> An additional tip
>
> Noise cancels out (partially) in parallel amps, this works especially
> well in preamps where you can hang several amps to an input signal and
> (with series resistors) tie all outputs together
Sorry, I just can't accept this statement in regard to front end
noise. In fact I am sure the opposite is true, as when you parallel
amplifiers, you necessarily reduce the input signal level to each by the
split factor, 3dB for two, and make the noise WORSE by that amount.
>Peter van Hoof wrote:
>
>> An additional tip
>>
>> Noise cancels out (partially) in parallel amps, this works especially
>> well in preamps where you can hang several amps to an input signal and
>> (with series resistors) tie all outputs together
>
> Sorry, I just can't accept this statement in regard to front end
>noise. In fact I am sure the opposite is true, as when you parallel
>amplifiers, you necessarily reduce the input signal level to each by the
>split factor, 3dB for two, and make the noise WORSE by that amount.
>
> Cheers,
> Paul B.
I think you are assuming that the input signal to each preamp would be
halved because the input impedances of the amps are now in parallel. This
is a faulty analysis, though, and would only be true if the input impedance
of the amplifier was comparable to the source impedance of the transducer.
In fact, the amplifier impedance can be _much_ higher than the transducer
impedance.
For example, a piezo transducer at 40 KHz can have an impedance of about
2000 ohms. But a TL082 op-amp in non-inverting mode has 10**12 ohms input
impedance, essentially infinite compared to that measly 2K ohms; putting
additional op-amps in parallel would have very little effect on the signal
that each amp would see.
--BN
----------------------------------------------------------------------------
Country Robot "Modular robot components
69 S. Fremont Ave. # 2 for education and industry"
Pittsburgh, PA 15202
----------------------------------------------------------------------------
On Mon 01 Dec, Peter van Hoof wrote:
>
>> An additional tip
>>
>> Noise cancels out (partially) in parallel amps, this works especially
>> well in preamps where you can hang several amps to an input signal and
>> (with series resistors) tie all outputs together
>
> Sorry, I just can't accept this statement in regard to front end
>noise. In fact I am sure the opposite is true, as when you parallel
>amplifiers, you necessarily reduce the input signal level to each by the
>split factor, 3dB for two, and make the noise WORSE by that amount.
If you use N transistors in parallel in your frontend, then noise will go
down by SQRT(N) -- same as having a transistor of N times the die area.
Frank
------------------------------------------------------------------------
Frank A. Vorstenbosch Phone: +44-181-636 3391
Electronics & Software Engineer or: +44-181-636 3000
Eidos Technologies Ltd., Wimbledon, London Mobile: +44-976-430 569
This is quite untrue if you understand the math behind it
-The output signals from the amps are added together nicely because they are all in phase
-The noise generated in the preamps you can compare to random numbers rangin from positive to negative **add them all up and voila** the average will be less than the peak to peak noise of a single preamp
> An additional tip
>
> Noise cancels out (partially) in parallel amps, this works especially
> well in preamps where you can hang several amps to an input signal and
> (with series resistors) tie all outputs together
Sorry, I just can't accept this statement in regard to front end
noise. In fact I am sure the opposite is true, as when you parallel
amplifiers, you necessarily reduce the input signal level to each by the
split factor, 3dB for two, and make the noise WORSE by that amount.
You go it right! Paralleling transistors in the input stage gives lower
noise because the input parasitic noise resistances (base spreading
resistance, base and emitter bulk resistances and ohmic contacts for
bipolars, related parameters for FETs) are reduced. This is the now well
known, patented secret used in low noise hi-fi IC amps from Signetics
(Dolby) and others.
> ----------
> From: Frank A. Vorstenbosch[SMTP:favKILLspamEIDOS.CO.UK]
> Sent: Monday, December 01, 1997 1:49 AM
> To: .....PICLISTKILLspam.....MITVMA.MIT.EDU
> Subject: Re: Serial Sonar Unit (SSU) - Noise performance
>
> On Mon 01 Dec, Peter van Hoof wrote:
> >
> >> An additional tip
> >>
> >> Noise cancels out (partially) in parallel amps, this works
> especially
> >> well in preamps where you can hang several amps to an input signal
> and
> >> (with series resistors) tie all outputs together
> >
> > Sorry, I just can't accept this statement in regard to front end
> >noise. In fact I am sure the opposite is true, as when you parallel
> >amplifiers, you necessarily reduce the input signal level to each by
> the
> >split factor, 3dB for two, and make the noise WORSE by that amount.
