Bode plot

In electrical engineering and control theory, a Bode plot is a graph of the frequency response of a system. It is usually a combination of a Bode magnitude plot, expressing the magnitude (usually in decibels) of the frequency response, and a Bode phase plot, expressing the phase shift.
As originally conceived by Hendrik Wade Bode in the 1930s, the plot is an asymptotic approximation of the frequency response, using straight line segments.
Overview.
Among his several important contributions to circuit theory and control theory, engineer Hendrik Wade Bode, while working at Bell Labs in the 1930s, devised a simple but accurate method for graphing gain and phase-shift plots. These bear his name, "Bode gain plot" and "Bode phase plot". "Bode" is often pronounced although the Dutch pronunciation is Bo-duh. ().
Bode was faced with the problem of designing stable amplifiers with feedback for use in telephone networks. He developed the graphical design technique of the Bode plots to show the gain margin and phase margin required to maintain stability under variations in circuit characteristics caused during manufacture or during operation. The principles developed were applied to design problems of servomechanisms and other feedback control systems. The Bode plot is an example of analysis in the frequency domain.
Definition.
The Bode plot for a linear, time-invariant system with transfer function formula_1 (formula_2 being the complex frequency in the Laplace domain) consists of a magnitude plot and a phase plot.
The Bode magnitude plot is the graph of the function formula_3 of frequency formula_4 (with formula_5 being the imaginary unit). The formula_4-axis of the magnitude plot is logarithmic and the magnitude is given in decibels, i.e., a value for the magnitude formula_7 is plotted on the axis at formula_8.
The Bode phase plot is the graph of the phase, commonly expressed in degrees, of the transfer function formula_9 as a function of formula_10. The phase is plotted on the same logarithmic formula_10-axis as the magnitude plot, but the value for the phase is plotted on a linear vertical axis.
Frequency response.
This section illustrates that a Bode Plot is a visualization of the frequency response of a system.
Consider a linear, time-invariant system with transfer function formula_12. Assume that the system is subject to a sinusoidal input with frequency formula_10,
that is applied persistently, i.e. from a time formula_15 to a time formula_16. The response will be of the form
i.e., also a sinusoidal signal with amplitude formula_18 shifted in phase with respect to the input by a phase formula_19.
It can be shown that the magnitude of the response is
and that the phase shift is
A sketch for the proof of these equations is given in the appendix.
In summary, subjected to an input with frequency formula_10 the system responds at the same frequency with an output that is amplified by a factor formula_21 and phase-shifted by formula_22. These quantities, thus, characterize the frequency response and are shown in the Bode plot.
Rules for handmade Bode plot.
For many practical problems, the detailed Bode plots can be approximated with straight-line segments that are asymptotes of the precise response. The effect of each of the terms of a multiple element transfer function can be approximated by a set of straight lines on a Bode plot. This allows a graphical solution of the overall frequency response function. Before widespread availability of digital computers, graphical methods were extensively used to reduce the need for tedious calculation; a graphical solution could be used to identify feasible ranges of parameters for a new design.
The premise of a Bode plot is that one can consider the log of a function in the form:
as a sum of the logs of its zeros and poles:
This idea is used explicitly in the method for drawing phase diagrams. The method for drawing amplitude plots implicitly uses this idea, but since the log of the amplitude of each pole or zero always starts at zero and only has one asymptote change (the straight lines), the method can be simplified.
Straight-line amplitude plot.
Amplitude decibels is usually done using formula_25 to define decibels. Given a transfer function in the form
where formula_27 and formula_28 are constants, formula_29, formula_30, and formula_31 is the transfer function:
To handle irreducible 2nd order polynomials, formula_38 can, in many cases, be approximated as formula_39.
Note that zeros and poles happen when formula_10 is "equal to" a certain formula_27 or formula_28. This is because the function in question is the magnitude of formula_43, and since it is a complex function, formula_44. Thus at any place where there is a zero or pole involving the term formula_45, the magnitude of that term is formula_46.
