Is this correct? How would I simulate the noise gain with Multisim?.What you describe is reasonably well understood. How much of my understanding is correct?.what the minimal distance from the -1 the transfer function is at the frequency where it is closest. How stable your system is depends on how far it stays away from the -1 point, ie. If we hit this -1 point at any frequency then the whole system is unstable. This is the point that we are trying to avoid. If we put it in a feedback loop, where the feedback has a transfer function H(s), then the new transfer function with feedback becomes:įor an amplifier with feedback loop to be stable means the denumerator of this equation must not equal zero, this happens when: Yes it is stable, within the frequency range that you have sweept over. Is the presented front-end stable? If not, what part of the Bode plot tells me about the stability? What value do I need to adjust to stabilize the design?.The period between pulses is 300 ns.įirst of all, V_rev does nothing, it is connected in series with an ideal current source (which has infinite output impedance) and a capacitor (which does not couple DC), hence it might as well be 0V (or connected to gnd). After a delay of 100 ns the current source jumps in 1 ns rise time to a value of 440 uA. The pulsed current source is configured to start at 0 A. Is this correct? How would I simulate the noise gain with Multisim?Īs requested a transient analysis of a pulsed current source. For example, the standard inverting amplifier has a signal gain of -Rf/Ri while it has a noise gain of 1+Rf/Ri, thus, although if Rf/Ri falls we still have a unity amplification of the noise and the noise can be caught in a positive feedback loop.ģ. The reasoning behind using the noise instead of the signal gain (am I using the terms correctly in this context?) is that both gains can be different. What I believe I am missing is that usually the stability is defined in terms of noise gain. When the blue curve hits -180 degrees at about 230 MHz the constructive interference condition is satisfied, however, the magnitude of the frequency response is below unity, therefore, there is no positive feedback that makes the circuit ring.Ģ.
In the phase diagram of the Bode plot, the blue curve describes the difference between the voltage at the inverting input of the op-amp and the output of the op-amp. Is the presented front-end stable? If not, what part of the Bode plot tells me about the stability? What value do I need to adjust to stabilize the design?įrom my understanding, we need constructive interference at the inverting input of the op-amp as well as a gain larger than unity. In Multisim an AC sweep analysis gives the following Bode plot:ġ. I designed a front-end for the S5972 high-speed photodiode.
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