Transistor as a switch
We shall try to understand the operation of the transistor as a switch by analysing the behaviour of the base-biased transistor in
CE configuration as shown in Fig.(a). Applying Kirchhoff-s voltage rule to the input and output sides of this circuit, we get
`V_(BB) = I_B R_B + V_(BE)` and
`V_(CE) = V_(C C) -I_CR_C`
We shall treat `V_(BB)` as the dc input voltage `V_i` and `V_(CE)` as the dc output voltage `V_o`. So, we have
`V_i = I_B R_B + V_(BE)` and
`V_o = V_(C C) - I_CR_C`
Let us see how `V_o` changes as `V_i` increases from zero onwards. In the case of Si transistor, as long as input `V_i` is less than 0.6 V, the transistor will be in cut off state and current `I_C` will be zero.
Hence `V_o = V_(C C)`
When `V_i` becomes greater than 0.6 V the transistor is in active state with some current `I_C` in the output path and the output `V_o` decrease as the term `I_CR_C` increases. With increase of `V_i` , `I_C` increases almost linearly and so `V_o` decreases linearly till its value becomes less than about 1.0 V. Beyond this, the change becomes non linear and transistor goes into saturation state. With further increase in Vi the output voltage is found to decrease further towards zero though it may never become zero. If we plot the `V_o` vs `V_i` curve, [also called the transfer characteristics of the base-biased transistor (Fig.(b)], we see that between cut off state and active state and also between active state and saturation state there are regions of non-linearity showing that the transition from cutoff state to active state and from active state to saturation state are not sharply defined.
Let us see now how the transistor is operated as a switch. As long as `V_i` is low and unable to forward-bias the transistor, `V_o` is high (at `V_(C C) ).` If `V_i` is high enough to drive the transistor into saturation, then `V_o` is low, very near to zero. When the transistor is not conducting it is said to be switched off and when it is driven into saturation it is said to be switched on. This shows that if we define low and high states as below and above certain voltage levels corresponding to cutoff and saturation of the transistor, then we can say that a low input switches the transistor off and a high input switches it on. Alternatively, we can say that a low
input to the transistor gives a high output and a high input gives a low output. The switching circuits are designed in such a way that the
transistor does not remain in active state.
CE configuration as shown in Fig.(a). Applying Kirchhoff-s voltage rule to the input and output sides of this circuit, we get
`V_(BB) = I_B R_B + V_(BE)` and
`V_(CE) = V_(C C) -I_CR_C`
We shall treat `V_(BB)` as the dc input voltage `V_i` and `V_(CE)` as the dc output voltage `V_o`. So, we have
`V_i = I_B R_B + V_(BE)` and
`V_o = V_(C C) - I_CR_C`
Let us see how `V_o` changes as `V_i` increases from zero onwards. In the case of Si transistor, as long as input `V_i` is less than 0.6 V, the transistor will be in cut off state and current `I_C` will be zero.
Hence `V_o = V_(C C)`
When `V_i` becomes greater than 0.6 V the transistor is in active state with some current `I_C` in the output path and the output `V_o` decrease as the term `I_CR_C` increases. With increase of `V_i` , `I_C` increases almost linearly and so `V_o` decreases linearly till its value becomes less than about 1.0 V. Beyond this, the change becomes non linear and transistor goes into saturation state. With further increase in Vi the output voltage is found to decrease further towards zero though it may never become zero. If we plot the `V_o` vs `V_i` curve, [also called the transfer characteristics of the base-biased transistor (Fig.(b)], we see that between cut off state and active state and also between active state and saturation state there are regions of non-linearity showing that the transition from cutoff state to active state and from active state to saturation state are not sharply defined.
Let us see now how the transistor is operated as a switch. As long as `V_i` is low and unable to forward-bias the transistor, `V_o` is high (at `V_(C C) ).` If `V_i` is high enough to drive the transistor into saturation, then `V_o` is low, very near to zero. When the transistor is not conducting it is said to be switched off and when it is driven into saturation it is said to be switched on. This shows that if we define low and high states as below and above certain voltage levels corresponding to cutoff and saturation of the transistor, then we can say that a low input switches the transistor off and a high input switches it on. Alternatively, we can say that a low
input to the transistor gives a high output and a high input gives a low output. The switching circuits are designed in such a way that the
transistor does not remain in active state.