Bipolar junction transistors (BJTs) are often used in discrete amplifiers because they are readily available, inexpensive, and provide excellent gain. However, in operation, bipolar junction transistor amplifiers must be stabilized against variation in device gain, supply voltage, and temperature environment. This is commonly accomplished with either a passive or active DC bias network.

sTransistor is a utility to aid DC biasing transistor amplifiers. It enables the user to switch between twelve of the most common passive and active DC bias configurations and then synthesize a resistor network to support the desired bias state. sTransistor supports a transistor small signal model conditions in calculating the bias conditions. In addition, when appropriate, the bias stiffness can be varied.

The basic start screen is shown above right and to the right. The buttons on the far left support switching between different bias conditions. The bias circuit is shown in the middle and the small signal transistor model is shown below. Above the transistor parameters any specific notes are provided. The user can modify the values shown in the square boxes to meet the desired bias conditions and then press the calculate button in the upper left side of the configuration screen. This will calculate the remaining values.

If for any reason the user wishes to reset or start over, simply pushing the same configuration button on the far left will reset screen to initial conditions.

The twelve DC bias configurations include:
  • a non-stabilized DC bias configuration often used as a switch
  • the voltage feedback bias which uses voltage feedback from the collector to stabilize the amplifier
  • the voltage feedback with voltage source configuration
  • the voltage feedback with current source configuration
  • the universal configuration which has elements of both an emitter stabilized and voltage feedback stabilized configuration
  • the emitter follower
  • the classic voltage divider which uses feedback from the emitter for device stabilization
  • an improved bootstrap configuration
  • a voltage feedback with voltage source where a diode is used
  • an NPN active bias configuration
  • a PNP active bias configuration
  • and a current mirror
sTransistor is available in the Apple App store at: sTransistor.