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Topspice simulation - experts advice needed - RF Cafe Forums
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| balajisu
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Post subject: Topspice simulation - experts advice needed
Posted: Wed Aug 23, 2006 1:50 pm
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| Colonel |
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Joined: Wed Jul 06, 2005
5:54 pm Posts: 31 Location: Germantown,MD
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Simulating a Voltage Regulator spice model - Performing
Transient analysis with Voltage regulator loaded
with current source set with 20KHx pulse input.
Voltage regulator is a fixed 5V regulator. When
I measure the regulator output after the simulation
it seems to be stable. When i load the regulator
with a LPF and then run a similar simulation I see
the regulator output to be ringing? can
anyone throw some light on whats happening in this
situation? Any help in this regard would
be much appreciated Thanks
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fred47 |
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Post subject: TopSPICE & VR output ringing
Posted: Mon Aug 28, 2006 11:03 pm
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| General |
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Joined: Wed Feb 22, 2006
3:51 pm Posts: 119 |
Hi! There are two common structures for voltage
regulators. In one, the output comes from the emitter
of a transistor (NPN for a positive voltage, PNP
for a negative voltage). In the other, the output
comes from the collector of a transistor (PNP for
a positive voltage, NPN for a negative voltage).
The first one is more widely used, but the second
is the structure of choice for LDO's (Low DropOut
voltage regulators). These two behave rather differently,
because feedback is more local for the emitter output,
and more global (read "slower") for the collector
output. So it's important to know which style
of regulator you have. BJT's are often the
active element of choice, due to die size considerations.
Also, just as in RF, the input impedance
of your LPF (the one whose connection leads to ringing)
is important - is it capacitive or inductive at
the ringing frequency? The oscillating emitter
follower is a famous and rather widespread problem,
so the simulator is probably not lying. So
let us hear more from you! Good Luck,
Fred
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balajisu |
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Post subject:
Posted: Tue Aug 29, 2006 9:43 am
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| Colonel |
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Joined: Wed Jul 06, 2005
5:54 pm Posts: 31 Location: Germantown,MD
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Thanks Fred for all the inputs. The regulator
model that I have been using is a UA7805 fixed positive
voltage regulator from TI. I believe it is a emitter
follower. Not sure about the input impedance of
the filter. Shall get back to you on this one.
Can you throw some light on why oscillations
occur in a emitter follower? Thanks for your
time Best regards Balaji
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balajisu |
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Post subject:
Posted: Tue Aug 29, 2006 11:18 am
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| Colonel |
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Joined: Wed Jul 06, 2005
5:54 pm Posts: 31 Location: Germantown,MD
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Fred The oscillation frequency of the ripples
were at 25 - 30 MHz. I looked at the S11 for
the Filter ( 1MHz to 100 MHz), At the lower end
it was inductive and moved over to capacitive. @
the oscillation frequency its looks like an open.
Its more like a crossover point from being inductive
to capacitive. Looking forward for your
feedback regards Balaji
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fred47 |
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Post subject: Emitter Follower Oscillation
Posted: Fri Oct 20, 2006 2:57 pm
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| General |
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Joined: Wed Feb 22, 2006
3:51 pm Posts: 119 |
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Posted 11/12/2012
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