Polarization transistor

1) Problematic relative to the determination of the job point:

The straight one of cargo in continuous determines the point of job in continuous, then to varying of marks them of income has the oscillation of marks them of escape in the escape characteristic, in all this but attention must be made that it is always remained in the active zone and that they are respects the following limits to you:

to) Current maximum it is tied to the dimension of the threads that they join reoforo and semiconductor

b) Tension massima is tied to the break-down and to the reach-through therefore it depends also on the logon

c) Power massima is tied to the adopted method of dissipation

d) V_BE the maximum

in a generalized manner the job point is chosen so as to to halve the straight one of cargo in continuous, after which it is looked at which it would have to be the slope of the straight one of alternated cargo in, passing for the job point and trasla sin when the new point of job does not divide it in two equal parts. It is observed that the two straight ones have various slopes in how much in continuous consider R_C and R_and while in alternated they consider R_C and R_L .

 

2) Problematic relative to the common emettitore with fixed polarization:

It is come true with a npn whose current of base_{the B} is obtained through one resistance R_B connected to the V_CC . Fixed R_B determines_{the B} however the job point can vary is with the temperature that replacing the transistor, in order to obviate to this disadvantage is necessary that_{the B} can vary as it happens in the circuit for the automatic polarization.

 

3) Sources that determine the instability of_{the C} :

to)       the current of inverse saturation_{the CO} doubles every increase of 10° of the temperature

b)       V_{the BE} diminishes of 2,5mV/°C

c)       b increases with the temperature

4) Difference between stabilization and thermal compensation:

The stabilization is come true with elements resists to you and is time to render minimums the effects of the temperature on the job point while the compensation is time to cancel such effects, has the only disadvantage that is more expensive.

 

5) Describe the circuit for the automatic polarization:

One places on the base a resistivo partitore and a R resistance_and on the emettitore, through it current slides one that is equal to the sum of_{the B} and_{the C} which depends on the current of inverse saturation_{the CO} that varies with the temperature.

If _{the B} increases _{the C} diminishesthe current which pilot the amplification and therefore _{the C} diminishes .

 

6) Factors of sensibility:

_{The C} depends on the temperature tramite b, V_BE e I_CO , its variation can be written in which the

One finds that these factors of sensibility come diminish if the resistance is increased to you on the emettitore, which cannot but to be much elevating in how much otherwise is necessary one feeding tension much elevating for having one current of decent intensity.

Often a condenser in parallel to the R is placed_and in order to make that the stabilization only happens for the variations due to the temperature which have frequency much lowland and for the variations of it does not mark them of income, is had therefore that in presence of marks them of income, the R_and comes bypassata from condenser C_and , often in parallel to this last one that is much large one it places a smaller condenser which has the task of bypassarlo to the elevated frequencies more where the electrolytic one does not answer adequately.

 

7) Sensibility of_{the C} to the variations of_{the CO} :

With reference to the circuit to automatic polarization the equation of the mesh of income is written and the obtained _B from the equationof the transistor is replaced in it , carrying b to the numerator obtains , rewrites it considering the variations of_{the CO} and consequently of_{the C} and placing ha , such factor of sensibility is considered mostly for the transistors to the germanium.

 

8) Sensibility of_{the C} to the variations of V_{the BE} :

Writing it it is for V_{the BE1} that for V_{the BE2} that _{the C1} and one determinerespective one_{the C2} , considering the single terms that varies and embezzling member to member have and therefore , such factor of sensibility is considered mostly for the transistors to the silicon.

 

9) Sensibility of_{the C} to the variations of b :

The equation of the mesh of income is written and is replaced in it, collecting, considering the sun variations and being , embezzling 1 to both is had the members obtains that being b > > 1 he is negligible.

 

10) Describe the circuit that executes the thermal compensation regarding_{the CO} :

A diode valve polarized between base and emettitore is placed inversely, in it therefore the current of saturation _Or the slides that is function of the temperature therefore like_{the CO} in the transistor whose current of base is therefore I_B= II₀ that replaced in the equation of the transistor and therefore_{the C} turns out independent from the variations of_{the CO} with the temperature.

 

11) Describe the circuit that executes the thermal compensation regarding V_BE :

A transistor with collector and base cortocircuita to you is placed between the base and the emettitore of the transistor to compensate, in this way is had , that determines that if the transistors are identical as pure the resistances on their collectors, has_{the C1} =_{the C2} but of the rest_{the C1} can is rendered constant in fact has with obvious conditions. The circuit however works better if they are the currents_{the B} of the transistors to being equal and V_{the BE} therefore on the bases do not become part the resistances ₂R and ₃R= R₂ , in such a way if the transistors is equal has_{the C1}=_{the C2} and of the rest_{the C1} can be deducted from the equation to the external mesh is had in fact is observed moreover that therefore if is had and therefore the transistor is polarized to the center of the characteristic.