B² Spice

Case Study: Amplifier Max Power Testing in B² Spice

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Measuring PSRR
PSRR stands for Power Supply Rejection Ratio. It is the measure of an amplifier’s ability reject or prevent the power supply noise at its output. It is usually express in dBs, which means that a ratio is implied. The ratio is between a circuit's change at its output relative to the disturbance at the power supply that caused it. In other words, we take the power supply noise’s value and then divide it by the same noise at the output. For example, 2V of noise at the power supply rail might give rise to only 2mV of noise that the output, which would represent a 1000 to 1 reduction in noise, or a 60dB improvement, as PSRR in dB = 20Log(Vps/Vout).

The first problem we face is that the voltage sources used in SPICE are problem free, in that they can source infinite current, exhibit no impedance, and add no noise. The last problem is easily taken care of in B2 A/D Spice.

In the voltage source’s prosperities setup dialog box above, the DC value is set to 45 volts and the “noise” is set to 2 volts at 120Hz. Now when we run a Transient Sweep test with the amplifier’s input shorted to ground, we can measure the amount of signal at 120Hz at the output. We could try to let B2 A/D Spice display the PSRR figure for us by creating a new plot, but it is unlikely to prove as accurate as doing it by hand, as the waveforms might differ in phase or symmetry. And since our goal is the find the worst case failing, some judgment is required in choosing the right data for our formulas. For example, in the graph below, we see that the negative going part of the output’s waveform is greater than the positive going part. Thus, we use the absolute value of the negative trough to divide into the 1 volt peak of noise.