Corrected amplitude plot.
To correct a straight-line amplitude plot:
Note that this correction method does not incorporate how to handle complex values of formula_27 or formula_28. In the case of an irreducible polynomial, the best way to correct the plot is to actually calculate the magnitude of the transfer function at the pole or zero corresponding to the irreducible polynomial, and put that dot over or under the line at that pole or zero.
Straight-line phase plot.
Given a transfer function in the same form as above:
the idea is to draw separate plots for each pole and zero, then add them up. The actual phase curve is given by
formula_52.
To draw the phase plot, for each pole and zero:
Example.
To create a straight-line plot for a first-order (one-pole) lowpass filter, one considers the transfer function in terms of the angular frequency:
The above equation is the normalized form of the transfer function. The Bode plot is shown in Figure 1(b) above, and construction of the straight-line approximation is discussed next.
Magnitude plot.
The magnitude (in decibels) of the transfer function above, (normalized and converted to angular frequency form), given by the decibel gain expression formula_71:
then plotted versus input frequency formula_10 on a logarithmic scale, can be approximated by two lines and it forms the asymptotic (approximate) magnitude Bode plot of the transfer function:
These two lines meet at the corner frequency. From the plot, it can be seen that for frequencies well below the corner frequency, the circuit has an attenuation of 0 dB, corresponding to a unity pass band gain, i.e. the amplitude of the filter output equals the amplitude of the input. Frequencies above the corner frequency are attenuated – the higher the frequency, the higher the attenuation.
Phase plot.
The phase Bode plot is obtained by plotting the phase angle of the transfer function given by
versus formula_10, where formula_10 and formula_74 are the input and cutoff angular frequencies respectively. For input frequencies much lower than corner, the ratio formula_84 is small and therefore the phase angle is close to zero. As the ratio increases the absolute value of the phase increases and becomes –45 degrees when formula_85. As the ratio increases for input frequencies much greater than the corner frequency, the phase angle asymptotically approaches −90 degrees. The frequency scale for the phase plot is logarithmic.
Normalized plot.
The horizontal frequency axis, in both the magnitude and phase plots, can be replaced by the normalized (nondimensional) frequency ratio formula_75. In such a case the plot is said to be normalized and units of the frequencies are no longer used since all input frequencies are now expressed as multiples of the cutoff frequency formula_74.
An example with zero and pole.
Figures 2-5 further illustrate construction of Bode plots. This example with both a pole and a zero shows how to use superposition. To begin, the components are presented separately.
Figure 2 shows the Bode magnitude plot for a zero and a low-pass pole, and compares the two with the Bode straight line plots. The straight-line plots are horizontal up to the pole (zero) location and then drop (rise) at 20 dB/decade. The second Figure 3 does the same for the phase. The phase plots are horizontal up to a frequency factor of ten below the pole (zero) location and then drop (rise) at 45°/decade until the frequency is ten times higher than the pole (zero) location. The plots then are again horizontal at higher frequencies at a final, total phase change of 90°.
Figure 4 and Figure 5 show how superposition (simple addition) of a pole and zero plot is done. The Bode straight line plots again are compared with the exact plots. The zero has been moved to higher frequency than the pole to make a more interesting example. Notice in Figure 4 that the 20 dB/decade drop of the pole is arrested by the 20 dB/decade rise of the zero resulting in a horizontal magnitude plot for frequencies above the zero location. Notice in Figure 5 in the phase plot that the straight-line approximation is pretty approximate in the region where both pole and zero affect the phase. Notice also in Figure 5 that the range of frequencies where the phase changes in the straight line plot is limited to frequencies a factor of ten above and below the pole (zero) location. Where the phase of the pole and the zero both are present, the straight-line phase plot is horizontal because the 45°/decade drop of the pole is arrested by the overlapping 45°/decade rise of the zero in the limited range of frequencies where both are active contributors to the phase.
