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PICList Thread
'[EE]: Can a speaker be Input Devices'
2003\06\02@114705 by Alan B. Pearce

face picon face
>Can a speaker act as input devices to an Voice Activation circuit ?
>I want to built a Voice Activation circuit without using Mic.
>Is this possible ?
>
>Any voltage frustration from the speaker that can trigger a transistor ?

Yes you can do that. Think in terms of a dynamic microphone, or the simple
little intercoms you can buy which do exactly this. The output level may
well be very low as the sound pressure to voltage efficiency may not be as
high as a microphone.

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2003\06\02@115742 by Olin Lathrop

face picon face
>> Can a speaker act as input devices to an Voice Activation circuit ?
>> I want to built a Voice Activation circuit without using Mic.
>> Is this possible ?
>>
>> Any voltage frustration from the speaker that can trigger a transistor
>> ?

I didn't see the original message for some reason, but yes a speaker can
be used as a microphone.  As Alan mentioned, some intercom systems work on
this principal.

However, you won't get enough voltage directly from a speaker to turn on a
normal silicon bipolar transistor without physically banging on the
speaker.  You will need some amplification before it can "trigger a
transistor", no matter how frustrated the speaker gets.


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2003\06\02@115748 by Picdude

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face
Funny you should ask.  Ran into this this weekend...
  http://www.mitedu.freeserve.co.uk/Circuits/Misc/Sp-mic.htm

Cheers,
-Neil.


On Sunday 01 June 2003 13:03, Quah scribbled:
> Can a speaker act as input devices to an Voice Activation circuit ?  I
> want to built a Voice Activation circuit without using Mic. Is this
> possible ?
>
> Any voltage frustration from the speaker that can trigger a transistor ?
>
> Thanks.
> James

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2003\06\02@123356 by Olin Lathrop

face picon face
> Funny you should ask.  Ran into this this weekend...
>    http://www.mitedu.freeserve.co.uk/Circuits/Misc/Sp-mic.htm

That looks like a pretty crappy circuit.  All the gain is in a single
stage and depends on the I/V characteristics of the first transistor's B-E
junction.  Yuk.  This could be cleaned up with some feedback.

I'm also not convinced that loading the speaker to its characteristic
impedence is the best way to use it as a microphone.  Maybe it's true, but
it's not immediately obvious to me.  I can see some reasons for doing it
either way.


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2003\06\03@035902 by hael Rigby-Jones

picon face
> -----Original Message-----
> From: Olin Lathrop [SMTP:spam_OUTolin_piclistTakeThisOuTspamEMBEDINC.COM]
> Sent: Monday, June 02, 2003 5:33 PM
> To:   .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject:      Re: [EE]: Can a speaker be Input Devices
>
> > Funny you should ask.  Ran into this this weekend...
> >    http://www.mitedu.freeserve.co.uk/Circuits/Misc/Sp-mic.htm
>
> That looks like a pretty crappy circuit.  All the gain is in a single
> stage and depends on the I/V characteristics of the first transistor's B-E
> junction.  Yuk.  This could be cleaned up with some feedback.
>
> I'm also not convinced that loading the speaker to its characteristic
> impedence is the best way to use it as a microphone.  Maybe it's true, but
> it's not immediately obvious to me.  I can see some reasons for doing it
> either way.
>
>
I suspect a standard common emitter amplifier would work just as well but I
doubt it really matters.  The quality obtainable from a small transistor
radio type speaker is pretty dire anyway.

Mike


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2003\06\03@134650 by Mike Singer

picon face
Olin Lathrop wrote:
> > Funny you should ask.  Ran into this this weekend...
> >    http://www.mitedu.freeserve.co.uk/Circuits/Misc/Sp-mic.htm
>
> That looks like a pretty crappy circuit.  All the gain
> is in a single stage and depends on the I/V characteristics
> of the first transistor's B-E junction.  Yuk.  This could
> be cleaned up with some feedback.

Olin, have you red the text below the circuit?

"The first transistor operates in common base mode."

The feedback at variable voltage achieved due to collector
current changes make voltage changes on emitter resistor.
The base is fixed for variable voltage - look at the big
capacitor. Negative feedback is right before you. At the
time of germanium transistors era "common base mode" was
just common.

And not "all the gain is in a single stage" - it should be
"all the _voltage_ gain is in a single stage".
Current gain is in the second stage - emitter follower.

Great circuit, by the way.
Do not confuse newbie's if possible, please. They trust you.

