BJT Circuit Analysis

BJT Circuit Analysis

? Assuming that the transistor is in the active region, solve for the voltages and currents --- why this assumption?

? In general, the problem requires solution of a set of nonlinear equations:

VIN 2V

+

RB 100E3

RC 1E3 Q1

IS=1e-16

= 100

VCC + 5V

Lecture 12-1

BJT Circuit Analysis

? SPICE solves the system of nonlinear equations to obtain the voltages and currents

? Is this circuit in the active region?

IB

12.206 ?A VIN

+ 2V

RB

Q1

100E3 Default

VBE

+

779.365 mV

-

IC 1.221 mA

RC

1E3

VOUT

+

3.779 V

-

VCC

+ 5V

Lecture 12-2

BE Diode Characteristic

? We can effectively use a simplified model for the diode if we know the approximate operating range of the BE diode characteristic

0.000 pA

VBE

+ 0V

Q1 Default

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 3 mA 2

1

0 IE

Lecture 12-3

BE Diode Characteristic

? Note that "VON" changes if we're analyzing an order of magnitude less current

? So how do we know what the real "VON" is?

0.000 pA

VBE

+ 0V

Q1 Default

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.3 mA 0.2

0.1

0.0 IE

Lecture 12-4

Simplified BJT Circuit Analysis

? Assuming VBE is 0.78 volts, we can approximate this circuit solution by hand analysis

VIN 2V

+

RC 1E3

RB 100E3

Q1

IS=1e-16

= 100

VCC + 5V

Lecture 12-5

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