Gain margin and phase margin.
Bode plots are used to assess the stability of negative feedback amplifiers by finding the gain and phase margins of an amplifier. The notion of gain and phase margin is based upon the gain expression for a negative feedback amplifier given by
where AFB is the gain of the amplifier with feedback (the closed-loop gain), β is the feedback factor and "A"OL is the gain without feedback (the open-loop gain). The gain "A"OL is a complex function of frequency, with both magnitude and phase. Examination of this relation shows the possibility of infinite gain (interpreted as instability) if the product β"A"OL = −1. (That is, the magnitude of β"A"OL is unity and its phase is −180°, the so-called Barkhausen stability criterion). Bode plots are used to determine just how close an amplifier comes to satisfying this condition.
Key to this determination are two frequencies. The first, labeled here as "f"180, is the frequency where the open-loop gain flips sign. The second, labeled here "f"0 dB, is the frequency where the magnitude of the product | β "A"OL | = 1 (in dB, magnitude 1 is 0 dB). That is, frequency "f"180 is determined by the condition:
where vertical bars denote the magnitude of a complex number (for example, formula_90), and frequency "f"0 dB is determined by the condition:
One measure of proximity to instability is the gain margin. The Bode phase plot locates the frequency where the phase of β"A"OL reaches −180°, denoted here as frequency "f"180. Using this frequency, the Bode magnitude plot finds the magnitude of β"A"OL. If |β"A"OL|180 = 1, the amplifier is unstable, as mentioned. If "A"OL|180 < 1, instability does not occur, and the separation in dB of the magnitude of |β"A"OL|180 from |β"A"OL| = 1 is called the "gain margin". Because a magnitude of one is 0 dB, the gain margin is simply one of the equivalent forms: formula_92.
Another equivalent measure of proximity to instability is the phase margin. The Bode magnitude plot locates the frequency where the magnitude of |β"A"OL| reaches unity, denoted here as frequency "f"0 dB. Using this frequency, the Bode phase plot finds the phase of β"A"OL. If the phase of β"A"OL( "f"0 dB) > −180°, the instability condition cannot be met at any frequency (because its magnitude is going to be < 1 when "f = f"180), and the distance of the phase at "f"0 dB in degrees above −180° is called the "phase margin".
If a simple "yes" or "no" on the stability issue is all that is needed, the amplifier is stable if "f"0 dB < "f"180. This criterion is sufficient to predict stability only for amplifiers satisfying some restrictions on their pole and zero positions (minimum phase systems). Although these restrictions usually are met, if they are not another method must be used, such as the Nyquist plot.
Optimal gain and phase margins may be computed using Nevanlinna–Pick interpolation theory.
Examples using Bode plots.
Figures 6 and 7 illustrate the gain behavior and terminology. For a three-pole amplifier, Figure 6 compares the Bode plot for the gain without feedback (the "open-loop" gain) "A"OL with the gain with feedback "A"FB (the "closed-loop" gain). See negative feedback amplifier for more detail.
In this example, "A"OL = 100 dB at low frequencies, and 1 / β = 58 dB. At low frequencies, "A"FB ≈ 58 dB as well.
Because the open-loop gain "A"OL is plotted and not the product β "A"OL, the condition "A"OL = 1 / β decides "f"0 dB. The feedback gain at low frequencies and for large "A"OL is "A"FB ≈ 1 / β (look at the formula for the feedback gain at the beginning of this section for the case of large gain "A"OL), so an equivalent way to find "f"0 dB is to look where the feedback gain intersects the open-loop gain. (Frequency "f"0 dB is needed later to find the phase margin.)
Near this crossover of the two gains at "f"0 dB, the Barkhausen criteria are almost satisfied in this example, and the feedback amplifier exhibits a massive peak in gain (it would be infinity if β "A"OL = −1). Beyond the unity gain frequency "f"0 dB, the open-loop gain is sufficiently small that "A"FB ≈ "A"OL (examine the formula at the beginning of this section for the case of small "A"OL).