Mike.

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2003\06\03@181822 by Olin Lathrop

face picon face
> "The first transistor operates in common base mode."

Yes, I know.

> The feedback at variable voltage achieved due to collector
> current changes make voltage changes on emitter resistor.

That is only DC feedback to set the bias point of the first transistor.  I
agree this works.  I should have been more clear in that I was talking
about AC (the sound) feedback.  This circuit doesn't have any.  And, since
the output is not very linear in response to input changes, there will be
much distorion.  I stand by my comment that this is a "crappy" circuit.

> The base is fixed for variable voltage - look at the big
> capacitor.

Yes Mike, I understand how common base works.  Actually this circuit has
about the same response characteristics as common emitter would with the
emitter tied to ground (assuming a suitable DC bias arrangement).  In
either case, small AC input signal is presented directly accross the B-E
junction of the first transistor.  The small voltage signals there cause
large (but non-linear) changes in collector current, which is where all
the voltage gain comes from.

> Negative feedback is right before you.

Again, only DC feedback for bias purposes.

>  And not "all the gain is in a single stage" - it should be
> "all the _voltage_ gain is in a single stage".
> Current gain is in the second stage - emitter follower.

OK, you got me there.  I thought voltage gain was implicit from the
discussion, but I should have been more specific.

> Great circuit, by the way.

Not!  See above.


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2003\06\03@183610 by Bob Blick

face
flavicon
face
Olin Lathrop said:
> Yes Mike, I understand how common base works.  Actually this circuit has
> about the same response characteristics as common emitter would with the
> emitter tied to ground (assuming a suitable DC bias arrangement).  In
> either case, small AC input signal is presented directly accross the B-E
> junction of the first transistor.  The small voltage signals there cause
> large (but non-linear) changes in collector current, which is where all
> the voltage gain comes from.
>
>> Negative feedback is right before you.
>
> Again, only DC feedback for bias purposes.

Hi Olin,

I agree with Mike here, you need to study the circuit a little more.
Common-base amps are pretty linear. The negative feedback is degenerative,
but it results in low distortion.

Build it if you don't believe me, put a sine wave in and look at the
output, then pick up a good textbook and read. If you have trouble finding
a good textbook, google for "cascode amps", somewhere in there should be a
description somewhere about the distortion characteristics of common-base
amps.

Cheerful regards,

Bob

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2003\06\04@025507 by Mike Singer

picon face
Olin Lathrop wrote:
> > "The first transistor operates in common base mode."
>
> Yes, I know.
>
> > The feedback at variable voltage achieved due to collector
> > current changes make voltage changes on emitter resistor.
>
> That is only DC feedback to set the bias point of the
> first transistor.  I agree this works.  I should have
> been more clear in that I was talking about AC (the
> sound) feedback.  This circuit doesn't have any.


Can't agree fully with you, Olin.

( http://www.mitedu.freeserve.co.uk/Circuits/Misc/Sp-mic.htm )

"DC feedback for bias purposes" is getting by collector and
base resistors.
"AC (the sound) feedback" you were talking about is getting
by emitter resistor.


> I stand by my comment that this is a "crappy"
> circuit.
...
> Yes Mike, I understand how common base works.  Actually this circuit
> has about the same response characteristics as common emitter would
> with the emitter tied to ground (assuming a suitable DC bias
> arrangement).  In either case, small AC input signal is presented
> directly accross the B-E junction of the first transistor.

In a "common emitter mode", if we assume this DC bias arrangement
suitable (collector and base resistors and capacitor), then, anyway, we
need emmiter resistor for the linearity.

And to feed the base of the transistor with an AC input signal
we need extra resistor from the base to capacitor and some
capacitor between AC input signal and the base. The base
resistor will spoil stability, extra capacitor is extra part.

In "common base mode" even emitter resistor could be substituted
with loudspeaker's coil resistance.


> > Great circuit, by the way.

> Not!  See above.


Mike.

P.S. Hmm, Peter's got better score :-)

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2003\06\04@083621 by Dave Tweed

face
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Olin Lathrop <EraseMEolin_piclistspam_OUTspamTakeThisOuTEMBEDINC.COM> wrote:
> Yes Mike, I understand how common base works.  Actually this circuit has
> about the same response characteristics as common emitter would with the
> emitter tied to ground (assuming a suitable DC bias arrangement).  In
> either case, small AC input signal is presented directly accross the B-E
> junction of the first transistor.  The small voltage signals there cause
> large (but non-linear) changes in collector current, which is where all
> the voltage gain comes from.

