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Sci.Electronics.Basics -> quick emitter follower question

There are 25 messages in this thread.
You are currently looking at messages 1 to 20.


Author: tempus fugit
Date: 12:48 13-09-06

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
Author: John Popelish
Date: 13:02 13-09-06


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.
Author: tempus fugit
Date: 13:21 13-09-06

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" <jpopelish@rica.net> wrote in message
news:lPKdnVs8sswaqpXYnZ2dnUVZ_r-dnZ2d@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.
Author: Ban
Date: 14:20 13-09-06

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



Author: Eeyore
Date: 17:35 13-09-06



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
Author: John Larkin
Date: 17:47 13-09-06

On Wed, 13 Sep 2006 20:20:16 +0200, "Ban" <bansuri@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
Author: Michael Black
Date: 19:28 13-09-06

Eeyore (rabbitsfriendsandrelations@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
Author: tempus fugit
Date: 20:41 13-09-06

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" <et472@FreeNet.Carleton.CA> wrote in message
news:eea464$rpk$1@theodyn.ncf.ca...
> Eeyore (rabbitsfriendsandrelations@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
>
Author: Bob Eld
Date: 22:57 13-09-06


"tempus fugit" <toccata@no.spam.ciaccess.com> wrote in message
news:a98d7$450835ed$d1d89e7c$28509@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
Author: John Larkin
Date: 01:24 14-09-06

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

>
>"tempus fugit" <toccata@no.spam.ciaccess.com> wrote in message
>news:a98d7$450835ed$d1d89e7c$28509@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
Author: Eeyore
Date: 09:28 14-09-06



tempus fugit wrote:

> 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?

It's daft ! For starters you'l get a 'DC thump' every time the follower stage is
switched on or off.

JFETs make very very acceptable audio switches when used correctly.

Graham
Author: Bob Eld
Date: 11:54 14-09-06


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:fophg2lf80cr3vrn8n1pep194num1691u9@4ax.com...
> On Thu, 14 Sep 2006 02:57:19 GMT, "Bob Eld" <nsmontassoc@yahoo.com>
> wrote:
>
> >
> >"tempus fugit" <toccata@no.spam.ciaccess.com> wrote in message
> >news:a98d7$450835ed$d1d89e7c$28509@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

Don't you mean Ear-Queers? They like non-linear circuits. Some of them even
prefer single ended triode power stages sans feedback...go figure. Some
distorions can be perceived as quite musical. After all if you are just
adding to or rearranging a few harmonics in an already harmonically rich
signal, who's to say it doesn't sound better?
Author: tempus fugit
Date: 13:20 14-09-06

Thanks again for all the replies.


>
> It's daft ! For starters you'l get a 'DC thump' every time the follower
stage is
> switched on or off.

OK this answers one of my other possible problems - I wasn't sure if the
switching on/off was going to cause a bigger noise than the relays popping.
I had planned on ramping up the voltage to the follower, but I don't know if
that would help or not.


>
> JFETs make very very acceptable audio switches when used correctly.


I'm intrigued. Any suggestions on how to use one for this application?

Thanks
Author: Eeyore
Date: 13:48 14-09-06



tempus fugit wrote:

> Thanks again for all the replies.
>
> >
> > It's daft ! For starters you'l get a 'DC thump' every time the follower
> stage is
> > switched on or off.
>
> OK this answers one of my other possible problems - I wasn't sure if the
> switching on/off was going to cause a bigger noise than the relays popping.
> I had planned on ramping up the voltage to the follower, but I don't know if
> that would help or not.
>
> >
> > JFETs make very very acceptable audio switches when used correctly.
>
> I'm intrigued. Any suggestions on how to use one for this application?

Dead easy.

Use a nice low Ron jfet like a J108 and pull it off with a negative voltage on
the gate.
http://www.fairchildsemi.com/pf/J1/J108.html

Use an open-collecter arrangement to drive the gate with say 10M from gate to
source. The signal path is from drain to source ( bidirectional ). Use a 10k
'pull down' R on both in and out to avoid any DC levels getting into the switch.

If finding a negative voltage is a problem use a p-channel device like a J174
and pull the gate positive.
http://www.fairchildsemi.com/pf/J1/J174.html

The n-channel parts have lower Ron as do the lower numbered devices in each
family. The tradeoff is a higher Vgs to turn them off.

Graham
Author: Eeyore
Date: 13:49 14-09-06



Bob Eld wrote:

> "John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote
>
> > On the other hand, audio folks love open-loop cathode followers, which
> > are far more nonlinear than any reasonably biased emitter follower.
> >
> > John
>
> Don't you mean Ear-Queers? They like non-linear circuits. Some of them even
> prefer single ended triode power stages sans feedback...go figure. Some
> distorions can be perceived as quite musical. After all if you are just
> adding to or rearranging a few harmonics in an already harmonically rich
> signal, who's to say it doesn't sound better?

Audiophools(tm)

Graham
Author: Eeyore
Date: 13:50 14-09-06



John Larkin wrote:

> On Wed, 13 Sep 2006 20:20:16 +0200, "Ban" <bansuri@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.

