FrequencyResponseofFirstOrder
RC
Circuits
11.21
no amplitude distortion). The entire input waveshape will get bodily shifted in time-axis by a definite
delay and will appear as output in that case. Therefore, either zero time delay for all frequencies or
constant time delay for all frequencies will prevent phase distortion. A constant time delay implies that
the phase delay must be a
linear function of
w
.
The conditions to be satisfied by a circuit such that there is no waveshape distortion
when a signal passes through it must be evident now – its frequency response must
haveagainthatisflatwith
w
andaphasewhichiseitherzeroorlinearon
w
,
i.e.,
ofthe
form
f
=
-
k
w
where
k
isarealnumber.
Obviously only a memoryless circuit can satisfy this. Hence a circuit which contains at least
one inductor or capacitor will cause waveshape distortion in general. Similarly, we conclude that a
memoryless circuit cannot function as a
filter; we will need inductors and capacitors for that.
We observe that, in the example we analysed in this section, the input contained three sinusoids of
1 rad/s, 3 rad/s and 5 rad/s and the output contained exactly three sinusoidal components with the
same frequencies as in the input. In short, the circuit did not change the frequency of sinusoids.
Neither did it generate a sinusoid with a frequency that was not there in the input. This, in fact, is
a property of any lumped linear time-invariant (LLTI) system. They can only scale, differentiate or
integrate signals. And these three mathematical operations cannot produce a sinusoid with a frequency
that is different from that of input.
Therefore, a single frequency sinusoid cannot suffer waveshape distortion in passing
throughalineartime-invariantcircuit.
However, a non-linear circuit can change the waveshape of a single frequency sinusoid. Apply
about 10mV of 1kHz sinusoidal voltage to a 741 Operational Amplifier non-inverting pin after
grounding the inverting pin. The Operational Amplifier is in the open loop and its large gain results
in output getting saturated. We will observe a waveform that is almost a square wave at the output.
That is
non-linear distortion. The waveshape distortion we observed in the example in this stion was
not due to non-linearity. It occurred due to differential treatment experienced by various sinusoidal
components in a mixture of sinusoids when they went through the circuit. The distortion which occurs
due to frequency response of a linear circuit is termed as
linear distortion in order to distinguish this
kind of distortion from distortion due to non-linearity.
Amplitude Distortion and
Phase Distortion are
the two inseparable components of
Linear Distortion.
One should not be under the impression that the Series
RC Circuit can function only as a low-
pass filter. In fact, the kind of filter realised by a given circuit will strongly depend on where exactly
is the input applied and where exactly is the output taken. A Series
RC Circuit excited by a voltage
source at the input with output taken across the capacitor is a low-pass filter. The same circuit with the
same excitation but with the output taken across the resistor is a high-pass filter that passes the high
frequency sinusoids to the output and curtails the low frequency components including DC.
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