Features of the saturated state of the transistor

Features of the saturated state of the transistor

　　For the transistor to be saturated, the base current must be large enough, i.e. IB≥IBS. When the transistor is saturated, the saturation voltage (UCES) between the collector and the emitter is very small, according to the relationship between the output voltage of the transistor and the output current UCE=EC-ICRC, so IBS=ICS/β=EC-UCES/β≈EC/βRC. When the transistor is saturated, the base current is large, for the silicon , the saturation voltage drop of the emission junction UBES=0.7V (germanium  UBES=-0.3V), while UCES=0.3V, it can be seen that UBE>0, UBC>0, that is, both the emitting junction and the collector junction are forward biased.

　　After the transistor is saturated, the saturation resistance RCE=UCES/ICS between C and E, uces is very small, ICS is large, so the saturation resistance RCES is very small. Therefore, after the triode is saturated, the interval between G and E is regarded as a short circuit, and the equivalent circuit of the NPN-type triode in the saturated state is shown in Figure 1a.

　　Features in the transistor cut-off state

　　To make the transistor in the cut-off state, the base current IB=0 must be required, at this time the collector IC=ICEO≈0 (ICEO is the penetration current, very small), according to the relationship between the output voltage of the transistor and the output current UCE=EC-ICRC, the voltage between the collector and the emitter UCE≈EC.

　　When the transistor is cut-off, the base current IB=0, and the voltage between the collector and the emitter UCE≈ECO can be seen, UBE≤0, UBC<0, that is, both the emitter junction and the collector junction are inversely biased. After the transistor is cut-off, the cut-off resistance RCE=UCE/IC between C and E, uces is very large, equal to the power supply voltage, ICS is extremely small, and the resistance RCE between C and E is very large, so after the transistor is cut-off, C and E are regarded as open circuits, and the equivalent circuit of the NPN type triode in the cut-off state is shown in Figure 1b.

　　Features in the amplified state of the transistor

　　To make the transistor in an amplified state, the base current must be: 01V or more, UBE>0, UBC<0, that is, the emission junction is forward biased and the collector junction is reverse biased.

　　When the transistor is amplified, IB corresponds to IC. As IB increases, the IC also increases, and when 1B doubles, the IC also doubles. Therefore, the IC is mainly controlled by the IB and changes, and the change of the IC is much greater than the change of IB, that is, the collector IC=β×IB.