Bode plot

; the straight-line approximations are labeled "Bode pole"; phase varies from 90° at low frequencies (due to the contribution of the numerator, which is 90° at all frequencies) to 0° at high frequencies (where the phase contribution of the denominator is −90° and cancels the contribution of the numerator).] A Bode plot, named after Hendrik Wade Bode, is usually a combination of a Bode magnitude plot and Bode phase plot:

A Bode magnitude plot is a graph of log magnitude versus frequency, plotted with a log-frequency axis, to show the transfer function or frequency response of a linear, time-invariant system.

The magnitude axis of the Bode plot is usually expressed as decibels, that is, 20 times the common logarithm of the amplitude gain. With the magnitude gain being logarithmic, Bode plots make multiplication of magnitudes a simple matter of adding distances on the graph (in decibels), since

: $log(a\; cdot\; b)\; =\; log(a)\; +\; log(b).$

A Bode phase plot is a graph of phase versus frequency, also plotted on a log-frequency axis, usually used in conjunction with the magnitude plot, to evaluate how much a frequency will be phase-shifted. For example a signal described by: "A"sin(ω"t") may be attenuated but also phase-shifted. If the system attenuates it by a factor "x" and phase shifts it by −Φ the signal out of the system will be ("A"/"x") sin(ω"t" − Φ). The phase shift Φ is generally a function of frequency.

Phase can also be added directly from the graphical values, a fact that is mathematically clear when phase is seen as the imaginary part of the complex logarithm of a complex gain.

In Figure 1(a), the Bode plots are shown for the one-pole highpass filter function:

::$mathrm\{T\_\{High(f)\; =\; frac\; \{j\; f/\; f\_1\}\; \{1\; +\; j\; f/f\_1\}\; ,$

where "f" is the frequency in Hz, and "f"₁ is the pole position in Hz, "f"₁ = 100 Hz in the figure. Using the rules for complex numbers, the magnitude of this function is

::$mid\; mathrm\{T\_\{High(f)\; mid\; =\; frac\; \{\; f/f\_1\; \}\; \{\; sqrt\{\; 1\; +\; (f/f\_1)^2\; ,$

while the phase is:

::$varphi\_\{T\_\{High\; =\; 90^circ\; -\; an^\{-1\}\; (f/f\_1).$

Care must be taken that the inverse tangent is set up to return "degrees", not radians. On the Bode magnitude plot, decibels are used, and the plotted magnitude is:

:$20log\_\{10\}\; mid\; mathrm\{T\_\{High(f)\; mid\; =20log\_\{10\}\; left(\; f/f\_1\; ight)$ :::::::  $-20\; log\_\{10\}\; left(\; sqrt\{\; 1\; +\; (f/f\_1)^2\; \}\; ight).$ In Figure 1(b), the Bode plots are shown for the one-pole lowpass filter function:

::$mathrm\{\; T\_\{Low\; (f)\; =\; frac\; \{1\}\; \{1\; +\; j\; f/f\_1\}.$

Also shown in Figure 1(a) and 1(b) are the straight-line approximations to the Bode plots that are used in hand analysis, and described later.

The magnitude and phase Bode plots can seldom be changed independently of each other — changing the amplitude response of the system will most likely change the phase characteristics and vice versa. For minimum-phase systems the phase and amplitude characteristics can be obtained from each other with the use of the Hilbert transform.

If the transfer function is a rational function with real poles and zeros, then the Bode plot can be approximated with straight lines. These asymptotic approximations are called straight line Bode plots or uncorrected Bode plots and are useful because they can be drawn by hand following a few simple rules. Simple plots can even be predicted without drawing them.

The approximation can be taken further by "correcting" the value at each cutoff frequency. The plot is then called a corrected Bode plot.

Rules for hand-made Bode plot

The premise of a Bode plot is that one can consider the log of a function in the form::$f(x)\; =\; A\; prod\; (x\; +\; c\_n)^\{a\_n\}$

as a sum of the logs of its poles and zeros::$log(f(x))\; =\; log(A)\; +\; sum\; a\_n\; log(x\; +\; c\_n).$

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 the $20\; log\_\{10\}(X)$ version. Given a transfer function in the form

:$H(s)\; =\; A\; prod\; frac\{(s\; +\; x\_n)^\{a\_n\{(s\; +\; y\_n)^\{b\_n$

where $x\_n$ and $y\_n$ are constants, $s\; =\; jomega$, $a\_n,\; b\_n\; >\; 0$, and "H" is the transfer function:

* at every value of s where $omega\; =\; x\_n$ (a zero), increase the slope of the line by $20\; cdot\; a\_n\; dB$ per decade.
* at every value of s where $omega\; =\; y\_n$ (a pole), decrease the slope of the line by $20\; cdot\; b\_n\; dB$ per decade.
* The initial value of the graph depends on the boundaries. The initial point is found by putting the initial angular frequency "ω" into the function and finding |"H"("j"ω)|.
* The initial slope of the function at the initial value depends on the number and order of zeros and poles that are at values below the initial value, and are found using the first two rules.

