Question on NPN Logic Inverter From Textbook

[rstofer]

Assuming an hFE of 40, I did a little MATLAB thing to figure out what the base resistor should be.  I am also going to assume V_ceSat = 0.2V and Vb = 0.7V.  In any event, I come up with around 20k Ohms for the base resistor to get the device into saturation.  If I were to actually build the circuit, I would use 10k just to make absolutely sure I got the transistor into saturation.

Code: [Select]

[font=courier]
format shortEng
format compact

Vcc     = 5.0       % given
Vsignal = 1.0       % given
VceSat  = 0.2       % assumed
Vb      = 0.7       % assumed

R2      = 8200      % given
VR2     = Vcc - VceSat
IR2     = VR2 / R2

Ib      = IR2 / 40  % assumed hFE = 40
VR1     = Vsignal - Vb
R1      = VR1 / Ib
[/font]
The results look like:

Code: [Select]

[font=courier]
Vcc =
     5.0000e+000
Vsignal =
     1.0000e+000
VceSat =
   200.0000e-003
Vb =
   700.0000e-003
R2 =
     8.2000e+003
VR2 =
     4.8000e+000
IR2 =
   585.3659e-006
Ib =
    14.6341e-006
VR1 =
   300.0000e-003
R1 =
    20.5000e+003
[/font]

This little arithmetic problem will also run in the free Octave (I believe based on other tests).

Biasing to use the transistor as a common emitter amplifier is an entirely different matter.  Here the issue is just to create an inverter.