In an amplifier circuit where feedback is present, there is other information contained in the phase curve of the Bode plot that needs to be considered to ensure amplifier stability. More complicated filters with multiple cutoff stages can also be analyzed in a similar way. For an Nth order filter, where N filter stages are cascaded in series, these roll-off and attenuation values would be multiplied by N.įor a passive filter circuit, this basically summarizes the important points in a Bode plot. For the 1st-order low pass filter shown above, the magnitude curve has standard -20 dB/decade rolloff and -3 dB attenuation at the 45 degree phase shift. When looking at these filters, we normally focus on the magnitude curve as this will have standard roll-off values and attenuation at the 45 degree phase shift point. Magnitude and phase (Bode plot) for a 1st-order low pass filter. The same characteristics would be seen for a high pass filter, although the magnitude curve would be reversed. Here, when the phase shift in the Bode plot is 45 degrees, the magnitude curve passes through approximately -3 dB. The phase plot shows how the phase shift develops when the source frequency starts to enter the cutoff region. The image below shows the Bode plot for a 1st-order low pass filter (top: magnitude, bottom: phase). Presence of poles or zeros: These frequencies appear as peaks and valleys in a Bode plot magnitude curve, respectively, and the phase curve may pass through certain points at these frequencies.Īs filters and amplifiers follow similar ideas, it helps to look at these two types of circuits to see what information is contained in the Bode plot phase curve. One can determine this from looking at the magnitude and phase of the Bode plot. Presence of feedback: If feedback is present in the circuit, the circuit may transition to instability. Type of circuit: Filters and amplifiers are similar, but slightly different effects can occur depending on the circuit topology and whether active components are used. The information one finds in a Bode plot depends on a few factors: In short, the frequency response for any LTI system can be summarized using a Bode plot. Important Phase Points in a Bode PlotĪ Bode plot is a simple way to show some important information in the transfer function for a linear time-invariant (LTI) system. To better see what the phase in a Bode plot says about your circuits, let’s analyze these systems briefly and show how you can identify important quantities in Bode plot phase curves. However, an often unconsidered aspect of phase in a Bode plot is its effect on stability, which arises in negative feedback amplifiers. In a filter or amplifier, the phase curve in the Bode plot indicates separation between the input and output signals, which becomes quite important when we consider the role of feedback in a circuit. This simple plot is just the transfer function of a circuit plotted on a logarithmic scale, but the two portions of this plot tell you important information about signal behavior. Among the various tools used to understand reactive circuits, a primary tool is the Bode plot. Reactive circuits need careful analysis to determine how they interact with signals of different frequencies.
The phase is one of two pieces of information shown in a Bode plot, where the output voltage is shifted in time with respect to the input voltage.Īs reactive components, inductors and capacitors induce a phase shift in a filter or amplifier circuit, creating a phase shift that can be seen in a Bode plot. When a phase shift is introduced into a filter or amplifier, the magnitude of the phase shift and attenuation/gain can be determined over a range of frequencies. Reactive components in LTI systems will produce a phase shift between the current and voltage in the component.
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