# Voltage Divider Bias circuit usage?

#### Petrucciowns

Joined Jun 14, 2009
62
Can anyone give me an example of what a Voltage Divider Bias circuit would be used for. All I can find is theory, but not an example of its operation.

Thanks.

#### beenthere

Joined Apr 20, 2004
15,808
Focus divider for electromagnetic lens focusing in TV CRT.

#### hobbyist

Joined Aug 10, 2008
887
Example: of it's operation..

VCC=10 V.
Rload = 10K ohms.
Zin = > 2K ohms.
Av. = 10
RB1 is the voltage divider resistor from ground to the base of the transistor.
RB2 is the resistor from supply to base, to form a "voltage divider"

Make RC 1/10 Rload = 1Kohms
make RE = 100 ohms. Av. = RC / RE
VC = 5 V.
IC = VC / RC = 5 / 1000 = 5mA.
5mA X RE = VRE = 0.5V.
VBE = 0.7V.
VBE + VRE = VRB1
0.7 + 0.5 = 1.2V. = VRB1

Make RB1 = 2K ohms to satisfy Zin parameter.

ID = divider current = VRB1 / RB1 = 1.2V / 2K = 600 uA.

RB2 = (VCC - VRB1) / ID
(10 - 1.2) / 600 uA. = 14.7K ohms.

make it 15K ohms.

So RB1 and RB2 form the voltage divider to provide a base voltage of 1.2V.
so that in turn can cause a emitter voltage of 0.5V. wich ultimately determines the amount of collector current that willk flow through the transistor.

Now some checking.

VCC X RB1 / (RB1 + RB2) = 1.17V. = voltage at the base.
1.17V. - 0.7 V. = 0.476V. at the emitter.
so 0.467V / 100 = 4.7mA of collector current, assuming emitter asnd collector current are the same in the calculations.

Hope this is a good example to put with the theories you learned.

I keep rereading your post and think maybe this isn't the example your looking for afterall. I don't know.

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