>
> If you use N transistors in parallel in your frontend, then noise will
> go
> down by SQRT(N) -- same as having a transistor of N times the die
> area.
>
> Frank
> ----------------------------------------------------------------------
> --
> Frank A. Vorstenbosch Phone: +44-181-636
> 3391
> Electronics & Software Engineer or: +44-181-636
> 3000
> Eidos Technologies Ltd., Wimbledon, London Mobile: +44-976-430
> 569
>
> If you use N transistors in parallel in your frontend, then noise will
> go down by SQRT(N) -- same as having a transistor of N times the die
> area.
Yes, but if your input is impedance-matched, your input SIGNAL will be
divided by N, so your signal-to-noise ratio will be SQRT(N) WORSE for
the exercise.
You remind me of something I saw somewhere about using power output
devices as front-end amplifiers but I think the advantage of that was
actually in overload capability! Fact is, we DON'T use large die area
devices but rather tiny little GaAsFETs in RF work.
Now, another mail to me personally suggested that the amplifiers in
question were so high in impedance compared to the transducer that
paralleling them would not alter the signal voltage! Fine! If you are
using such a bad impedance match, the MOST effective way to improve
signal-to-noise ratio is stunningly easy - use a step-UP transformer to
correctly match the impedance!
Peter van Hoof writes again:
> This is quite untrue if you understand the math behind it
> -The output signals from the amps are added together nicely because
> they are all in phase
> -The noise generated in the preamps you can compare to random numbers
> rangin from positive to negative **add them all up and voila** the
> average will be less than the peak to peak noise of a single preamp
Yes, Peter, that is the significance of the SQRT(N) in the above
discussion and applies to what happens at the common OUTPUT, but you are
still failing to understand what is happening at the INPUT where you
will be LOSING signal much faster! The term is "signal-to-noise ratio".
> I think you are assuming that the input signal to each preamp would be
> halved because the input impedances of the amps are now in parallel.
> This is a faulty analysis, though, and would only be true if the input
> impedance of the amplifier was comparable to the source impedance of
> the transducer. In fact, the amplifier impedance can be _much_ higher
> than the transducer impedance.
REALLY? Well, in that case, wouldn't it be far more sensible to match
the impedance better by using a step-UP transformer before resorting to
parallel amplifiers?
Hint: That's how they do it in microphone pre-amplifiers. They do
however not infrequently trade off poorer noise figure for lower cost
and/ or slightly flatter frequency response by omitting the transformer
and using a second pre-amp module to provide a balanced input. This is
however, just that, a trade-off.
Hmmm. I thought I was replying to a list, but it turns out it was
direct. You may care to redirect it or not, and I will reply to the
others.
>> halved because the input impedances of the amps are now in parallel.
>> This is a faulty analysis, though, and would only be true if the input
>> impedance of the amplifier was comparable to the source impedance of
>> the transducer. In fact, the amplifier impedance can be _much_ higher
>> than the transducer impedance.
>
> REALLY? Well, in that case, wouldn't it be far more sensible to match
>the impedance better by using a step-UP transformer before resorting to
>parallel amplifiers?
>
> Hint: That's how they do it in microphone pre-amplifiers. They do
>however not infrequently trade off poorer noise figure for lower cost
>and/ or slightly flatter frequency response by omitting the transformer
>and using a second pre-amp module to provide a balanced input. This is
>however, just that, a trade-off.
>
I defy you to come up with a transformer (or any magnetics, for that
matter) that has anything approaching within several orders of magnitude a
2000 ohm to 10**12 ohm impedance difference.
Besides, I'd say the impedance mismatch is the reason why the amplifier is
useful--it doesn't load the transducer significantly. The object is not to
maximize power transfer from the transducer to the amp, but to amplify the
signal.
--BN
----------------------------------------------------------------------------
Country Robot "Modular robot components
69 S. Fremont Ave. # 2 for education and industry"
Pittsburgh, PA 15202
----------------------------------------------------------------------------
Now, back to the topic in the subject.
What about instead of the normal approach of timing a pulse, with
sensitive amps, ...
Using the more Pic-friendly method of connecting recievers, and
transmitters more or less directly to pins, and getting the gain
instead from Many pulses.
