Page 141 - Op Amps Design, Applications, and Troubleshooting

124 AMPLIFIERS

In our case, we need an emitter-collector breakdown voltage greater than
15 volts. The exact collector current will be computed later, but we need to esti-
mate a worst-case value so that we can select the transistor. For this purpose, we
can assume that the entire +15 volts of the supply are felt across the 50-ohm
load. Ohm's Law tells us the value of load current.

The actual collector current will be less than this, but this is a good value to use for
initial transistor selection.
Now we need to determine the required current gain (j3) of the transistor. In
data sheets, this is generally labeled as h FE. We can again make a rough estimate
for purposes of transistor selection. If we divide the load current computed above
by half of the short-circuit output current of the op amp, we will have a good place
to start. The following computation gives us the minimum value of j8 that our
transistor should have.

The short-circuit current for a 741 is listed as 20 milliamps, and so in the present

The power dissipation of the transistor can be estimated with the following equation:

2
V
P D(estimate) = -££• (2.54)

where V cc is the positive supply voltage. For the present case, the estimated
power dissipation of the transistor is