Hi;

I'm thinking about using a simple 1 transistor emitter follower in an audio
design (the audio signal will pass thru it). Does the noise figure of the
transistor need to be taken into account, or will there be no noise added
since there is no amplification happening (sort of like the signal just
'passes thru' the transistor untouched)?

Thanks

tempus fugit wrote:
> Hi;
> 
> I'm thinking about using a simple 1 transistor emitter follower in an audio
> design (the audio signal will pass thru it). Does the noise figure of the
> transistor need to be taken into account, or will there be no noise added
> since there is no amplification happening (sort of like the signal just
> 'passes thru' the transistor untouched)?

The transistor adds noise to any signal applied to it.  That noise 
does not get voltage amplified by the follower, but it still gets 
added to the signal.

OK, that's kind of what I thought too, but I thought I should make sure
before choosing a suitable transistor.

Thanks John.


"John Popelish" <j...@rica.net> wrote in message
news:l...@adelphia.com...
> tempus fugit wrote:
> > Hi;
> >
> > I'm thinking about using a simple 1 transistor emitter follower in an
audio
> > design (the audio signal will pass thru it). Does the noise figure of
the
> > transistor need to be taken into account, or will there be no noise
added
> > since there is no amplification happening (sort of like the signal just
> > 'passes thru' the transistor untouched)?
>
> The transistor adds noise to any signal applied to it.  That noise
> does not get voltage amplified by the follower, but it still gets
> added to the signal.

tempus fugit wrote:
> Hi;
>
> I'm thinking about using a simple 1 transistor emitter follower in an
> audio design (the audio signal will pass thru it). Does the noise
> figure of the transistor need to be taken into account, or will there
> be no noise added since there is no amplification happening (sort of
> like the signal just 'passes thru' the transistor untouched)?
>
> Thanks


Not only will the transistor add some noise, it will also add a lot of 
distortion. around 1% for 600mV peak to peak. The distortion raises 
proportional to the input level and is independent from the bias point.
The noise is completely irrelevant compared to this phenomenon. If you want 
the signal "untouched", you should use an opamp as buffer, OP27 comes to 
mind.
-- 
ciao Ban
Apricale, Italy 

tempus fugit wrote:

> Hi;
>
> I'm thinking about using a simple 1 transistor emitter follower in an audio
> design (the audio signal will pass thru it). Does the noise figure of the
> transistor need to be taken into account, or will there be no noise added
> since there is no amplification happening (sort of like the signal just
> 'passes thru' the transistor untouched)?

There certainly will be noise added but not a lot.

Graham

On Wed, 13 Sep 2006 20:20:16 +0200, "Ban" <b...@masterweb.it>
wrote:

>tempus fugit wrote:
>> Hi;
>>
>> I'm thinking about using a simple 1 transistor emitter follower in an
>> audio design (the audio signal will pass thru it). Does the noise
>> figure of the transistor need to be taken into account, or will there
>> be no noise added since there is no amplification happening (sort of
>> like the signal just 'passes thru' the transistor untouched)?
>>
>> Thanks
>
>
>Not only will the transistor add some noise, it will also add a lot of 
>distortion. around 1% for 600mV peak to peak. The distortion raises 
>proportional to the input level and is independent from the bias point.
>The noise is completely irrelevant compared to this phenomenon. If you want 
>the signal "untouched", you should use an opamp as buffer, OP27 comes to 
>mind.

What's the mechanism? With enough DC voltage across the emitter
resistor and a light load, one can approach constant emitter current.
All that's left is Early voltage effects, pretty small usually.

John

Eeyore (r...@REMOVETHIS.hotmail.com) writes:
> tempus fugit wrote:
> 
>> Hi;
>>
>> I'm thinking about using a simple 1 transistor emitter follower in an audio
>> design (the audio signal will pass thru it). Does the noise figure of the
>> transistor need to be taken into account, or will there be no noise added
>> since there is no amplification happening (sort of like the signal just
>> 'passes thru' the transistor untouched)?
> 
> There certainly will be noise added but not a lot.
> 
> Graham
> 

And of course, the real issue is the application.  One worries about
noise with weak signals, with larger signals the signal swamps out
the noise.

A lot of places where emitter followers are used, the issue of noise
will never come up.

On the other hand, one reason you see transformers in low level audio
signals even today is because they are at a point where the signal
is weak, and introducting noise at that point is not a good thing.

So they have those matching transformers between microphones and
the preamp, and between those moving coil phono cartridges and the phono
preamp, since they need to step up the voltage at those points without
adding any noise.  Once the signal is stepped up by the transformer,
the noise of the following preamp is less important.

  Michael

Thanks for all the replies.

Michael, since you mentioned the application.....

