Page 142 - Op Amps Design, Applications, and Troubleshooting

High-Current Amplifier 125

By scanning a transistor data book (selector guides in particular), a transistor
that satisfies the above requirements can be found. For illustration purposes, let us
select a 2N3440 transistor. The data sheet for this common device is presented as
Appendix 9. Its critical parameters follow:

1. fioih FE 160
2. Base-emitter voltage drop (V EE) 1.3 volts (maximum)
3. Emitter-collector breakdown voltage 250 volts (minimum)
4. Maximum collector current 1.0 amps
5. Power dissipation 10 watts
6. Thermal resistance, junction to case (6j C) 17.5°C/W
7. Thermal resistance, junction to air (8 JA) 175°C /W
8. Maximum junction temperature 200°C

These values exceed our rough, worst-case requirements. Now let us extend our
estimate to include the determination of a heat sink (see Appendix 10 for a more
complete discussion). We will assume that 50°C will be the highest expected ambi-
ent temperature. The required thermal resistance (0j A) can be estimated as follows:

Since the required value of 8 fA is greater than the transistor's 6 fc, this transis-
tor can be used for this application. However, since the required value of thermal
resistance (Qj A) is less than the 6 JA for the transistor, a heat sink will be needed to
ensure safe operation. The required thermal resistance (6 SA) of the heat sink can be
estimated as follows:

Note that the case-to-sink thermal resistance (6 CS) was estimated as 2°C/W. Since
the required thermal resistance is only slightly lower than the transistor's own
thermal rating, it should be easy to find or make a satisfactory heat sink. There are
many transistor/heat sink combinations that are adequate for a given application.
Final selection must include cost and availability considerations.

Determine the Maximum Output Voltage of the Op Amp. The maxi-
mum op amp output voltage is simply V 5AT. Appendix 1 lists the data sheet for a
741. The worst-case saturation voltage is listed as 10 volts for resistive loads of 2 to
10 kilohms. If the load on the op amp is over 10 kilohms, the saturation voltage is