You could also use a constant current load too.

Graham
Author: John Larkin
Date: 16:55 14-09-06

On Thu, 14 Sep 2006 18:50:39 +0100, Eeyore
<rabbitsfriendsandrelations@REMOVETHIS.hotmail.com> wrote:

>
>
>John Larkin wrote:
>
>> On Wed, 13 Sep 2006 20:20:16 +0200, "Ban"
<bansuri@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.
>
>You could also use a constant current load too.
>
>Graham

Even better. At 10 mA emitter current, output impedance will be under
3 ohms, so a reasonable load will hardly pump the b-e junction
nonlinear.

John
Author: Eeyore
Date: 18:33 14-09-06



John Larkin wrote:

> On Thu, 14 Sep 2006 18:50:39 +0100, Eeyore
> <rabbitsfriendsandrelations@REMOVETHIS.hotmail.com> wrote:
> >John Larkin wrote:
> >> On Wed, 13 Sep 2006 20:20:16 +0200, "Ban" 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.
> >
> >You could also use a constant current load too.
> >
> >Graham
>
> Even better. At 10 mA emitter current, output impedance will be under
> 3 ohms, so a reasonable load will hardly pump the b-e junction
> nonlinear.
>
> John

With Re as low as that you'll most likely be seeing the reflected ( terminology ? )
impedance of the previous stage in fact.

Graham
Author: Ban
Date: 00:21 15-09-06

John Larkin wrote:
> On Thu, 14 Sep 2006 18:50:39 +0100, Eeyore
> <rabbitsfriendsandrelations@REMOVETHIS.hotmail.com> wrote:
>
>>
>>
>> John Larkin wrote:
>>
>>> On Wed, 13 Sep 2006 20:20:16 +0200, "Ban"
<bansuri@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.
>>
To drive the output load the stage needs a varying current, which in return
requires a slightly different Vbe. The smaller the load, the more the
variation in Vbe, which has an exponential characteristic. The inherent
feedback helps linearizing, but for lower R_load and higher capacitive
loading distortion shows up immediately.


>> You could also use a constant current load too.
>>
>> Graham

That would be a cure, but then any load impedance will annulate the effort.

>
> Even better. At 10 mA emitter current, output impedance will be under
> 3 ohms, so a reasonable load will hardly pump the b-e junction
> nonlinear.
>
> John

The output impedance is (1/gm + Rg/Hfe)||Re. With 50k input impedance we
will hardly come below 100R.
So even if the simple follower has a very high input and low output
impedance, its properties cannot be used simultaneously. Those disadvantages
can be overcome with some more sophisticated circuitry like the
White-follower or this differential input stage:
+---+------o
| | +Vb
/ \ |
( 1m) |
\_/ |
| |
| |/
+-|
| |>
| |
.---)---+ out
| | +------o
|/ \| |
o---| |-+
|> <| |
| | |
'-+-' |
| |
.-. |
| | / \
| | (10m)
'-' \_/
| | -Vb
+-----+------o
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
But for driving long cables I would recommend rather the opamp buffer
solution with a complementary output stage.
--
ciao Ban
Apricale, Italy



Author: tempus fugit
Date: 09:33 15-09-06

Hi Ee

I actually discovered the J105 and was thinking of trying to use it somehow
already.

I haven't really worked with JFETs much before, and am only now becoming
familiar with the open collector thing (which would be open drain in this
case?). Could we move this discussion to ABSE and I can post a schematic of
what I think it should be set up like? I'm also unsure of the biasing
required.

Thanks


"Eeyore" <rabbitsfriendsandrelations@REMOVETHIS.hotmail.com> wrote in
message news:450995D1.5395472A@REMOVETHIS.hotmail.com...
>
>
> tempus fugit wrote:
>
> > Thanks again for all the replies.
> >
> > >
> > > It's daft ! For starters you'l get a 'DC thump' every time the
follower
> > stage is
> > > switched on or off.
> >
> > OK this answers one of my other possible problems - I wasn't sure if the
> > switching on/off was going to cause a bigger noise than the relays
popping.
> > I had planned on ramping up the voltage to the follower, but I don't
know if
> > that would help or not.
> >
> > >
> > > JFETs make very very acceptable audio switches when used correctly.
> >
> > I'm intrigued. Any suggestions on how to use one for this application?
>
> Dead easy.
>
> Use a nice low Ron jfet like a J108 and pull it off with a negative
voltage on
> the gate.
> http://www.fairchildsemi.com/pf/J1/J108.html
>
> Use an open-collecter arrangement to drive the gate with say 10M from
gate to
> source. The signal path is from drain to source ( bidirectional ). Use a
10k
> 'pull down' R on both in and out to avoid any DC levels getting into the
switch.
>
> If finding a negative voltage is a problem use a p-channel device like a
J174
> and pull the gate positive.
> http://www.fairchildsemi.com/pf/J1/J174.html
>
> The n-channel parts have lower Ron as do the lower numbered devices in
each
> family. The tradeoff is a higher Vgs to turn them off.
>
> Graham
>
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