DC Analysis of NPN VDB Circuit

[eev_carl]

Hi,

I built this circuit and modeled it in Multisim, but my calculations for a DC analysis don't match. 

I was expecting VBB = 10 * 4.7 / (4.7 * 10) = 3.2 V.  VE = 3.2 - 0.7 = 2.5 V.  IE = 2.5 / 220 = 11.4 mA.  I get 2.30 V at the base though on both my real and simulated circuits.

Is there something wrong with my selection of resistors?  I tried same algorithm with other values and everything meshed.

Thanks,
Carl

[ledtester]

VBB = 10 * 4.7 / (4.7 * 10) = 3.2 V.

You probably mean 10V * 4.7kR / (4.7kR + 10kR).

You are not accounting for the base-emitter current.

According to the Multisim analysis, the base current is 267uA. Note that 267uA*4.7k = 1.25V which is approximately the difference you're seeing.

[T3sl4co1l]

Notice what I_E * (R3) is.

Tim

[eev_carl]

Thanks all. 

After reading the responses, I found a section in my textbook that said my simplified analysis was only good for *well-designed* circuits with the base current much less than the VDB current.  In what I posted, they're the same order of magnitude (IB=281uA, IVDB=489uA).

The VDB resistor values I wrote were working produced a much larger difference (IB=29uA, IVDB=797uA) for example.

So, bad design on my first pass