Why two back to back diodes cannot function as a transistor?

Consider two diodes connected back to back in the configuration shown below

back to aback diodes

back to back diodes

It is obvious that if one junction is forward biased then other junction will be reverse biased consider for example diode D1 is forward biased and diode D2 is reverse biased much like a NPN transistor in active region according to the junction voltages only current order of reverse saturation current flows through the series junctions. This can be explained as follows: the reverse biased diode D2 at most will allow only currents order of reverse saturation currents. Since D1 and D2 are in series only currents order of reverse saturation currents flow through their junctions. It is obvious that this is not the case with the transistor in active region (because of the internal design of transistor). The forward current entering the base is sweeped across into collector by the electric filed generated by the reverse bias voltage applied across the base collector junction.

What is punch through in transistor?

As we increase the reverse bias voltage of base collector junction depletion width increases and effective base width decreases (since base is less doped compared to collector, depletion layer protrudes more into the base the collector hence effective base width gets affected), at some point effective base width approaches zero and transistor will breakdown .This phenomenon is called reach through or punch through.

What is the relation between BVcbo and BVceo?

ANS: BVcbo and BVceo are collector to base breakdown voltage with emitter open circuited and collector to base breakdown voltage with emitter open circuited. The relation between BVcbo and BVceo is

                                                       BVceo = BVcbo*(1/ βdc)^n

Where βdc is DC current gain and n is a constant ranging from 2 to 10.

What is significance of avalanche multiplication factor in transistor?

Avalanche Breakdown occurs due to avalanche multiplication of charge carriers. The reverse saturation current that crosses collector junction i.e. Ico becomes M*Ico where M is avalanche multiplication factor multiplied due to avalanche effect. If reverse bias voltage is increased much beyond BVcbo then the avalanche multiplication factor becomes infinitely large and hence the current through the transistor rises abruptly. Avalanche multiplication factor is a function of voltage between collector and base and is given by

                 M= 1 / (1-(Vcb /BVcbo)^n)

where BVcbo is collector to base breakdown voltages with emitter open circuited

What is signification of emitter degeneration resistance?

ANS:  Emitter degeneration resistance refers to addition of resistance in emitter lead which introduces negative feedback in the amplifier circuit. Some of the significant consequences of adding Emitter degeneration resistance are

a) The voltage gain decreases (output is taken across collector) hence the name degeneration resistance (loss in gain). But gain is more stable with respect to variation in transistor parameters. Due to reduction in voltage gain the allowable input signal excursion is large than the one without emitter resistance before the amplifier enters into nonlinear regime.

b) Bandwidth increases. This is because gain bandwidth product remains constant.

c) Input resistance increases.

d) Non linear distortion and noise levels get reduced.

e) It helps in stabilizing the q point of transistor.

What is significance of emitter bypass capacitor an input output blocking capacitors?

The input and output blocking capacitors acts as short circuit for AC signals and prevents DC signal transmission from DC power supply to AC signal source and to output by blocking DC signals.

 Emitter bypass capacitor acts as short circuit for entire bandwidth of AC signal to be amplified thereby shorting the emitter degeneration resistance (which reduces gain) so that the amplifier gain will be large. For the signals frequencies out of interest i.e. frequencies lying outside frequency spectrum of input AC signal to be amplified, bypass capacitor acts as open circuit so that the gain of amplifier gets reduced due to emitter degeneration resistance thereby improving the efficiency of amplifier. The DC biasing currents and voltages are unaffected by bypass and blocking capacitors as they act as open circuits for DC signals (Impedance offered by capacitor is Xc = 1 / (w*C) where w is frequency of AC signal, w=0 for DC signal, C is capacitance).

Common emitter amplifier circuit

Common emitter amplifier circuit

Equivalent Common Emitter circuit for AC signals

Equivalent Common Emitter circuit for AC signals

Equivalent Common Emitter circuit for DC signals

Equivalent Common Emitter circuit for DC signals

What is the condition to eliminate thermal run away in BJT?

The condition to eliminate the thermal runaway in BJT is to bias the transistor in such a way that the Vce < Vcc/2, where Vcc is the DC power supply, Vce is collector to emitter voltage. This is derived on the fact that the rate at which heat is removed from the junction should be more compared to the rate at which heat is dissipated at the junction.

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