Learn EASY| What are transistor switching times (5 important topics)

Transistor Switching Times

We studied that transistor is used as an amplifier when it is in active region. In this section, we will consider the application of the transistor as a switch. A transistor when operated as a switch is either in ON position or OFF condition. When it is ON, the transistor is in the saturation region and when it is OFF, it is in the cutoff region. 

The transistor has many applications as a switch. For example, a digital computer uses thousands of transistor switches. Hence the speed with which a transistor operates

as a switch is of paramount importance. Now let us consider the time response of a simple transistor as a switch.

        

Transistor switching times in graph

Fig(a) shows the circuit diagram of a transistor as a switch. The switch is controlled by the current applied to the base. The response of the transistor for input pulse Vi is shown in Fig(b). This input pulse makes the transition between the voltage levels V2 and Vi. At the transistor is in the cutoff and at V the transistor is in saturation. 

Though the input pulse makes a transition from V to V2 at time t = 0, the output does not immediately respond to the input signal. Instead, there is a time-lapse between the voltage transition of input and output. Similarly, when the input waveform switches from V2 and Vi at time T, the output does not respond immediately and it takes some time to reach the voltage, Vi. Now we will define some of the terminologies used in transistor switching times.

 Transistor switching times are

1.Turn on Time

2.Delay time

3.Rise time

4.Turn off 

5.Fall time

Turn on Time

The delay between the leading edge of input voltage and the time the collector current takes to reach 90% of the maximum value is called the turn-on-time. 

Delay time

It is the time required for the collector current to increase to 0.11e, sat. Three factors

contribute to the delay time. 

(1) A non-zero time is required to charge emitter junction capacitance when the transistor switches from off to on.

 (2) A non-zero time is required for the minority carriers to cross the base region and then to reach the collector region. 

(3) Some time is required for the collector current to rise to 10% of its

maximum.

Rise time

It is the time required for the collector current to increase from 0.11e, sat to 0.91e, sat.

During this interval, the transistor is operating in the normal active region. The rise time is due to the fact that the transistor collector current must traverse the active region before it reaches saturation from the cutoff.

Turn off time

The time required for the collector current to decrease from 1e sat to 0.11e, sat when

Vi goes negative is called turn-off time toff. Storage time is the elapsed time from the trailing edge of the input pulse to the point where Ie just starts to decrease towards zero. This storage time is due to the saturation excess charge that is stored in the base when the transistor is in saturation. The transistor takes some time to remove the excess charge and this time is called storage

time.

Fall time

The fall time ts is the time it takes for the collector current to decrease from le, sat to 0.1e, sat. The fall time is due to the fact that the transistor collector current must traverse the active region before it reaches cutoff from saturation.

Propagation time

The time it takes for a transistor switch to respond to an input signal is called propagation time Tpd.