# base bias for transistor help!

#### alitex

Joined Mar 5, 2007
139
hi all
how i can calculate the base bias for this transistor in this circuit and what the capacitor 1nf mean here??

#### alitex

Joined Mar 5, 2007
139

#### bertus

Joined Apr 5, 2008
20,451
Hello,

As far as I know the resistor is used for powering the mic and also biasing the transistor.
The capacitor is for decoupling the RF signal.

Did you also have a look at this thread?
Project: Mini Spy FM transmitter

Bertus

#### Audioguru

Joined Dec 20, 2007
11,249
The 4.7k resistor applies base current to the transistor, powers the Jfet transistor inside the electret microphone and is the load resistor for the Jfet transistor inside the electret microphone.

The 1nF capacitor grounds the base of the transistor at the radio frequency so that it is a common-base amplifier as part of the Colpitts oscillator.

#### alitex

Joined Mar 5, 2007
139
thank's to all...
but my question is how i can calculate the base voltage?
can i consider the capacitor as impedance and calculate it by xc=1/(2*?*f*c)?

Joined Dec 26, 2010
2,148
The capacitor impedance is not relevant to the DC potential on the base. This will depend on the voltage / current characteristics of an unspecified model of microphone.

These devices are sold as audio transducers, not DC voltage references. My guess would be that this would end up with about 1.5V across it, but even with a given make of mic. the bias may not be all that consistent. (Others may know better than me, but I wouldn't expect miracles from this simple circuit.)

#### Audioguru

Joined Dec 20, 2007
11,249
All other FM transmitter circuits use one resistor to power the electret mic (it uses about 0.5mA) and another resistor or two to bias the transistor. The audio from the mic is coupled to the transistor through a capacitor.