Figure 7 shows the corresponding phase comparison: the phase of the feedback amplifier is nearly zero out to the frequency "f"180 where the open-loop gain has a phase of −180°. In this vicinity, the phase of the feedback amplifier plunges abruptly downward to become almost the same as the phase of the open-loop amplifier. (Recall, "A"FB ≈ "A"OL for small "A"OL.)
Comparing the labeled points in Figure 6 and Figure 7, it is seen that the unity gain frequency "f"0 dB and the phase-flip frequency "f"180 are very nearly equal in this amplifier, "f"180 ≈ "f"0 dB ≈ 3.332 kHz, which means the gain margin and phase margin are nearly zero. The amplifier is borderline stable.
Figures 8 and 9 illustrate the gain margin and phase margin for a different amount of feedback β. The feedback factor is chosen smaller than in Figure 6 or 7, moving the condition | β "A"OL | = 1 to lower frequency. In this example, 1 / β = 77 dB, and at low frequencies "A"FB ≈ 77 dB as well.
Figure 8 shows the gain plot. From Figure 8, the intersection of 1 / β and "A"OL occurs at "f"0 dB = 1 kHz. Notice that the peak in the gain "A"FB near "f"0 dB is almost gone.
Figure 9 is the phase plot. Using the value of "f"0 dB = 1 kHz found above from the magnitude plot of Figure 8, the open-loop phase at "f"0 dB is −135°, which is a phase margin of 45° above −180°.
Using Figure 9, for a phase of −180° the value of "f"180 = 3.332 kHz (the same result as found earlier, of course). The open-loop gain from Figure 8 at "f"180 is 58 dB, and 1 / β = 77 dB, so the gain margin is 19 dB.
Stability is not the sole criterion for amplifier response, and in many applications a more stringent demand than stability is good step response. As a rule of thumb, good step response requires a phase margin of at least 45°, and often a margin of over 70° is advocated, particularly where component variation due to manufacturing tolerances is an issue. See also the discussion of phase margin in the step response article.
Bode plotter.
The Bode plotter is an electronic instrument resembling an oscilloscope, which produces a Bode diagram, or a graph, of a circuit's voltage gain or phase shift plotted against frequency in a feedback control system or a filter. An example of this is shown in Figure 10. It is extremely useful for analyzing and testing filters and the stability of feedback control systems, through the measurement of corner (cutoff) frequencies and gain and phase margins.
This is identical to the function performed by a vector network analyzer, but the network analyzer is typically used at much higher frequencies.
For education/research purposes, plotting Bode diagrams for given transfer functions facilitates better understanding and getting faster results (see external links).
Related plots.
Two related plots that display the same data in different coordinate systems are the Nyquist plot and the Nichols plot. These are parametric plots, with frequency as the input and magnitude and phase of the frequency response as the output. The Nyquist plot displays these in polar coordinates, with magnitude mapping to radius and phase to argument (angle). The Nichols plot displays these in rectangular coordinates, on the log scale.
Appendix.
Proof for the relation to frequency response.
This section shows that the frequency response is given by the magnitude and phase of the transfer function in Eqs.()-().
Slightly changing the requirements for Eqs.()-() one assumes that the input has been applied starting at time formula_93 and one calculates the output in the limit formula_94. In this case, the output is given by the convolution
of the input signal with the inverse Laplace transform of the transfer function formula_96. Assuming that the signal becomes periodic with mean 0 and period T after a while, we can add as many periods as we want to the interval of the integral
Thus, inserting the sinusoidal input signal one obtains
Since formula_96 is a real function this can be written as
The term in brackets is the definition of the Laplace transform of formula_101 at formula_102. Inserting the definition in the form formula_103 one obtains the output signal
stated in Eqs.()-().