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Dynamic Switching Operation of BJT

This topic assumes user's familiarity with Bipolar Junction Transistor (BJT). In this applet the dynamic operation of BJT is simulated, showing in particular that the switching times are related to the stored minority charge in Base and that the slow turnoff response of BJT is due to the saturation of BJT. This applet deals with the switching speed, and the associated physical processes, of BJT. This topic is usually an earlier topic in the study of digital circuits involving bipolar transistors.

Among the digital circuits involving BJT, the gate delay times improve from Resitor Transistor Logic (RTL), to Diode Transistor Logic (DTL), and to Transistor Transistor Logic (TTL) and Emitter Coupled Logic (ECL). The gate delay time of modern TTL can be as low as 1.5 ns, that of ECL is less than 1 ns in SSI (Small Scale Integration) and MSI (Medium Scale Integration) and even shorter in VLSI (Very Large Scale Integration). ECL finds application in digital communication circuits and other high speed circuits. The turn-on and turn-off delay times that you will see in this applet are much longer than 1 ns.

At present time TTL continues to be used although much of its application grounds has been taken up by CMOS. For low gate delay times (or for high speed operation), keeping BJT out of saturation is important. Both TTL and ECL avoid transistor saturation. Still emerging digital technologies are GaAs and BiCMOS. BiCMOS combines advantages of both CMOS and bipolar circuits and provides the means for realizing very dense, low-power, high-speed integrated circuits.

Purposes of this applet are to teach and learn the following:

  1. associate the aspects of dynamic switching operation of BJT to physical processes in the BJT,
  2. in particular, associate switching times to charge in Base,
  3. 'experience' the slow response of saturated BJT, and
  4. use the Schottky-diode clamp to remove saturation in BJT.
We employ a Resistor Transistor Logic inverter circuit in this applet. The inverter action is accomplished through the simple equation: Vo = Vcc - Rc * Ic.   When Vi is high, Ib is high and thus Ic is high. Therefore Vo is low. When Vi is low, Vo is high.

The main screen view is composed of five sections: i) top left part is the basic RTL circuit where the switching is done by user's mouse click in the input voltage area; ii) below the RTL circuit there are four graphs, Vi (input voltage), iB (Base current), Qb (total excess minority carrier charge in Base), and iC (Collector current); iii) top-center part of screen shows a BJT device schematic; iv) top-right part of screen is the excess minority charge profile in Base; and v) bottom-right part can show three different images where the image is selected by the Choice item in the right hand side of the top-bar.

The top-bar ('North' bar) has four items: first item to control Schokky-clamp on BJT, fourth item to select the image in the bottom-right part of screen, second item is a button to view (and chang if you wish) the circuit parameter values, and the third item is also a button to view (and change if you wish) the BJT parameters.

The bottom-bar ('South' bar) is to control the animation. The first two buttons are to speed up or down the animation [each time you click on the button, the speed changes], the third button is to pause or resume the animation, and the fourth button, "StepForward", is to move forward at discrete time steps. The fourth button is enabled only when the animation is in the PAUSE mode (by the third button).

In the RTL circuit the time elapsed is shown. The elapsed time is reset every time the input voltage is switched by your mouse click.

Follow tutorial, worksheet and other supplementary materials to play with this applet and learn.