In electronics, a Bode plot (or Bode diagram) is a plot of the circuit gain in decibels (or phase, in degrees) on a linear-scale -axis (recall decibels are themselves logarithmic values) plotted against frequency on a log-scale -axis. This is the standard way of plotting the frequency response of a circuit.
Gain plot
A first-order low-pass gain plot is given by the below. We can refer to this chart when drawing them out ( is the numerator, is the denominator). Some observations:
- The 20 dB per decade is mainly a result of the decibel scaling (). If our poles/zeros are duplicated (i.e., ) then we get 40 dB per decade. This scales as we continue.
- The contributions are summed up to get an overall Bode plot.
- What this means is that we should build multiple different plots then combine them.
- Why is this? We can take the logarithm of : by log properties multiplication becomes addition and division becomes subtraction.
- Putting everything in standard form (the rightmost format) is easiest to do. This lets us figure out the corner frequency pretty easily and gives us a uniquely easy point of reference for non-constant poles or zeroes (0 dB).
- It’s good practice to start from the left and combine moving right.
- There’s a pole at the corner frequency, which results in the corner of the plot. In practice, the gain is around 3 dB less than the usual flat gain to the left of the corner.
Phase plot
The first-order phase function is given by: We can refer to this plot when drawing phase contributions: