rating of the transistor Ic(max), VCE (max) and PD(max) should not be exceeded at any value ofĬonsider the fig shown in fig1. The value of the signal Ic when no signal is applied should be at least equal to the max.Vce voltage should not fall below VCE (sat) (0 for Si, 0 for Ge) for any part of the i/p signal.įor VCE less than VCE (sat) the collector base junction is not probably reverse biased.Junction must be reverse biased for all levels of i/p signal. Emitter base junction must be forward biased (VBE=0 Si, 0 for Ge) and collector base.There are four conditions to be met by a transistor so that it acts as a faithful ampr: Selecting proper supply voltages and resistance for obtaining desired operating point or Q point isĬalled as biasing and the ckt used for transistor biasing is called as biasing ckt. ToĮstablish an operating point (proper values of collector current Ic and collector to emitter voltage VCE)Īppropriate supply voltages and resistances must be suitably chosen in the ckt. That means an operating point has to be established in this region. If the o/p signal must be a faithful reproduction of the i/p signal, the transistor must be Gain is gm*Rcollector or 1amp/volt * 1,000 ohms or Av=1,000x.UNIT-IV TRANSISTOR BIASING AND STABILIZATION NEED FOR TRANSISTOR BIASING The Vce is 30 - 1K*26ma = 30 - 26 = 4 volts, so the bipolar is in "linear" region. Suppose we have 1Kohm resistor from collector to +30 volt, carrying 26mA. Or Barry Gilbert's writings on IP2 and IP3 for bipolars. Ignoring some distortion, which you can predict using Taylor Series. Thus 1 millivolt PP on the base causes 1milliAmp PP collector AC current. The gm of a bipolar, at 25 degrees Centigrade, and knowing kt/q is 0.026 volts,Īnd if the Collector current is 0.026 amps (26 milliAmps), the gm is 1 amp per volt. We can honor Lee deForest by using gm for this. Lets view the bipolar transistor similar to vacuum tubes or MOSFETS.as transconductors, where changes in Input Voltage cause changes in Output Current.īipolars are fun to use, because we know EXACTLY the transconductance for any bipolar, if we know the DC collector current (that is, with no input AC signal).įor shorthand, we lable this the 'gM' or 'gm', because vacuum tube databooks used the variable "mutual transconductance" to explain how the Grid voltage controlled Plate current. 3 factors of 58mV below 1uA at 0.4v)Ġ.226 2.718 nanoAmp (the 0.026v of physics gives E^1 more I)Ġ.254 8.000 nanoAmp (you'll find N*18mV in voltage references) And at higher currents, this fine table - showing 58 milliVolts more Vbe produces 10X more current - loses accuracy because the bulk silicon has a linear resistance and curve tracers will show that.Ġ.2 volts 1nanoAmp (approx. Melt the transistor (a known risk with bipolarsĬan we actually operate a bipolar transistor over 1uA to 10Amps collector current? Yes, if its a power transistor. Lets examine a table of Vbe versus Collector Current, for an imaginary bipolar:Ġ.458 10uA Notice 58mV more Vbe gives exactly 10X more current.Ġ.690 100mA [transistor is HOT, so the current may runaway and
#Transistor biased in active region how to
Have a look at 2N5302 which has Ube = 3V at Ice = 30A and Uce = 4V.Īt end of this posting, you'll know how to compute voltage gain of a bipolar. One thing for sure : in analogue applications, the dependence of Ice from Ube must definitely be considered, mainly for high power or high current transistors. The maximal Ube at the maximal Ice is easily 2.5V to 3V for huge power transistors and Ice's greater than 25A. However, as mentioned, the dependence of Ice or, better said, Rce from Ube is heavily non linear after a given point and, thus, the increase of Ice does not lead to a huge increase of Ube, yet, there is such.īelow 0.7V, the increase of Ice can be somewhat linear and this depends on the transistor.
In practice, transistors do get slightly open for any Ube > 0 and continue to increase their openness with the increase of Ube. In practice, Ube can range from 0V to 3V even more for high power transistors. In some low power transistors, this idealised Ube can be 0.6V or 0.65V. Transistors, in theory only, are fully closed for any Ube = 0.7V. Note: you can find the source for the diagram above here. The Ebers-Moll model for the emitter current in a bipolar transistor is:
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