I don't think you're applying the correct analysis to this circuit.

The speaker is a very low-impedance source, and its AC current output is
much more significant than its voltage output.

The common-base arrangement of the transistor acts to impose the AC
current of the speaker across the much larger load resistor connected
to the collector, providing voltage gain equal to the ratio of the
impedances.

The AC voltage across the B-E junction is on the order of 1 mV or less,
and the nonlinearity of this junction has little effect on circuit
performance.

The biggest problem with this circuit is the fact that the impedance of
the speaker varies quite a bit with frequency, which makes the voltage
gain of the circuit vary as well. The low-value emitter resistor helps
to limit this effect.

In general, the common-base amplifier is useful whenever you need to
match a low-impedance source to a high-impedance load.

-- Dave Tweed

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2003\06\04@085938 by Olin Lathrop

face picon face
part 1 3029 bytes content-type:text/plain; (decoded 7bit)

> I agree with Mike here, you need to study the circuit a little more.
> Common-base amps are pretty linear.

They can be, but this one isn't.

> The negative feedback is
> degenerative, but it results in low distortion.

Yes, that's usually the intent of properly done negative feedback.
However this circuit has no *AC* negative feedback.

For sake of discussion, lets forget about the second stage.  It is an
emitter follower that has roughly unity voltage gain, and is there to
provide a lower output impedence (power gain).  I don't have a problem
with that.

So, let's look at the first stage in detail.  I've attached the schematic
GIF to this message so we can all see the same thing (it's only
4.4Kbytes).

The 5.6Kohm resistor produces a voltage accross it proporional to the
current thru the transistor.  This means any supply voltage variations
show up in the output, by the way.

The 2.2Mohm resistor provides base bias to the transistor so that its
quiescent point is in the "on" region.  This resistor does provide some
negative feedback, but due to the capacitor this feedback only effects the
DC operating point, not the AC signal.  Note that the time constant
between the base bias resistor and the capacitor is about 8 minutes.  This
will clearly filter out any audio frequencies, in addition to providing an
annoyingly long startup delay.  So, from an AC point of view the base is
being held at a fixed voltage (which makes this a common base circuit).

The 8.2ohm emitter resistor is so small compared to the 5.6Kohm resistor
that it will have very little voltage accross it.  It therefore has little
effect on the DC operating point.  It's only purpose apparently is to load
the speaker with a fixed impedence.

Now let's look at AC characteristics.  The speaker, series capacitor, and
8.2ohm resistor together act as a low impedence source producing small
audio voltages when the speaker receives sounds.  This low impedence AC
voltage source is directly connected to the emitter.

So, for AC analisys, we've got a fixed voltage on the base and the input
signal is a low impedence voltage on the emitter.  In other words, the
input signal is presented directly accross the transistor's B-E junction.
This is essentially a diode, and the voltage from current response of a
diode is very non-linear.  The transistor has the added effect of
multiplying this diode current by the transistor gain and making that the
collector current.  The output voltage is therefore a scaled up signal
proportional to the current thru a diode resulting from the input voltage.

Yes, this has high gain, but the output signal will have significant
distortion, and any AC on the power supply will be added to it directly.
Much of these problems could be corrected with negative feedback, but this
circuit has no *AC* feedback path.

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2003\06\04@090723 by Olin Lathrop

face picon face
Mike Singer wrote:
> "AC (the sound) feedback" you were talking about is getting
> by emitter resistor.

Not really.  The 8.2ohm is so low compared to everything else in this
circuit that the speaker, series capacitor and 8.2ohm resistor are
essentially a voltage source from the rest of the circuit's point of view.
If there was a resistor in series with the speaker and the emitter, it
would provide some negative feedback, but this one is in parallel with the
speaker (for AC analisys).


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2003\06\04@092706 by Olin Lathrop

face picon face
Dave Tweed wrote:
> The speaker is a very low-impedance source, and its AC current output is
> much more significant than its voltage output.

You just contradicted yourself.  "Very low impedance source" implies that
the voltage will be fixed, and the current whatever it needs to be.
Because of the very low impedence, I think the speaker acts much more like
a voltage than current source in this circuit.

> The AC voltage across the B-E junction is on the order of 1 mV or less,
> and the nonlinearity of this junction has little effect on circuit
> performance.

