small signal NPN transitor for muting microphone

[Phil Allison]

"Kevin White".

620mV.

** Try measuring some - as I suggested.

Data sheets are often great works of fiction.

A 2N3904 collector current at that VBE is about 0.2mA

** Not with a Vce of near zero.

and would
be highly sensitive to any temperature difference between the diode and
the transistor.

** Huh ?

Also because of the large ratio between forward and
reverse Hfe this could also cause significant distortion.

** It would be better to use a Schottky diode instead.





......... Phil

 

[Ian Du Rieu]

What I am trying to do is, mute the microphone by using the transistor as

explained in the circuit. I can un-mute it by closing the switch. All this
works fine on one unit I have modified. I need to modify about 200 units.

My question is, can you see any problem with the circuit?
BTW new parts I will be adding, are:
one 4.7K register, one Transistor, one diode and a switch.

Why not use a MOSFET? It will handle the small AC signals perfectly,
provided you don't yell into the mic & cause the MOSFET's diode to turn
on.

Cheers,
Ian Du Rieu
The Leon Audio Company

 

[Phil Allison]

"Ian Du Rieu"

Why not use a MOSFET?

** That darn body diode plus higher cost.

It will handle the small AC signals perfectly,
provided you don't yell into the mic & cause the MOSFET's diode to turn
on.

** Junction fets are sometimes used ( ie J111, 112, 113 ) - but have on
resistances 50 to 100 times greater than a small signal transistor.



......... Phil

 

[Jim Thompson]

On Mon, 23 Jan 2006 22:10:04 +0100, martin griffith

[snip]

There are similar ccts in Studer tape (B62 and A80RC decks) for
switching the EQ circuits around. I dont have the manuals with me, but
ISTR they used the transistors "upside down" !



martin

That is correct. The drop of a bipolar device, when acting as a
switch, is a function of inverse beta, the higher the inverse beta the
lower the drop.

When a transistor is inverted, then "inverse" beta is actually the
value of the "forward" beta.

Clear as mud, but I hope it covered the ground ?

...Jim Thompson

 

[[email protected]]

When bipolar transistors are are saturated they exhibit an offset
voltage. If the emitter and collector are reversed in your circuit,
then when the bipolar is on it will show virtually no offset voltage,
i.e. the voltage between the emitter and collector is nearly zero.
If it were me, I would not do it this way.
I would use a 2N7000 CMOS transistor which is similar to the
2N2222, except the input impedance of the 2N7000 is infinite.
Next use a 1uF capacitor to couple the 2N7000 drain to the signal
channel. This may prevent a click when the mute is engaged.
If you do get a click, then I would go to the VACTEC
LED/Photoresistor module and also use the 1uF capacitor. The VACTEC
photoresistor may be turned on softly (slowly) and this will allow the
cap to charge slowly, below the bandpass of the channel (<10Hz) thus
avoiding a click.

Charles Gilbert
[email protected]
http://www.NonDigital.Netfirms.com