BJT as Electronic Switch

The transistor can be used as an electronic switch, in addition to an amplifier. As a switch, we use the cutoff and saturation regions of BJT operation.

Cutoff Region. If 0.5 I v or so, the EBJ will conduct negligible current. Also, the CBJ will be reversed biased with a large VCC. Consequently,
iB»0,ic»0,and I E »=0
which means
v O = V CC
These are the cutoff conditions and the BJT is in the “off” state.
Saturation Region. For the “on” state of the switch, we increase vI until the BJT saturates. This occurs when the EBJ and the CBJ are both forward biased. Due to asymmetries in the device fabrication, the voltage drops are different for these two forward-biased junctions.

These are only approximate values for saturated BJTs. The actual values of V CE|set and V EC|sat depend heavily on i C Equivalent circuit models for these saturated BJTs are:
So, with vI “large,” then

With V0= CE|sat then

Remember that because the BJT is no longer operating in the active region, C B i c i B Instead, if the BJT is operating in the saturation mode

Example N24.1. The BJT in the circuit below has 50 <bea
Designing at “electronic switch” has essentially two parts: cutoff and saturation. Cutoff is easy to design. Just make V1 <0.5V or so. Saturation is a bit more difficult to design. We need vI sufficiently large so that the collector current becomes large enough for the CBJ to become forward biased. For this problem, assume the BJT is saturated so that V CE| sat =0.2 V . Therefore,

To saturate the BJT with the smallest b we need to provide

This is IB just on the edge of saturation (EOS). For an “overdrive factor (ODF)” of 10 means we want to force 10 times this current into the base of the BJT:

Therefore, since

Now, with this design and the transistor saturated, what is the “forced” b

This value is much smaller that bmin =50, as expected. Anotherway to compute b forced is to notice:

such that

Using (7) for this example,

Lastly, what happens if b is increased from 50 to 150 as stated in the problem? Will the transistor stay saturated? Yes, it will. Actually, nothing changes in our saturated circuit as b varies.However, b forced becomes smaller indicating that the transistor is becoming more saturated.