Why connect amplifier common to shield - to reduce oscillations - Page 1
Forum section: electronics / projects

author: agentaero
author_rank: Newbie
timestamp: March 01, 2021, 02:03:38 pm
content: Hi all,I've been reading through "Noise Reduction Techniques In Electronic Systems" by Henry Ott and read through a passage I'm having some difficulty understanding. See the attached for the image snippet in question.The images are showing the parasitic capacitance between the amplifier terminals and shield. In the upper images the claim is made that C1s and C3s make a feedback loop that could result in oscillations. I am seeing this capacitance between output and input as rolling off the frequency response of the amp theoretically making it more stable. In saying that,I must be missing the proper way to view this situation and my supposition is wrong.In the lower image, it shows connecting the shield to the amp common. The passage states that making this connection short circuits C2s and the feedback is eliminated. So with that, I am certain that I don't follow the reasoning.Is anyone able to break this down? How is it a feedback loop is created in the upper two circuits and why might it result in oscillations/instability? Why does short-circuiting C2s eliminate this feedback?Thanks in advance!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2021, 02:14:42 pm
content: Well, are you imagining an inverting or noninverting amplifier, and what magnitude of gain?The catch is, in general, the phase might be anything. Especially since we'll be concerned about these paths at high frequencies, where phase shift is large and the reactance of these paths is small. Or more generally, coupling is large, and the phase shift along the paths might be large as well (resonant shields? or, what if that ground link, in the bottom case, is inductive?).Tim

author: agentaero
author_rank: Newbie
timestamp: March 01, 2021, 02:33:23 pm
content: Hi Tim. Thank you for the reply! Certainly appreciated.I've attached the short paragraph below. The assumption is that the amplifier is high gain.So I'm understanding that we can't really say anything about the phase of the signal since the shielding arrangement can't be known. This could create instability in our loop.But how would the shield link that short circuits C2s break that feedback loop. I can't figure it out.Thanks!!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2021, 03:03:35 pm
content: Well, rearrange it, it's just capacitance at the input, and at the output. Who cares, nothing special. At least, nothing new introduced by the amplifier module itself.And presumably, the source and load are fine with that: either the capacitive load of those cables, or at high frequency, their characteristic impedances. They have to be, regardless of whether the amplifier is added inline or not; it's a separate issue. That's what he's highlighting.Edit: heh, also interesting to note there's no "4". The terminals are numbered 1, 2 and 3. This I think implies the grounds are common to a point, not just a line. This eliminates one route for common mode interference. Presumably, the shield is common to that point as well, so that my earlier mention of inductance is moot -- the shield IS the common point, and we don't have to worry about resonances to it. (Or, at least at frequencies on the order of cable resonances; microwave voodoo like cavity resonances aside.)Tim

author: agentaero
author_rank: Newbie
timestamp: March 01, 2021, 07:04:57 pm
content: That clears it up for me - thank you very much!!And the edit is helpful too.Much appreciated Tim!Best,