I currently have a really slick switching system for my guitar effects
pedals. To make a long story short, the only problem with it is that it uses
relays to switch the effects in and out, and they give an audible "pop" when
switched. It's not horrible, but I was trying to find a way to make things
cleaner (I can't leave well enough alone). I first thought of using a simple
transistor or JFET as a switch in place of the relay, but a friend informed
me that it wasn't possible to have the audio pass through the transistor if
it was also being used as a switch (or is there a way?). We came up with the
idea of using a transistor as an amp with very little gain and switching the
amp on and off instead of a relay. Which brings us to this discussion.

Any thoughts on this line of reasoning?

Thanks


"Michael Black" <e...@FreeNet.Carleton.CA> wrote in message
news:eea464$rpk$1...@theodyn.ncf.ca...
> Eeyore (r...@REMOVETHIS.hotmail.com) writes:
> > tempus fugit wrote:
> >
> >> Hi;
> >>
> >> I'm thinking about using a simple 1 transistor emitter follower in an
audio
> >> design (the audio signal will pass thru it). Does the noise figure of
the
> >> transistor need to be taken into account, or will there be no noise
added
> >> since there is no amplification happening (sort of like the signal just
> >> 'passes thru' the transistor untouched)?
> >
> > There certainly will be noise added but not a lot.
> >
> > Graham
> >
>
> And of course, the real issue is the application.  One worries about
> noise with weak signals, with larger signals the signal swamps out
> the noise.
>
> A lot of places where emitter followers are used, the issue of noise
> will never come up.
>
> On the other hand, one reason you see transformers in low level audio
> signals even today is because they are at a point where the signal
> is weak, and introducting noise at that point is not a good thing.
>
> So they have those matching transformers between microphones and
> the preamp, and between those moving coil phono cartridges and the phono
> preamp, since they need to step up the voltage at those points without
> adding any noise.  Once the signal is stepped up by the transformer,
> the noise of the following preamp is less important.
>
>   Michael
>

"tempus fugit" <t...@no.spam.ciaccess.com> wrote in message
news:a98d7$450835ed$d1d89e7c$2...@PRIMUS.CA...
> Hi;
>
> I'm thinking about using a simple 1 transistor emitter follower in an
audio
> design (the audio signal will pass thru it). Does the noise figure of the
> transistor need to be taken into account, or will there be no noise added
> since there is no amplification happening (sort of like the signal just
> 'passes thru' the transistor untouched)?
>
> Thanks

Never add an open, simple emitter follower to a high quality audio circuit.
As was mentioned they will add distortion to the signal which may be
tolerable in a phone or other circuit where high quality audio is not
required. This happens because the internal emitter resistance, re, is a
function of the emitter current which in turn is a function of the base to
emitter voltage. This makes a variable, non-linear, voltage divider with the
load impedance. The nominal gain is one, but in reality is always less than
one and varies with the voltage level. That non-linearity causes excessive
distortion both harmonic and intermodulation.

Emitter followers are used inside vitually all bi-polar amplifiers, op-amps,
etc. But, they are ALWAYS inside feed back loops attended with current
sources and gain stages to reduce their inherent problems. If you want a
unity gain buffer make one with an appropriate low noise unity gain op-amp.
Bob

On Thu, 14 Sep 2006 02:57:19 GMT, "Bob Eld" <n...@yahoo.com>
wrote:

>
>"tempus fugit" <t...@no.spam.ciaccess.com> wrote in message
>news:a98d7$450835ed$d1d89e7c$2...@PRIMUS.CA...
>> Hi;
>>
>> I'm thinking about using a simple 1 transistor emitter follower in an
>audio
>> design (the audio signal will pass thru it). Does the noise figure of the
>> transistor need to be taken into account, or will there be no noise added
>> since there is no amplification happening (sort of like the signal just
>> 'passes thru' the transistor untouched)?
>>
>> Thanks
>
>Never add an open, simple emitter follower to a high quality audio circuit.
>As was mentioned they will add distortion to the signal which may be
>tolerable in a phone or other circuit where high quality audio is not
>required. This happens because the internal emitter resistance, re, is a
>function of the emitter current which in turn is a function of the base to
>emitter voltage. This makes a variable, non-linear, voltage divider with the
>load impedance. The nominal gain is one, but in reality is always less than
>one and varies with the voltage level. That non-linearity causes excessive
>distortion both harmonic and intermodulation.
>

Excessive?


>Emitter followers are used inside vitually all bi-polar amplifiers, op-amps,
>etc. But, they are ALWAYS inside feed back loops attended with current
>sources and gain stages to reduce their inherent problems. If you want a
>unity gain buffer make one with an appropriate low noise unity gain op-amp.
>Bob
>

On the other hand, audio folks love open-loop cathode followers, which
are far more nonlinear than any reasonably biased emitter follower.

John