Pulse Circuits - Transistor as a Switch

A transistor is used as an electronic switch by driving it either in saturation or in cut off. The region between these two is the linear region. A transistor works as a linear amplifier in this region. The Saturation and Cut off states are important consideration in this regard.

ON & OFF States of a Transistor

There are two main regions in the operation of a transistor which we can consider as ON and OFF states. They are saturation and cut off states. Let us have a look at the behavior of a transistor in those two states.

Operation in Cut-off condition

The following figure shows a transistor in cut-off region.

When the base of the transistor is given negative, the transistor goes to cut off state. There is no collector current. Hence I_C = 0.

The voltage V_CC applied at the collector, appears across the collector resistor R_C. Therefore,

V_CE = V_CC

Operation in Saturation region

The following figure shows a transistor in saturation region.

When the base voltage is positive and transistor goes into saturation, I_C flows through R_C.

Then V_CC drops across R_C. The output will be zero.

$$I_C = I_{C(sat)} \: = \: \frac{V_{CC}}{R_C} \: and \: V_{CE} = 0$$

Actually, this is the ideal condition. Practically, some leakage current flows. Hence we can understand that a transistor works as a switch when driven into saturation and cut off regions by applying positive and negative voltages to the base.

The following figure gives a better explanation.

Observe the dc load line that connects the I_C and V_CC. If the transistor is driven into saturation, I_C flows completely and V_CE = 0 which is indicated by the point A.

If the transistor is driven into cut off, I_C will be zero and V_CE = V_CC which is indicated by the point B. the line joining the saturation point A and cut off B is called as Load line. As the voltage applied here is dc, it is called as DC Load line.

Practical Considerations

Though the above-mentioned conditions are all convincing, there are a few practical limitations for such results to occur.

During the Cut off state

An ideal transistor has V_CE = V_CC and I_C = 0.

But in practice, a smaller leakage current flows through the collector.

Hence I_C will be a few μA.

This is called as Collector Leakage Current which is of course, negligible.

During the Saturation State

An ideal transistor has V_CE = 0 and I_C = I_C(sat).

But in practice, V_CE decreases to some value called knee voltage.

When V_CE decreases more than knee voltage, β decreases sharply.

As I_C = βI_B this decreases the collector current.

Hence that maximum current I_C which maintains V_CE at knee voltage, is known as Saturation Collector Current.

Saturation Collector Current = $I_{C(sat)} \: = \: \frac{V_{CC} - V_{knee}}{R_C}$

A Transistor which is fabricated only to make it work for switching purposes is called as Switching Transistor. This works either in Saturation or in Cut off region. While in saturation state, the collector saturation current flows through the load and while in cut off state, the collector leakage current flows through the load.

Switching Action of a Transistor

A Transistor has three regions of operation. To understand the efficiency of operation, the practical losses are to be considered. So let us try to get an idea on how efficiently a transistor works as a switch.

During Cut off (OFF) state

The Base current I_B = 0

The Collector current I_C = I_CEO (collector lekeage current)

Power Loss = Output Voltage × Output Current

$$= V_{CC} \times I_{CEO}$$

As I_CEO is very small and V_CC is also low, the loss will be of very low value. Hence, a transistor works as an efficient switch in OFF state.

During Saturation (ON) state

As discussed earlier,

$$I_{C(sat)} = \frac{V_{CC} - V_{knee}}{R_C}$$

The output voltage is V_knee.

Power loss = Output Voltage × Output Current

$$= \:V_{knee} \times I_{c(sat)}$$

As V_knee will be of small value, the loss is low. Hence, a transistor works as an efficient switch in ON state.

During Active region

The transistor lies between ON & OFF states. The transistor operates as a linear amplifier where small changes in input current cause large changes in the output current (ΔI_C).

Switching Times

The Switching transistor has a pulse as an input and a pulse with few variations will be the output. There are a few terms that you should know regarding the timings of the switching output pulse. Let us go through them.

Let the input pulse duration = T

When the input pulse is applied the collector current takes some time to reach the steady state value, due to the stray capacitances. The following figure explains this concept.

From the figure above,

Time delay(t_d) − The time taken by the collector current to reach from its initial value to 10% of its final value is called as the Time Delay.
Rise time(t_r) − The time taken for the collector current to reach from 10% of its initial value to 90% of its final value is called as the Rise Time.
Turn-on time (T_ON) − The sum of time delay (t_d) and rise time (t_r) is called as Turn-on time.

T_ON = t_d + t_r
Storage time (t_s) − The time interval between the trailing edge of the input pulse to the 90% of the maximum value of the output, is called as the Storage time.
Fall time (t_f) − The time taken for the collector current to reach from 90% of its maximum value to 10% of its initial value is called as the Fall Time.
Turn-off time (T_OFF) − The sum of storage time (t_s) and fall time (t_f) is defined as the Turn-off time.

T_OFF = t_s + t_f
Pulse Width(W) − The time duration of the output pulse measured between two 50% levels of rising and falling waveform is defined as Pulse Width.