Bias Circuit

The equation set above has three unknowns, IC, IB, and VBE. The equations can be combined to
obtain a function for IC,
Equation C.3
A good estimate can be obtained with using VBE
Equation C.4
0.6 V, giving simply
The simple expression is sufficient, for example, for selecting a value for RB in the circuit of
Fig. C.1(a) in the BJT amplifier project. We have also neglected the dependence of βDC on VBC
of (B.11). This is a reasonable approximation for establishing the nominal values for the
measurement circuit components.
The collector – emitter voltage, VCE, is then established with
Equation C.5
In designing a project circuit for a current sweep, RB and RC are selected for the highest
currents and highest DAQ output voltage. In the amplifier project, the circuit of Fig. C.1(b)
uses the resistors from the circuit of Fig. C.1(a). It thus has a unique bias variable and VCC
solution for a given design VCE requirement. This is based on (C.3) and (C.5) and is
Equation C.6
In the amplifier project, the project Mathcad file is used to find the solution for VCC [with an
educated guess for VBE, as in (C.4)]. If the result has VCC > 10 V, the limit from the DAQ, it is
necessary to decrease RC or increase RB. This may be necessary, as the transistor βDC is not
known with precision at the point of the selection of the resistor values. After measuring the
actual VCC, the result is used in the Mathcad file to compute a value for βDC from a circuit
solution.