I agree that the very small amplitude of the input signal makes the
nonlinearity of the B-E voltage to current characteristic less noticeable.


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2003\06\04@110826 by Jack Smith

picon face
In the interest of putting some numbers out for debate, I just ran a SPICE
simulation of this simple amplifier circuit.

I modeled the speaker as 4 ohms in series with 170uH and a 1 mV AC source.
My database does not have BC109C transistors, so I used two BC109B
transistors instead. The B's have a lower hFE (typical value about one-half
the C spec). I used a 4.7 k resistor for the output load. The Vcc is 12
volts. Other parameters are as shown in the original schematic.


Anyway, the result shows the 3 dB bandwidth is 70 Hz - 9.6 KHz.  Total
harmonic distortion at 1 KHz is 0.57% (through 9th harmonic).  2nd harmonic
is about 45 dB below the reference, 3rd is about 55 db below the reference.
A transient analysis shows no significant start-up distortion (looked at for
the first 5 mSec.) Overall gain into the 4.7K load is 103.5, or a hair over
40 dB if you use a sloppy definition of dB by ignoring the different
impedances.

The DC operating point analysis shows Q1 collector = 7.2 volts, Q2 emitter =
6.5 volts.

Assuming that SPICE reflects reality and my speaker model isn't too far out,
the circuit appears to function reasonably well.

Jack




{Original Message removed}

2003\06\04@115208 by Bob Blick

face
flavicon
face
Olin Lathrop said:

> So, for AC analisys, we've got a fixed voltage on the base and the input
> signal is a low impedence voltage on the emitter.  In other words, the
> input signal is presented directly accross the transistor's B-E
> junction. This is essentially a diode, and the voltage from current
> response of a diode is very non-linear.  The transistor has the added
> effect of
> multiplying this diode current by the transistor gain and making that
> the collector current.  The output voltage is therefore a scaled up
> signal proportional to the current thru a diode resulting from the input
> voltage.
>
> Yes, this has high gain, but the output signal will have significant
> distortion, and any AC on the power supply will be added to it directly.

Olin,

Think about it from a current sense as well, please. I think you are not
seeing the forest for the trees. Collector and base current in this
transistor are less than 1% different. The speaker is putting current into
the emitter. If the speaker is a voltage source in series with a
resistor(say it's the same as the external one, 8.2 ohms), then the
voltage gain is 5600/4.1 assuming an impossible zero resistance
transistor.

In reality the resistance in the transistor drops the gain a lot, but
overall it is a linear circuit, and it is very close to the way some low
impedence microphone preamps I've seen do it. They were not audiophile
grade, but were in good quality public address systems.

Also note that Jack Smith's Spice result of .57% THD shows we're talking
about a circuit that is respectable for its low parts count.

In answer to your latest challenge, ripple on the power supply will get
through, but it won't be amplified. Normally as long as it doesn't get
amplified it's accepted in a small circuit block like this.

When always using common-emitter connections it's all too easy to forget
the many different ways to use a transistor. This is one of them. Enjoy!

Cheerful regards,

Bob

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2003\06\04@121058 by Mike Singer

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part 1 1406 bytes content-type:text/plain; (decoded 7bit)

Olin Lathrop wrote:
> The 8.2ohm emitter resistor is so small compared
> to the 5.6Kohm resistor that it will have very
> little voltage accross it.  It therefore has little
> effect on the DC operating point.

You're right. I have not looked at resistors' values.

According to BC109C datasheet
http://www.fe.up.pt/~victorm/DataSheet/Transistors/BC%20107_108_109.pdf

DC current gain:
BC109C hFE min 420 typ 520 max 800
at IC = 2 mA; VCE = 5 V

For the significant negative feedback
Rk (symbols from the attached circuit) should be much
less (guess why, the winner will get few points:-)
than Re* hFE=8*500=4KOm

Let it be, say Rk=820 Ohm, much less than 4KOm

Now let's do some calcs to estimate Rb:

Collector current:  Ik=(Vdd-Vk)/Rk

Base current:      Ib=(Vk-Ve-0.7)/Rb

DC current gain:   hFE= Ik/Ib

Assume Ve+0.7=1V

Thus Rb=(Vk-1)* hFE / Ik

Or Rb=(Vk-1)* hFE * Rk /(Vdd-Vk)

If we assume Vdd=6V, Vk=3V, hFE=500, Rk=820 Ohm

then we'll get  Rb=(3-1)* 500 * 820 /(6-3) = aprox 300Kom


Conclusion:
The circuit is "feedbacked" greatly with
Rb=300Kom and Rk=820 Ohm.


