As discussed in another article, the bipolar transistor is a good switching device because of its large transconductance Gm.  The same article showed the simple circuit in Figure 1 below, wherein a single NPN transistor is used as a switch for energizing or powering off the load resistor connected between the collector and the positive supply.

One problem with the simple switch circuit in Figure 1 is the fact that a stray capacitance exists between the transistor's collector and its grounded emitter, such that the switch-off time of the transistor is slower than its switch-on time.  This is because during switch-off, this stray capacitance has to charge first through the load resistor before the load current stops.  During switch-on, on the other hand, this stray capacitance needs to discharge to ground, which is easily accomplished by the conducting transistor. The slower charging up of the stray capacitance compared to its quick discharging is the reason why the switch-off of Figure 1's circuit is slower than its switch-on.

The circuit in Figure 2 addresses the limitation of the circuit in Figure 1.  Two output transistors are used in this circuit, driven by a single input transistor.  The output of this circuit is taken from the collector of the lower transistor.  Just like the circuit in Figure 1, this circuit is an inverting circuit, i.e., the output signal has a phase that's opposite that of the input signal.  Thus, the output is low if the input is high and the output is high if the input is low.

Figure 1. A simple switch using an NPN transistor	Figure 2. A circuit with two output transistors, allowing equally rapid switch-off and switch-on

If the input is high, the upper output transistor goes into cut-off because its base voltage is pulled down by the conducting input transistor.  Meanwhile, the lower output transistor saturates because the conducting input transistor is supplying its base with a higher current.  Such conditions immediately pulls down the collector of the lower output transistor to almost ground level, i.e., the output goes 'low'.

On the other hand, if the input is low, the input transistor stops conducting, causing the voltage at the base of the upper output  transistor to be pulled up by the positive supply, thereby turning it on.  Meanwhile, the non-conducting input transistor prevents the base of the lower output transistor from receiving any current, driving it into cut-off.  With the lower output transistor in cut-off and the upper output transistor conducting, the output of the circuit is pulled up towards the positive supply, i.e., the output goes 'high.'

The circuit in Figure 2 allows the output to switch off as fast as its switch-on, since the conducting lower output transistor immediately pulls the output to ground during switch-off.

See Also:   Bipolar Transistor;  The BJT as a Switch;  Analog Electronics;  More Articles