input
Figure: Exponential input Figure: The low-pass RC circuit.
LOW-PASS RC CIRCUIT (Contd.)

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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.

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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.

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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

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HIGH-PASS RC CIRCUIT (Contd.)
 Pulse Input
 Square-Wave Input
 A square wave may be treated as a series of positive and
negative steps.

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HIGH-PASS RC CIRCUIT (Contd.)
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.

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

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HIGH-PASS RC CIRCUIT (Contd.)
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.

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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.

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

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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.

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

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

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RLC CIRCUITS (Contd.)

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

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RLC CIRCUITS (Contd.)
 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

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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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