The saturation region model for the bjt shown figure

372	iB	C H A P T E R	t h e
372		C
B		C
		iC
		vCE
		vCE	can be significantly simplified by using simple piecewise-linear models for the diodes.
	vBE	iE
		-
		-	with a voltage source (from Figure 4.33a). In the model in Figure 7.49c, the dependent
		(a)

B	iB	iB′	C
					OFF state model cutoff: when the base current iB is zero, both the diodes are OFF, and
					so is the current source. Figure 7.50a depicts the corresponding BJT model in the cutoff

					iB = iB′

(b)		vCE > vBE − 0.4 V	(7.48)

the emitter diode is ON and the collector diode is OFF, and the active region results. In this region of operation, as illustrated in the active region BJT model in Figure 7.50b,

iB			+
		iC		The 0.6-V source models the corresponding 0.6-V diode drop. Observe further that
		iC		iB = iB′
		βiB′		iB = iB′
		βiB′		in the active region because the base-to-collector diode is off. In the active region, BJTs
vBE	-	+ 0.6 V-


				The condition vCE > vBE − 0.4 V ensures that the base-collector diode stays OFF. The condition states that the collector voltage must not fall below the base voltage by more
		iE
		iE	-
	(c)	E
	(c)	E


	is equivalent to the simpler constraints	vCE > vBE − 0.4 V.
	vBC < 0.4 V		or

factor β, so that the collector current becomes iC = βiB				C H A P T E R		S E V E N						373
				0.4 V				C
				+ -				C
				B	iB			vCE
				+	iB			vCE
				vBE		-		iE			-
				vBE		-					-

iC =			(7.49)
	�βiB			0.4 V			C	+
	�βiB			+ -
The base-to-collector diode in Figure 7.49c helps model the onset of saturation. Speci-fically, saturation results when the both the base-to-collector and the base-to-emitter diodes are ON. When iB > 0, and the condition implied by Equation 7.48 is violated, that is, if vCE = vBE − 0.4 V				B	iB	iB′		βiB′

				vBE		-		iE		-

vBC = 0.4 V		or
								C	iC			+

forward-biased diodes). Because of their low resistance, the path currents are determined	B		+	vCE
by external circuit constraints. By summing voltages along the path E, B, C, we see that	+			vCE
		vBE
Our model is not yet complete. There is one additional state in which the emitter			- iE
diode is OFF and the collector diode is ON, as can happen when the base-to-collector			- iE	-
			E	-

transistor models in various regions of operation.