Best regards,
Mike.

P.S. Don't blame me hard if I'm wrong - I'm
not an EE guy neither from education point of view,
nor from the profession.

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part 3 2 bytes
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2003\06\04@175905 by Peter L. Peres

picon face
> distortion in common base preamp

The distortion generated with the small signals encountered will be
negligible wrt the distortion generated by the speaker proper. There are
few small speakers that get under 1% distortion and they are unlikely to
be used as microphones in such a circuit.

The amplitude on the collector will most likely be 100mVpkpk at most while
speaking directly into the speaker at 10 cm or so. This is a small
percentage of the output capability and distortion will be low. You can
model it in SPICE if you like.

3% distortion is acceptable for speech (some say 10%). A cheap small
speaker will give 5% distortion at least. Remember that the emf in the
voice coil depends linearly on the magnetic field in the gap and the field
depends on the (inverse) square of the apparent gap (wrt voice coil
position). 1% distortion in this context means gap accuracy of 0.01^2 =
0.0001. A 1mm gap would require machining and geometrical tolerance to 100
nanometers for that. Oops, not your average far eastern tiny speaker. Oh,
by the way, this assumes everything else is perfect, like suspension,
damping, centering, coil geometry, and acoustics.

The 0.01% distortion implied by some HiFi amps implies speakers with a
field that accurate and the gap parts should be machined to 1e-8 which is
20nm for a 2mm gap. Re-oops. Better don't look at your expensive speakers.
20nm surface quality and geometry is mirror grade (good enough for laser
mirror I think). And there are no shortcuts I know of here.

One of the less published reasons for which good speakers go for huge
magnets is this. Big magnets afford a larger gap and that puts precision
within machinability limits, besides increasing efficiency and
electrical coupling/damping. However big magnets are expensive. If the
manufacturers could get to these results otherwise, they would. The
ferro-fluid gap fillers etc play a role here but I think that an air gap
is cleaner.

Peter

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2003\06\05@011304 by ?q?Debbie=20Hynes?=

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part 1 602 bytes content-type:text/plain; charset=iso-8859-1 (unknown type 8bit not decoded)

Crikey, this is all getting somewhat heavy seeing as the cct is a just a puny
ol'audio amp. ;)

Attached (4k gif) is a schematic of a hydrophone amp I made yonks back. It used
a mylar spkr but didn't work too well under water coz it couldn't hack any
pressure. But it should function ok as a microphone?

Best - Debbie  :)

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2003\06\05@040556 by hael Rigby-Jones

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{Quote hidden}

I guess this is where the NTX flat panel speakers, and electrostatics have
an advantage?

Mike


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2003\06\05@081858 by Olin Lathrop

face picon face
Debbie Hynes wrote:
> Crikey, this is all getting somewhat heavy seeing as the cct is a just
> a puny ol'audio amp. ;)
>
> Attached (4k gif) is a schematic of a hydrophone amp I made yonks back.
> It used a mylar spkr but didn't work too well under water coz it
> couldn't hack any pressure. But it should function ok as a microphone?

This seems to be the same basic concept with a better DC bias arrangement
and transformer coupled output.  That makes sense, but I don't see what
advantage you are getting from R5 unless this circuit had to work with a
particular transformer you happen to have already.


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2003\06\05@154822 by ?q?Debbie=20Hynes?=

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--- Olin Lathrop <RemoveMEolin_piclistKILLspamspamEMBEDINC.COM> wrote: > Debbie Hynes wrote:
> > Attached (4k gif) is a schematic of a hydrophone amp I made yonks back.
> > It used a mylar spkr but didn't work too well under water coz it
> > couldn't hack any pressure. But it should function ok as a microphone?
>
> This seems to be the same basic concept with a better DC bias arrangement
> and transformer coupled output.  That makes sense, but I don't see what
> advantage you are getting from R5 unless this circuit had to work with a
> particular transformer you happen to have already.