To handle irreducible 2nd order polynomials, $ax^2\; +\; bx\; +\; c$ can, in many cases, be approximated as $(sqrt\{a\}x\; +\; sqrt\{c\})^2$.

Note that zeros and poles happen when ω is "equal to" a certain $x\_n$ or $y\_n$. This is because the function in question is the magnitude of H(jω), and since it is a complex function, $|H(jomega)|\; =\; sqrt\{H\; cdot\; H^*\; \}$. Thus at any place where there is a zero or pole involving the term $(s\; +\; x\_n)$, the magnitude of that term is $sqrt\{(x\_n\; +\; jomega)\; cdot\; (x\_n\; -\; jomega)\}=\; sqrt\{x\_n^2+omega^2\}$.

Corrected amplitude plot

To correct a straight-line amplitude plot:

* at every zero, put a point $3\; cdot\; a\_n\; mathrm\{dB\}$ above the line,
* at every pole, put a point $3\; cdot\; b\_n\; mathrm\{dB\}$ below the line,
* draw a smooth curve through those points using the straight lines as asymptotes (lines which the curve approaches).

Note that this correction method does not incorporate how to handle complex values of $x\_n$ or $y\_n$. 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:

:$H(s)\; =\; A\; prod\; frac\{(s\; +\; x\_n)^\{a\_n\{(s\; +\; y\_n)^\{b\_n$

the idea is to draw separate plots for each pole and zero, then add them up. The actual phase curve is given by $-mathrm\{arctan\}(mathrm\{Im\}\; [H(s)]\; /\; mathrm\{Re\}\; [H(s)]\; )$.

To draw the phase plot, for each pole and zero:

* if A is positive, start line (with zero slope) at 0 degrees
* if A is negative, start line (with zero slope) at 180 degrees
* at every $omega\; =\; x\_n$ (for stable zeros – ), increase the slope by $45\; cdot\; a\_n$ degrees per decade, beginning one decade before $omega\; =\; x\_n$ (E.g.: $frac\{x\_n\}\{10\}$)
* at every $omega\; =\; y\_n$ (for stable poles – ), decrease the slope by $45\; cdot\; b\_n$ degrees per decade, beginning one decade before $omega\; =\; y\_n$ (E.g.: $frac\{y\_n\}\{10\}$)
* "unstable" (right half plane) poles and zeros ($Re(s)\; >\; 0$) have opposite behavior
* flatten the slope again when the phase has changed by $90\; cdot\; a\_n$ degrees (for a zero) or $90\; cdot\; b\_n$ degrees (for a pole),
* After plotting one line for each pole or zero, add the lines together to obtain the final phase plot; that is, the final phase plot is the superposition of each earlier phase plot.

Example

A passive (unity pass band gain) lowpass RC filter, for instance has the following transfer function expressed in the frequency domain:

:$H(jf)\; =\; frac\{1\}\{1+j2pi\; f\; R\; C\}$

From the transfer function it can be determined that the cutoff frequency point "f"_c (in hertz) is at the frequency :$f\_mathrm\{c\}\; =\; \{1\; over\; \{2pi\; RC$:or (equivalently) at:$omega\_mathrm\{c\}\; =\; \{1\; over\; \{RC$ where $omega\_mathrm\{c\}=2pi\; f\_mathrm\{c\}$ is the angular cutoff frequency in radians per second.

The transfer function in terms of the angular frequencies becomes::$H(jomega)\; =\; \{1\; over\; 1+j\{omega\; over$omega_mathrm{c}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 $A\_mathrm\{vdB\}$::$A\_mathrm\{vdB\}\; =\; 20\; log|H(jomega)|\; =\; 20\; log\; \{1\; over\; left|1+j\{omega\; over$omega_mathrm{c ight :::$\{\}\; =\; -\; 20log\; left|1+j\{omega\; over$omega_mathrm{c ight| = -10log{left [1 + frac{omega^2}{omega_mathrm{c}^2} ight] }

when plotted versus input frequency $omega$ on a logarithmic scale, can be approximated by two lines and it forms the asymptotic (approximate) magnitude Bode plot of the transfer function:
* for angular frequencies below $omega\_mathrm\{c\}$ it is a horizontal line at 0 dB since at low frequencies the $\{omega\; over\; \{omega\_mathrm\{c\}$ term is small and can be neglected, making the decibel gain equation above equal to zero,
* for angular frequencies above $omega\_mathrm\{c\}$ it is a line with a slope of −20 dB per decade since at high frequencies the $\{omega\; over\; \{omega\_mathrm\{c\}$ term dominates and the decibel gain expression above simplifies to $-20\; log\; \{omega\; over\; \{omega\_mathrm\{c\}$ which is a straight line with a slope of −20 dB per decade.

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

: $varphi\; =\; -\; an^\{-1\}\{omega\; over\; \{omega\_mathrm\{c\}$

versus $omega$, where $omega$ and $omega\_mathrm\{c\}$ are the input and cutoff angular frequencies respectively. For input frequencies much lower than corner, the ratio $\{omega\; over\; \{omega\_mathrm\{c\}$ 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 $omega\; =omega\_mathrm\{c\}$. 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 $\{omega\; over\; \{omega\_mathrm\{c\}$. 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 $omega\_mathrm\{c\}$.

