Post by thread:[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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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 don't only ask, if you know just show us ! : http://www.piclist.com/techref/piclist/pcbcontest.htm On Wed, 10 Jul 2002, Thomas N wrote: {Quote hidden}> 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 > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Hi Thomas Why not use an Intersil analogue IC They are about $1 Mike ----- Original Message ----- From: "Thomas N" <spam_OUTthomasn101TakeThisOuTHOTMAIL.COM> To: <.....PICLISTKILLspam@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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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: {Quote hidden}> > 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. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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 -- Dwayne Reid <dwaynerKILLspamplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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 {Quote hidden}>From: Dwayne Reid <.....dwaynerKILLspam.....PLANET.EON.NET> >Reply-To: pic microcontroller discussion list <EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU> >To: PICLISTspam_OUTMITVMA.MIT.EDU >Subject: Re: [EE]: Analog switch using 2N3904 >Date: Thu, 11 Jul 2002 10:43:05 -0600 > >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 > >-- >Dwayne Reid <@spam@dwaynerKILLspamplanet.eon.net> >Trinity Electronics Systems Ltd Edmonton, AB, CANADA >(780) 489-3199 voice (780) 487-6397 fax > >Celebrating 18 years of Engineering Innovation (1984 - 2002) > .-. .-. .-. .-. .-. .-. .-. .-. .-. .- > `-' `-' `-' `-' `-' `-' `-' `-' `-' >Do NOT send unsolicited commercial email to this email address. >This message neither grants consent to receive unsolicited >commercial email nor is intended to solicit commercial email. > >-- >http://www.piclist.com hint: PICList Posts must start with ONE topic: >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > _________________________________________________________________ Join the world s largest e-mail service with MSN Hotmail. http://www.hotmail.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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: {Quote hidden}>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. > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics