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'[EE]: Analog switch using 2N3904'
2002\07\11@025028 by Thomas N

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Hello everyone,

I have two audio channels feeding the one audio amplifier.  I want to add an analog switch between the signal and the amplifier so I can turn the switch on when I want the audio from a channel to go to the amplifier.  I came up with a circuit, but it has some problem.

My circuit is like this:  
The input signal is AC-couple by a 100n capacitor.  The other end of this capacitor is connected to the Collector of the 2N3904.  There is also a voltage divider (two 10k ohms) at the Collector of the transistor to bias the Collector voltage at 1/2 VCC.  The Emiter is the output of the switch and it has 10k to Ground.  The base of the transistor is connected to a 10k resistor.  This is the control signal to turn on the switch.
The switch work great, but with one bad thing; when the switch is turned on, the DC bias at the output of the switch is at about 1/2 VCC.  When the switch is turned off, this bias goes down to ground.  Due to the bias voltage change, there is a glitch on a input of the amplifier and cause a "click" sound.  My problem is right here.  How to keep this bias so there is no "click" sound.

Do you have any idea how I can improve the circuit so my output bias voltage remains at 1/2 VCC when the trasistor is turned on or off?  Do you have any similar analog switch using a transistor?  Anything would be helpful.

Thank you in advance!
Thomas

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2002\07\11@035541 by Vasile Surducan

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Thomas, maybe you can inspire yourself from the way I had polarised a fet
in www.geocities.com/vsurducan/electro/FET/Volume.htm
Is not a switcher but is fairly close.

regards,
Vasile

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On Wed, 10 Jul 2002, Thomas N wrote:

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2002\07\11@042136 by Mike Pink

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Hi Thomas

Why not use an Intersil analogue IC They are about $1

Mike


----- Original Message -----
From: "Thomas N" <spam_OUTthomasn101TakeThisOuTspamHOTMAIL.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, July 11, 2002 7:49 AM
Subject: [EE]: Analog switch using 2N3904


Hello everyone,

I have two audio channels feeding the one audio amplifier.  I want to add an
analog switch between the signal and the amplifier so I can turn the switch
on when I want the audio from a channel to go to the amplifier.  I came up
with a circuit, but it has some problem.

My circuit is like this:

The input signal is AC-couple by a 100n capacitor.  The other end of this
capacitor is connected to the Collector of the 2N3904.  There is also a
voltage divider (two 10k ohms) at the Collector of the transistor to bias
the Collector voltage at 1/2 VCC.  The Emiter is the output of the switch
and it has 10k to Ground.  The base of the transistor is connected to a 10k
resistor.  This is the control signal to turn on the switch.

The switch work great, but with one bad thing; when the switch is turned on,
the DC bias at the output of the switch is at about 1/2 VCC.  When the
switch is turned off, this bias goes down to ground.  Due to the bias
voltage change, there is a glitch on a input of the amplifier and cause a
"click" sound.  My problem is right here.  How to keep this bias so there is
no "click" sound.

Do you have any idea how I can improve the circuit so my output bias voltage
remains at 1/2 VCC when the trasistor is turned on or off?  Do you have any
similar analog switch using a transistor?  Anything would be helpful.

Thank you in advance!
Thomas

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2002\07\11@050211 by Roman Black

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Hi Thomas, yes it will work but you need to remove
the voltage divider from the collector and put it on
the emitter instead. The collector needs no resistors,
you can just cap couple it (but use larger cap like
10uF!!).

For it to work well you need to keep the voltage divider
fairly beefy, say 1k resistors, and a MUCH higher
resistor to the base, like 56k. This is so the base will
float with respect to the emitter volts which are signal
dependant. I recommend keeping signal less than 1/2 Vcc
from peak to peak, to give you overhead and obviously
your transistor base driver must be full Vcc or close to
it. This circuit will do what you want, be very high
impedance when off and bias properly at 1/2 Vcc. :o)

PS. Any reason you didn't use a 4016 quad analog switch?
They're cheap you know... :o)
-Roman


Thomas N wrote:
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2002\07\11@050420 by Roman Black

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Vasile Surducan wrote:
>
> Thomas, maybe you can inspire yourself from the way I had polarised a fet
> in www.geocities.com/vsurducan/electro/FET/Volume.htm
> Is not a switcher but is fairly close.

Nice digital pot! Cool that the holding capactor
charge is good from 4 hours to 24 hours! :o)
-Roman

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2002\07\11@125741 by Dwayne Reid

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At 11:49 PM 7/10/02 -0700, Thomas N wrote:
>Hello everyone,
>
>I have two audio channels feeding the one audio amplifier.  I want to add
>an analog switch between the signal and the amplifier so I can turn the
>switch on when I want the audio from a channel to go to the amplifier.  I
>came up with a circuit, but it has some problem.
>
>My circuit is like this:
>
>The input signal is AC-couple by a 100n capacitor.  The other end of this
>capacitor is connected to the Collector of the 2N3904.  There is also a
>voltage divider (two 10k ohms) at the Collector of the transistor to bias
>the Collector voltage at 1/2 VCC.  The Emiter is the output of the switch
>and it has 10k to Ground.  The base of the transistor is connected to a
>10k resistor.  This is the control signal to turn on the switch.

Replace the transistor with a J175 FET.  Remove the bias network from the
collector (which is now drain).  Ground the source.  Feed the control
signal into the gate of the FET via a 1M resistor.  Put a 10K resistor in
series with your input signal, tied to the drain.  Take the output from the
Drain terminal.

The FET now acts as a shunt: it mutes the signal by shunting it to
ground.  The attenuation is the ratio between the ON resistance of the FET
and the series resistor.  If the attenuation is not deep enough, cascade 2
sections.

With 0V on the gate, the FET conducts and mutes the signal.  Applying 12V
to the gate turns the FET off and allows the signal to pass.

You can do something very similar with a N-channel MOSFET *if* your input
signal is less than 0.5V P-P.  In this case, applying the gate voltage
mutes the signal.

I use both techniques in my equipment.  The MOSFET version is used for
wireless mic switches (Ref mics and such), the JFET version is used in much
of my intercom equipment.  The nice thing is that you can tailor the
attenuation slope simply by adding a capacitor between G & S, although I
use actual ramp generators in my intercom stuff.  This eliminates any
'click' when the switch mutes or un-mutes the audio signal.

dwayne

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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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2002\07\11@132424 by Thomas N

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Thank you everyone for your inputs.  The reason why I use the transistor
instead of an IC is because I don't have enough room to put the IC, and I am
CHEAP!  I will play around with it a little more and see if I can get the
switch I want.
Regards,
Thomas


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2002\07\12@040010 by Peter L. Peres

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Use two transistors, with the other also feeding the output resistor, but
its control turned on in antiphase to the control of the real switch. When
you finish doing this and it works, count the parts, then go buy a CD4066
for $0.5 and put it in your circuit instead of it, and never look back.

Peter

On Wed, 10 Jul 2002, Thomas N wrote:

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2002\07\12@093339 by Roman Black

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Thomas N wrote:
>
> Thank you everyone for your inputs.  The reason why I use the transistor
> instead of an IC is because I don't have enough room to put the IC, and I am
> CHEAP!  I will play around with it a little more and see if I can get the
> switch I want.
> Regards,
> Thomas

Did you see my post? Your circuit is 99% done, just
lose the collector resistor, move the divider to the
emitter, and it works. :o)
-Roman

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