An example with pole and zero

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 a 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 A_FB 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. [ Ordinarily, as frequency increases the magnitude of the gain drops and the phase becomes more negative, although these are only trends and may be reversed in particular frequency ranges. Unusual gain behavior can render the concepts of gain and phase margin inapplicable. Then other methods such as the Nyquist plot have to be used to assess stability.] 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 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"_0dB, 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, | "a" + "j" "b" | = [ "a"² + "b"²] ^1/2 ), and frequency "f"_0dB 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 |β"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: 20 log₁₀( |β"A"_OL|₁₈₀) = 20 log₁₀( |"A"_OL|₁₈₀) − 20 log₁₀( 1 / β ).

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"_0dB. Using this frequency, the Bode phase plot finds the phase of β"A"_OL. If the phase of β"A"_OL( "f"_0dB) > −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"_0dB 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"_0dB < "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.cite book
author=Thomas H. Lee
title=The design of CMOS radio-frequency integrated circuits
page=§14.6 pp. 451-453
year= 2004
edition=Second Edition
publisher=Cambridge University Press
location=Cambridge UK
isbn=0-521-83539-9
url=http://worldcat.org/isbn/0-521-83539-9] cite book
author=William S Levine
title=The control handbook: the electrical engineering handbook series
page=§10.1 p. 163
year= 1996
edition=Second Edition
publisher=CRC Press/IEEE Press
location=Boca Raton FL
isbn=0849385709
url=http://books.google.com/books?id=2WQP5JGaJOgC&pg=RA1-PA163&lpg=RA1-PA163&dq=stability+%22minimum+phase%22&source=web&ots=P3fFTcyfzM&sig=ad5DJ7EvVm6In_zhI0MlF_6vHDA]

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"_0dB. 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"_0dB is to look where the feedback gain intersects the open-loop gain. (Frequency "f"_0dB is needed later to find the phase margin.)

Near this crossover of the two gains at "f"_0dB, 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"_0dB, 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"_0dB and the phase-flip frequency "f"₁₈₀ are very nearly equal in this amplifier, "f"₁₈₀ ≈ "f"_0dB ≈ 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"_0dB = 1 kHz. Notice that the peak in the gain "A"_FB near "f"_0dB is almost gone. [The critical amount of feedback where the peak in the gain "just" disappears altogether is the "maximally flat" or Butterworth design.] cite book
author=Willy M C Sansen
title=Analog design essentials
page=§0517-§0527 pp. 157-163
year= 2006
publisher=Springer
location=Dordrecht, The Netherlands
isbn=0-387-25746-2
url=http://worldcat.org/isbn/0-387-25746-2] Figure 9 is the phase plot. Using the value of "f"_0dB = 1 kHz found above from the magnitude plot of Figure 8, the open-loop phase at "f"_0dB 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 frequency where the open-loop gain flips sign "f"₁₈₀ does not change with a change in feedback factor; it is a property of the open-loop gain. The value of the gain at "f"₁₈₀ also does not change with a change in β. Therefore, we could use the previous values from Figures 6 and 7. However, for clarity the procedure is described using only Figures 8 and 9.] ). 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.

As an aside, it should be noted that 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.cite book
author=Willy M C Sansen
title=§0526 p. 162
isbn=0-387-25746-2
url=http://worldcat.org/isbn/0-387-25746-2] 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. 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 usage of applications for plotting Bode diagrams for given transfer functions helps better understanding and faster getting of results (see external links).

References and notes

See also

* Nyquist plot
* Nichols plot
* Analog signal processing
* Phase margin

External links

* [http://jegyzet.sth.sze.hu/ftp/!BSc/Szabalyozastechnika/BodePl.pdf Detailed explanation of Bode Plots]
* [http://www.facstaff.bucknell.edu/mastascu/eControlHTML/Freq/Freq5.html Explanation of Bode plots with movies and examples]
* [http://www.swarthmore.edu/NatSci/echeeve1/Ref/LPSA/Bode/BodeHow.html How to draw piecewise asymptotic Bode plots]
* [http://lims.mech.northwestern.edu/~lynch/courses/ME391/2003/bodesketching.pdf Summarized drawing rules] (PDF)
* [http://www.uwm.edu/People/msw/BodePlot/ Bode plot applet] - Accepts transfer function coefficients as input, and calculates magnitude and phase response
* [http://mickpc.pbwiki.com/index.php?wiki=BodePlot Bode Plotting on the HP49]
* [http://www.abc.chemistry.bsu.by/vi/fit.htm Circuit analysis in electrochemistry]
* [http://www.en-genius.net/includes/files/acqt_013105.pdf Tim Green: "Operational amplifier stability"] Includes some Bode plot introduction
* [http://www.bode.cg.yu Bode Plotter] Drag and Drop Bode diagram plotting tool by grafical definition of poles and zeros on polar diagram.
* Gnuplot code for generating Bode plot: