A designer wants a common-emitter stage biased at on a supply

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Semiconductor devices and analogue circuits

Electronics for Measurement, Ch Transistors: a current controlled by a third terminal

Problem

A designer wants a common-emitter stage biased at

I_C = 1.0\,\mathrm{mA}

on a

9.0\,\mathrm{V}

supply, with

R_C = 4.7\,\mathrm{k}\Omega

and

V_{BE} = 0.6\,\mathrm{V}

, using the data sheet's nominal

\beta_F = 150

. The design, exactly as written down, reads: “

I_B = I_C/\beta_F = 1.0\,\mathrm{mA}/150 = 6.67\,\mathrm{\mu A}

. A single resistor from the

9.0\,\mathrm{V}

rail to the base sets this current:

R_B = (V_{CC}-V_{BE})/I_B = (9.0-0.6)/6.67\,\mathrm{\mu A} = 1.26\,\mathrm{M}\Omega

, taken as the nearest E12 value,

1.2\,\mathrm{M}\Omega

. Since

R_B

fixes

I_B

and

I_C = \beta_F I_B

, the collector current is

1.0\,\mathrm{mA}

regardless of which transistor of this type is fitted.” The data sheet gives

\beta_F

from

70

300

for this transistor type, and the transistor saturates once

V_{CE}

falls to about

0.2\,\mathrm{V}

(with the emitter grounded,

V_C = V_{CE}

). ((a)) Find the error in the argument. ((b)) Calculate the actual collector current at

\beta_F = 70

, and the actual collector current at

\beta_F = 300

(checking whether the transistor stays out of saturation in each case).

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