i think you mean the 4066?
tino





Roman Black <fastvid@EZY.NET.AU>
Gesendet von: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
11.07.02 11:00
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        Thema:  Re: [EE]: Analog switch using 2N3904


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:
>
> 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.

> 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.

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