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1、Switching Characteristics2 Switching a transistor from one state to another is strongly related to the frequency characteristics just discussed.Switching is considered to be a large-signal change,whereas the frequency effects assumed only small changes in the magnitude of the signal.23 Consider an n
2、pn transistor in the circuit shown in Figure a switching from cutoff to saturation,and then switching back from saturation to cutoff.We describe the physical processes taking place in the transistor during the switching cycle.Switching CharacteristicsChange between cutoff to saturationInput base dri
3、ve for transistor switching.3Circuit used for transistor switching.t0:Assume that cutoff VBE=VBBt2the base drive continues to supply base current,driving the transistor into saturation and establishing the final minority carrier distribution in the device.5Switching CharacteristicsCollector current
4、versus time during transistor switching.Input base drive for transistor switching.The charge QBis the excess charge stored in a forward-active transistor,and QBX and Qcare the extra charges stored when the transistor is biased in saturation.The switching of the transistor from saturation to cutoff i
5、nvolves removing all of the excess minority carriers stored in the emitter,base,and collector regions.Charge storage in the base and collector at saturation and in the active modeSwitching Characteristicst=t3，the base voltage VBBwitches to a negative value of-VR.The reverse base current pulls the ex
6、cess stored carriers from the emitter and base regions.Storage time ts：when the transistor is biased in saturation,both the BE and BC junctions are forward biased.The charge QBXin the base must be removed to reduce the forward-biased BC voltage to 0 V before the collector current can change.This tim
7、e delay is called the storage time and is denoted by ts.The storage time is the time between the point at which VBBswitches to the time when the collector current is reduced to 90 percent of its maximum saturation value.Switching Characteristics Fall time tf：during which the collector current decrea
8、ses from the 90 percent to the 10 percent value the BCjunction is reverse biased but excess carriers in the base are still being removed,and the BE junction voltage is decreasing The switching-time response of the transistor can be determined by using the EbersMoll model.The frequency-dependent gain
9、 parameters must be used,and normally the Laplace transform technique is used to obtain the time response.Switching CharacteristicsCollector current versus time during transistor switching.Input base drive for transistor switching.Schottky-Clamped TransistorOne method frequently employed increase th
10、e switching speed is the use of a Schottky-clamped transistorSchottky-clamped transistor is a normal npn bipolar device with a Schottky diode connected between base and collectorThe circuit symbol for the Schottky-clamped transistorWhen the transistor is biased in the forward-active mode,the BC junc
11、tion is reverse biased;hence,the Schottky diode is reverse biased and effectively out of the circuit.The characteristics of the Schottky-clamped transistoror simply the Schottky transistorare those of the normal npn bipolar device.Schottky-Clamped Transistor When the transistor is driven into satura
12、tion,the BC junction becomes forward biased;hence,the Schottky diode also becomes forward biased.The effective turn-on voltage of the Schottky diode is approximately half that of the pn junction The excess minority carrier concentration in the base and collector at the BC junction is an exponential function of VBC If VBC is reduced from 0.5 to 0.3 V,for example,the excess minority carrier concentration is reduced by over three orders of magnitude.Schottky-Clamped Transistor