introduction BJTs look like old-fashioned electronic components, but they solve many problems due to their low cost and superior parameters. We can see new applications that were not possible in the past due to the high cost of these components, such as the possibility of replacing more powerful transistors (with or without heat sinks) with multiple parallel low-power transistors in some cases Benefit from it. In general, low-power transistors are faster, have higher operating frequencies, lower noise, and lower total harmonic distortion than higher-power transistors, especially those with bulk heat sinks, and their Packaging more convenient manual and automatic welding. Many of the transistors that consume about 1W of power consume a package similar to the TO-92. Most of these transistors are relatively inexpensive, can be purchased in large quantities, and TO-92 package is very easy to use. The heat generated from these packages is easily dissipated efficiently by cooling fans and even normal air convection. In addition, we can take advantage of the larger copper surface area around these transistors to increase their power consumption. Different packages for these electronic components, their data sheets and documentation, record a large number of cooling information and calculation methods, so we will not discuss in detail here. Power transistor packages such as the TO-126 and TO-220 are large and heavy-weight packages that are difficult to mount on a PCB and require additional heat sinks to take full advantage of the full performance and reliability of these power transistors. These packages and heat sinks can block the flow of cooling air and the use of additional heat sinks creates mechanical and electrical issues such as poor heat sinks in the vibrating equipment and the need for galvanic isolation. Transistor circuit Let us consider the following pairs of NPN / PNP transistors often used in audio drives: TIP29 / TIP30 (NPN / PNP, 40V, 1A, 2W, Ftmin = 3MHz, TO- 220), BD139 / BD140 (NPN / PNP, 80V, 1.5A, 1.25W, Ftmin> 3MHz or not specified, TO-126) BC639 / BC640 (NPN / PNP, 80V, 1A, 0.8W, Ft = 130MHz / 50MHz, TO-92) BC327 / BC337 (NPN / PNP, 45V, 0.8A, 0.625W, Ft (typ) BC550 / BC560 (NPN / PNP, 45V, 0.1A, 0.5W, Ftmin = 100MHz / 100MHz, TO-92) Some of these transistors may vary from manufacturer to manufacturer, and some may not be common to all manufacturers. We can see that the power consumption of the two parallel BC639s is about 1.6W, which exceeds the power consumption of a single BD135 / 137/139 1.25W.