It did - the o/p transformer was a mini AF PCB type. R5 was supposed to improve
the LF response somewhat. It was a real junkbox project. :)
I thought a long run of cable underwater would pick up 50Hz hum pretty bad but
it didn't so the diff o/p wasn't really necessary.
Debbie

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2003\06\06@074233 by Dave Tweed

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Olin Lathrop <spamBeGoneolin_piclistSTOPspamspamEraseMEEMBEDINC.COM> wrote:
> Dave Tweed wrote:
> > The speaker is a very low-impedance source, and its AC current output is
> > much more significant than its voltage output.
>
> You just contradicted yourself.  "Very low impedance source" implies that
> the voltage will be fixed, and the current whatever it needs to be.
> Because of the very low impedence, I think the speaker acts much more like
> a voltage than current source in this circuit.

Not at all. I didn't say it was a current source, I said that it's output
current is more significant. The point is that the available electrical
signal energy comes from a low-impedance source, and a common-base
amplifier is the better way to make use of it.

Nor does "very low impedance" imply that it's an ideal voltage source (zero
impedance). You can model the speaker as a voltage source in series with a
low-value resistor, or as a current source in parallel with that resistor,
but in the overall analysis, you can't treat it as an ideal source of
either type at all. The impedance of the emitter circuit, while very low,
is very significant in the analysis, and it *does* provide a certain amount
of negative feedback that controls the overall gain.

> > The AC voltage across the B-E junction is on the order of 1 mV or less,
> > and the nonlinearity of this junction has little effect on circuit
> > performance.
>
> I agree that the very small amplitude of the input signal makes the
> nonlinearity of the B-E voltage to current characteristic less noticeable.

Do you also agree that since the transistor is operating in its linear
region, the collector current (both DC and AC components) is nearly
identical to the the emitter current? The base current will be whatever
it needs to be, as dictated by the I-V characteristic of the B-E junction,
in order to make this happen. Any distortion of this current caused by the
nonlinearity of the junction is simply added to the collector current, not
amplified by the transistor, so its effect relative to the signal is
actually reduced by the gain of the transistor.

-- Dave Tweed

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2003\06\06@165547 by Peter L. Peres

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> I guess this is where the NTX flat panel speakers, and electrostatics
> have an advantage?

I think so. Also ribbon microphones are among the best dynamic microphones
for similar reasons. But you can easily buy a better condenser mike than a
ribbon mike for the same money.

I've always wanted to build a valved amp with direct coupled electrostatic
speakers. Eventually I will do that. I have eyed some valves that have Ua
to 7kV so direct drive will not be a problem.

Peter

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2003\06\06@174435 by ?q?Debbie=20Hynes?=

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--- "Peter L. Peres" <KILLspamplpspamBeGonespamACTCOM.CO.IL> wrote: > > I guess this is where the
NTX flat panel speakers, and electrostatics
> > have an advantage?
>
> I think so. Also ribbon microphones are among the best dynamic microphones
> for similar reasons. But you can easily buy a better condenser mike than a
> ribbon mike for the same money.

Would you say that a speaker used as a microphone would be more sensitive than
a conventional microphone?  The speaker has a greater collecting area so, if
you were looking to detect really faint sounds, would a speaker-mic be the way
to go?

Heh, that argument make sense?
Debbie  :)

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2003\06\07@073152 by Peter L. Peres

picon face
>> I think so. Also ribbon microphones are among the best dynamic
>> microphones for similar reasons. But you can easily buy a better
>> condenser mike than a ribbon mike for the same money.
>
> Would you say that a speaker used as a microphone would be more
> sensitive than a conventional microphone?  The speaker has a greater
> collecting area so, if you were looking to detect really faint sounds,
> would a speaker-mic be the way to go?
>
>  Heh, that argument make sense? Debbie :)

I do not think so. The speaker has much more damping than the microphone.

Everyone has used stereo headphones as microphones in a pinch, at least
once. The experience is enlightening (or endarkening) even if the
headphones are of top quality and relatively high impedance (can be 200
ohms or more). Speakers microphones do not make. A $150 headphone driver
used as a mike will sound muffled and very unnatural with too much bass
response. Membrane mass, damping etc all play a role. Sensitivity is low
(even for drivers rated 110 dB/W and above). A $10 dynamic microphone will
outperform it ...

Peter

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2003\06\07@172315 by William Chops Westfield

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> Would you say that a speaker used as a microphone would be more
> sensitive than a conventional microphone?  The speaker has a greater
> collecting area so, if you were looking to detect really faint sounds,
> would a speaker-mic be the way to go?

No, of course not.  What you gain in area is more than destroyed by greater
mass and stiffness/friction and less sensitivity.  (ie the sensitive element
is heavier and harder to move, and you have to move it MORE to get the same
sort of signal out.)

BillW

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