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I am designing a visible light communication receiver that uses LMH6629 is the transimpedance. I did a (Multisim) simulation of the circuit on page 28 to confirm the 200MHz bandwidth indicated on that page but my simulation indicates a much lesser bandwith (~20MHz) . Below is the frequency response of the circuit, the Multisim model, and the snapshot of the circuit. Can you please help me figure out if I am doing something wrong ?
Hello Samir Cherian.
Thank you for your helpful quick reply.
You are right about overshoots in the transient response of the circuit. I adjusted C1 and it produces very good results (no peaking) . I thought ~20MHz was the "practical" bandwidth of the circuit since that was the point where the amplitude stopped its flatness. Beyond that point the transient response of the circuit was pretty bad.
After C1 adjustment though, the circuit was overall good up until for signals that are 80+MHz fast. Below is a transient response for 150MHz signal. Seems this time there is about 2ns difference at when they both peak (input signal leads). Also for the first few ps, the circuit looks unstable. Do you have any thought on what's possible causing this?
New AC signal Analysis,
Thank you
Hello Festus,
The time-difference between the signals is because of the limited bandwidth and open loop gain of the amplifier. With regards to the initial non linearity, i am not entirely sure what could be causing that. Couple of thoughts are:
1. Simulation tool trying to find the correct bias point.
2. Insufficient phase-margin in the circuit causing transient issues. This is probably less likely given the flatness of your amplitude response.
A good check for stabiilty/phase margin is to apply a single square pulse with very fast rise/fall time and check the circuits response to it. If there is a lot of overshoot/ringing then this could indicate insufficient phase margin. Can you please check that as well. If the response comes back clean then I think you are okay and can disregard the initiial non-linearity in the sinusoidal response.
-Samir
Samir,
With a 1ps rise/fall time150MHz input signal, there aren't lots of overshoots/undershoots except a tiny overshoot at the beginning (see the graph below). However this affect the output Vpp (decreases) but overall the circuit is stable around 150MHz.. By the way, would this also be applicable (using a fast square input signal) when stability/phase margin at small frequencies though?
Samir,
Thank you for your patience with me and your continuous support on the issue. I have a small question that goes like this: If you perform AC signal analysis of the circuit and it shows a flat response over a range of some frequencies [0-150MHz] (see Figure 1 below) and then, you perform a transient analysis on the same circuit using an input sinusoidal signal with a 100MHz frequency and the response shows clipped amplitude (see figure 2 below) , doesn't that itself somehow contradictory since the AC signal analysis previously stated that your amplitude will not be affected ?
Figure A:
Figure B:
Thank you,
S. Festus-
Oh I see. I think I understand why it happened like that. But, let's say you run a test and this happens:
In the image below (fig 1), I have 1ps risetime 130MHz square wave signal as input. I am expecting, based on BW = 0.35/ Rs , to get an output signal with a rise time close to 2.69ns theoretically. My actual output rise time is around 2.6ns which is pretty good on that end, but still the signal looks "triangular shaped" which is way off from the original square wave input we had. Does that suggest insufficient bandwidth of the circuit? or Does that mean I am pushing the bandwidth envelope too far? because If I lower the frequency to 50MHz but keep the same rise time (1ps), the output resembles more the original square wave - a little distorted but still looks better than the triangular shaped one , but also with slower rise time (3.7ns) (fig2)
What would you consider the practical bandwidth of this circuit (The AC signal analysis shows small attenuation at 130MHz)?
Figure 1:
Figure 2: