Bipolar Junction Transistor Switching Characteristic
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Bipolar Junction Transistor Switching Characteristic

Figure 122.7 illustrates a simple BJT switch (122.7a) and its collector charateristic (122.7b).

(a) Base on the switch-triad model, identify the signal, The on state and the off state of the BJT.

(b) How is the switching characteristic of the BJT implemented given that VCC = 5 V? Assume a transistor-transistor logic (TTL) as follows:
For Vin, logic low (0) = 0 V; logic high (1) = 5 V
For Vout, logic low (0) = 0 V; logic high (1) = 5 V

The strings: S7P5A51 (Physical Change).

The math:
Pj Problem of Interest is of type change (physical change). Transistors are primarily used for signal amplification and switching. Both are change problems.

(a) Signal is voltage. For Vout, on state (logic high) occurs when BJT is in the cuttoff region and off state (logic low) occurs when BJT is in the saturation region.

(b) Load-line equation at the collector circuit:
VCE = VCC - iCRC -------(1)
The load-line is the line through points A and B in figure 122.7(b).
So, from equation (1), Vout = VCE.

When input voltage, Vin = 0 V (logic low), BJT is in the cutoff region.
So, little or no current flows
So, Vout = VCE = VCC
So, output voltage, Vout = logic high (point A on the load-line)

Base current, iB of 50 μA and base resistor 89 kΩ with an offset voltage Vγ = 0.6, will drive BJT into the saturation region.
So, Vout = VCEsat ~ 0.2 V = logic low (point B on the load-line).

So, whenever, Vin is logic high (1), Vout becomes logic low (0).
Whenever, Vin is logic low (0), Vout becomes logic high (1).

Thus the simple BJT switch of figure 122.7(a) is implemented. This BJT is called the inverter because a logic low at the input results in a logic high at the output, and vice versa.

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