26-09-2013, 04:22 PM
LINEAR WAVESHAPING
LINEAR WAVESHAPING.pdf (Size: 2.67 MB / Downloads: 76)
Introduction:
If a circuit is designed with components like R ,L and C then it is called linear circuit.
When sinusoidal signal is applied ,the shape of the signal is preserved at the output with
or without change in the amplitude and shape. But a non-sinusoidal signal alters the
output when it is transmitted through a linear circuit.
The process whereby the form of non-sinusoidal signals such as step, pulse,
square wave, ramp and exponential is altered by transmission through a linear network
is called linear wave shaping.
HighPass RC Circuit as Differentiator
If the time constant of the RC highpass circuit is very much smaller than the time period of the
input signal, then the circuit behaves as a differentiator. Then the voltage drop across R is very small
when compared to the drop across C.
Practical clamping circuits
If a square wave is applied as input to a clamping circuit, the output reaches the steady-
state value after a few cycles. Hence for the input in Fig.(a) the output of the clamping
circuit is given in (b).
TRANSISTOR SWITCHING TIMES
Let the input to the transistor switch be a pulse of duration T.
When a pulse is applied, because of stray capacitances, collector current will not reach
the steady state value instantaneously. To know exactly when the device switches into the
ON state and also into the OFF state we define the following switching times of the
transistor
Breakdown Voltages:
In a transistor switch, the voltage change which occurs at the collector with switching is
nominally equal to the collector supply voltage. Since this voltage change will be used to
operate other circuits and devices VCC should be made as large as possible. However, by
increasing the value of VCC, the reverse- bias voltage on the collector base diode may
become so large that avalanche breakdown may occur in the collector diode. The leakage
current IC0 will now become MIC0 where M is the avalanche multiplication factor. M
depends on VCB.
MULTIVIBRATORS
Multivibrators are cross-coupled two-stage regenerative amplifiers acting as switching
circuits . Multivibrators are broadly classified as
1. Bistable multi or binary or Flip-Flop
2. Monostable multi or One shot multi or univibrator
3. Astble multi or Freerunning multi
Multivibrators are extensively used in digital and switching applications. A bistable
multivibrator remains in one of the stable states until we are asked to change. Hence this
circuit is essentially used as a memory element in digital circuits..
A monostable multi has only one stable state and one quasi-stable state. Initially the multi
is in stable state. After the application of a trigger, the multi goes into the quasistable
state and stays there for a finite time and will return back to the initial stable state. Such a
circuit is used as a gate.
Other type of multivibrator is an Astable multi which has two quasistable states. This
means that change of state occurs in the multi simultaneously. So, the output of this multi
is a squarewave. The output of an astable multi is normally used as a clock signal in
digital circuits.
Methods of triggering a binary:
To change the binary from one stable state to the other, a pulse of short duration
with sufficient amplitude (called trigger) of proper polarity should be applied at the
input(output) of an active device the circuit. The trigger can be a dc trigger or it can be a
pulse trigger. There are two triggering methods to change the state of multivibrator.
1.Unsymmetrical triggering
2.Symmetrical triggering.