![]() ![]() How do you calculate saturation of a transistor? Transistor Saturation The voltage between the collector and emitter terminals under conditions of base current or base-emitter voltage beyond which the collector current remains essentially constant as the base current or voltage is increased. Saturation voltage, collector-emitter (VCE(sat)) What is saturation voltage of a transistor? Additionally, it means that V B is higher than V C and V E. And forward biased is when the V BE and V BC are more than zero. Also, this mode makes the transistor’s diodes become forward biased. When a transistor is in saturation mode, it’s “On.” Plus, it behaves like a short circuit between the collector and emitter. What happens when transistor is saturated? When in saturation, the current may be high, but Vce is very small, keeping the power dissipated by the transistor very low. When in cutoff, there is no current flow between collector and emitter thus P = Vce ∗ Ic = 0. ![]() When the transistor is either in saturation or cutoff modes, it dissipates little power. How do you know if a transistor is saturated or active? That’s because Vce is as low as it’s going to go for that particular Ic. In a nutshell, “saturation” for a BJT is the point where a further increase in base current will not result in a corresponding increase in collector current. In other words, V B must be higher than both V E and V C. That means V BE must be greater than 0, and so must V BC. In saturation mode both of the “diodes” in the transistor are forward biased. A transistor in saturation mode acts like a short circuit between collector and emitter. Use of the current gain beta.Īny circuit that depends on a specific value of the current gain β is a bad circuit because that value varies for a given transistor as well as between different transistors of the same type.Saturation is the on mode of a transistor. It increases for larger emitter currents because the larger number of electrons injected into the base exceeds the available holes for recombination so the fraction which recombine to produce base current delines even further. The proportionality β can take values in the range 20 to 200 and is not a constant even for a given transistor. The usual symbols used to express the transistor current relationships are shown. Normal transistor action results in a collector-to-emitter current which is about 99% of the total current. More about transistor regions Transistor action Transistor Action More on transistor regions The base-emitter voltage V BE varies slightly with the collector-emitter voltage V CE at constant collector current I C : ΔV BE ≈ -0.001ΔV CE.The base-emitter voltage V BE is temperature dependent, decreasing about 2.1 mV/C.The effective AC series resistance of the emitter is about 25/I C ohms.An increase in base-emitter voltage V BE by about 60 mV will increase the collector current I C by about a factor of 10.A base emitter voltage V BE of about 0.6 v will "turn on" the base-emitter diode and that voltage changes very little, The value of β is not highly dependable since it depends on I C, V CE, and the temperature.īase-Emitter Junction Details Some useful "rules of thumb" which help in understanding transistor action are (from Horowitz & Hill): A further useful relationship is I C=βI B This relationship is stable over a wide range of voltages and currents. The saturation current is characteristic of the particular transistor (a parameter which itself has a temperature dependence). The collector current is related to this voltage by the Ebers-Moll relationship (sometimes labeled the Shockley equation): The base-emitter voltage V BE can be considered to be the controlling variable in determining transistor action. ![]() In saturation (collector a few tenths of a volt above emitter), large current useful for " switch on" applications.In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications.Cut off (no collector current), useful for switch operation.Transistor Operating Details Transistor OperationA transistor in a circuit will be in one of three conditions ![]()
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