Basically, take a maximal length shift register, connect that to
the output, now connect the reciever to the PIC, which counts on
it's fingers every time it hears a little bit of noise, and works
out when it may have been sent, hence the range.
>Paul B. wrote:
>
>>> halved because the input impedances of the amps are now in parallel.
>>> This is a faulty analysis, though, and would only be true if the input
>>> impedance of the amplifier was comparable to the source impedance of
>>> the transducer. In fact, the amplifier impedance can be _much_ higher
>>> than the transducer impedance.
>>
>> REALLY? Well, in that case, wouldn't it be far more sensible to match
>>the impedance better by using a step-UP transformer before resorting to
>>parallel amplifiers?
>>
>> Hint: That's how they do it in microphone pre-amplifiers. They do
>>however not infrequently trade off poorer noise figure for lower cost
>>and/ or slightly flatter frequency response by omitting the transformer
>>and using a second pre-amp module to provide a balanced input. This is
>>however, just that, a trade-off.
>>
>
>I defy you to come up with a transformer (or any magnetics, for that
>matter) that has anything approaching within several orders of magnitude a
>2000 ohm to 10**12 ohm impedance difference.
>
>Besides, I'd say the impedance mismatch is the reason why the amplifier is
>useful--it doesn't load the transducer significantly. The object is not to
>maximize power transfer from the transducer to the amp, but to amplify the
>signal.
>
>--BN
While it is true that an op amp is primarily a voltage amplifier and it is
not meant to be impedance matched, one could get getter gain and SN ratio
by using a step up transformer between a signal source and an op amp. Lets
say that the source has a 10K impedance and the op amp has a 10**12 imput
Z. Using a 10K:100K impedance match xfrmr will not change the impedance
match situation dramatically, but it seems to me that it will give 10
times more gain and improve the signal to noise ratio almost as much (since
the op amp's internal noise is presumably much larger than that generated
in the signal source or transformer).
On Tue, 2 Dec 1997 22:28:23 +1000 "Paul B. Webster VK2BZC"
<paulbspam_OUTmidcoast.com.au> writes:
>Frank A. Vorstenbosch wrote:
Regardless of all this bickering about amplifiers, examine the big
picture to see if the application's (sonar ranger attached to robot)
performance will indeed be limited by amplifier noise. Acoustic and
electrical interference from the machine and environment will likely be
predominant sources of noise, even using a preamp of only moderate
performance.
For long range, quiet environments, it may likely be more practical and
less expensive to increase the transmitter power and/or acoustic
"antenna" gain than to go to great lengths with exotic preamps. These
approaches also counteract environmental noise, a more sensitive receiver
doesn't.
This whole discussion reminds me of an article in a Dutch magazine , as I remember correctly it was the 4/1/87 issue of the Electuur it describes an interesting preamplifier with extremely low noise. the circuit had some disadvantages though, gain was -1 ( if I remember well the main component in the output stage was a 4 amp slow blow fuse :-)
Is this in mm/dd/yy or dd/mm/yy format? It may decide whether or
not one believes anything that is written in the magazine :-)
regards,
Mike Watson
> describes an interesting preamplifier with extremely low noise. the
> circuit had some disadvantages though, gain was -1 ( if I remember well
> the main component in the output stage was a 4 amp slow blow fuse :-)
>
> Peter
>
I'm getting a range of 10 feet using a single comparator from an LM339.
The output of the LM339 is digital. I'm not good at analog, so I
converted to digital as simply as I could.
The ultrasonic transducer is Mouser Electronics part number 251-1603.
> I'm getting a range of 10 feet using a single comparator from an LM339.
> The output of the LM339 is digital. I'm not good at analog, so I
> converted to digital as simply as I could.
The schematic of the input circuit is on the web page now. My apologies
to the 14 people who it the webpage before there was anything usefull.
Hit reload if you were one of the early ones.
I made the schematic with xpaint on my Linux box. Are there any real
packages for doing schematics that will run on Linux? How about under
DOSEMU and/or WINE? My email address is spamBeGonepaulhspamBeGonehamjudo.com, no need to
involve the whole list on Linux trivia. I'll put the answers on my
website, unless requested not to.
Note: I get the digest, so I won't see any posts to the list until
tomorrow morning.
MIDCOAST.COM.AU]
