Transistor vhf

1) Circuit equivalent to p- hybrid:

Draft of a model for the transistor that is used prettamente in vhf but that it reduces to the model to hybrid parameters in the case of vlves. In truth also the model to hybrid parameters can be used in vhf but in order to make that must be previewed one dependency from the frequency of the been involved resistances.

The circuit is constituted gives:

to)       one resistance r_bb' @ 100W between an inner point to the B' base and the clip of B base

b)       a resistance r_{b' c} @ 4 MW between one inner point to the B' base and the clip of collector C, it is associated to the Early effect that varies the thickness of the base and with it_and and therefore_{the C} .

c)       a resistance r_{b' and} @ 1KW between an inner point to the B' base and the clip of emettitore and, it is associated to the fact that the increase of the minority bearers of the base is translate in an increase of the current of recombination in the same one.

d)       one resistance r_ce @ 100KW tra the clip of emettitore and the clip of collector

and)       a generator of current of value g_mV_{b' and} place between the collector and the emettitore that for small variations of the v_{b' and} represents the proportionality between the current of minority bearers iniettata in the base (…and with it_{the C} ) and the v_{b' and} .

One only differs from the model of Giacoletto for the absence of the abilities introduced from the transistor.

 

2) Model of Giacoletto:

In adding to the model p- hybrid one ability C is had_and between one inner point to the B' base and the clip of emettitore and, such splice is in directly polarized kind therefore draft of one spread ability. One ability C _C betweenone inner point to the B' base is had moreover and the clip of collector C, such splice is in polarized kind inversely therefore draft of one transition ability.

 

3) Value of transconduttanza g_m for the model of Giacoletto:

it is looked at therefore the unusual second relation which a largeness differentiates is given them from the relationship of two largenesses continuous which_{the c} and V_{the T} that is the tension that is worth 26mV to ambient temperature.

 

4) Value of the resistance r_{b' and} for the model of Giacoletto:

 

5) Value of the resistance r_bb' for the model of Giacoletto:

it represents the difficulty that the bearers meet to enter cross-sectionally in the base, it becomes important in vhf for the two following reasons:

to)       increasing the repetition speed del it marks them, affinchè the bearers before reach sul collector that the negative semiwave arrives is necessary to reduce the flight time, to example reducing the amplitude della base.

b)       it is a resistance that looks at all the rest of the model in parallel, when they with frequency C knows itself_and becomes one short circuit therefore is only this resistance to limit the current.

It is observed that being r_bb' < < r_{b' and} this formula is a lot imprecisa and moreover pu² to write itself .

 

6) Gain in current short circuit for the model of Giacoletto:

In the circuit to p- hybrid closed in short circuit they come neglected resistances r_{b' c} and r_ce while condenser C_C is come to find in parallel to C_and from which dividend for g is had_{b' and} and being ottiene dove it is the frequency of advanced cut of the circuit, in fact for f = f_b is had that the module of the current amplification becomes 0,707 times the amplification of the maximum current h_fe that one has to the vlves.

It is observed that the product banda*guadagno is equal to one constant that does not depend on the assembly for which also he will be equal to the same constant, varrà after all the relation that being to @1 and b @100 implies that .

 

7) Frequency of transition f_T :

Draft of the frequency for which the current amplification short circuit to common emettitore, catches up the unitary value, it is worth once and therefore that the job point has been fixed is turned out to be a constant amount and from the first equality it looks at that it is equal to the product of the bandwidth for the amplification and extension the famous concept according to which if in an amplifier increases the bandwidth, consequently the amplification diminishes and viceversa. The frequency fvaried _T with the current of collector_{the C} passing to ambient temperature from 150Mhz to 300Mhz.