Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 333

current is concentrated in a very small area. The high
current, together with the voltage across the transistor,
causes intense heating, melting a hole from the collector
to the emitter. This causes a short circuit and internal
breakdown of the transistor.
The fundamental limitation to the use of transistors
is the breakdown voltage (BVcer). The breakdown
voltage is not sharp so it is necessary to specify the
value of collector current at which breakdown will
occur. This data is obtained from the data sheet of the
manufacturer.

Transistor Load Lines. Transistor load lines are used
to design circuits. An example of circuit design uses a
transistor with the following characteristics:

Maximum collector current 10 mA
Maximum collector voltage –22 V
Base current 0 to 300 μA
Maximum power dissipation 300 mW

The base current curves are shown in Fig. 12-31A.
The amplifier circuit is to be Class A, using a
common-emitter circuit, as shown in Fig. 12-31B. By
proper choice of the operating point, with respect to the
transistor characteristics and supply voltage, low-distor-
tion Class A performance is easily obtained within the
transistor power ratings.
The first requirement is a set of collector-current,
collector-voltage curves for the transistor to be
employed. Such curves can generally be obtained from
the data sheets of the manufacturer. Assuming that such
data is at hand and referring to Fig. 12-31A, a curved
line is plotted on the data sheet, representing the
maximum power dissipation by the use of the equation

(12-31)

or

(12-32)

where,
Ic is the collector current,
Pc is the maximum power dissipation of the transistor,
Vc is the collector voltage.

At any point on this line at the intersection of VcIc,

the product equals 0.033 W or 33 mW. In determining

the points for the dissipation curve, voltages are

selected along the horizontal axis and the corresponding

current is equated using:

(12-33)

The current is determined for each of the major

collector-voltage points, starting at 16 V and working

backward until the upper end of the power curve inter-

sects the 300μA base current line. After entering the

value on the graph for the power dissipation curve, the

area to the left of the curve encompasses all points

within the maximum dissipation rating of the transistor.

The area to the right of the curve is the overload region

and is to be avoided.

The operating point is next determined. A point that

results in less than a 33 mW dissipation is selected

somewhere near the center of the power curve. For this

example, a 5 mA collector current at 6 V, or a dissipa-

tion of 30 mW, will be used. The selected point is indi-

Ic

Pc

Vc

=-----

Vc

Pc

Ic

=-----

Figure 12-31. Load-line calculation curves.

Collector-to-Emitter Voltage (VCE)

Collector Current (

I)–mAc

A. Common-emitter-collector family of curves, with

load line and maximum dissipation power curve.

IC

IB

VCE

RB

RE

RS

RL

B. Amplifier circuit used for load-line calculations.

IC 1 – max ConstantDissipation Curve

Maximum

Operating Point

Collector-To-Emitter Voltage

OriginalPoint VC 1 – max

C. Load line moved to right for maximum

power output. Dotted lines are the original

load line and operating point.

Collector Current

Output

Input

+

C Vdc

12

10

8

6

4

2

0

0 2 4 6 8 10 12 14 16 18 20

270 MA^240 MA 210 MA

180 MA

150 MA

120 MA

90 MA

60 MA

30 MA

0 MA

Load lineMaximum dissapation33 mW

Base current

300 MA

IC

PC

VCE

=---------