Base common

In electronics, a bases common is a type of Amplificateur electronics using a bipolar Transistor. The basic term commune comes owing to the fact that the electrode “bases” transistor is connected to the mass. In this configuration, the entry signal is applied to the transmitter, the output signal being recovered to the collector.

This circuit is not employed more in low frequency, but it is often used for the amplifiers which require an impedance of exceptionally low entry, such as for example them Microphonous S with mobile reel. This circuit is very much used in the high frequency amplifiers, such as for example for VHF or the UHF because its input capacitance is not increased by the Effet Miller, person in charge of the fall of the high frequency profit of the assembly with transmitting commun run. It is also often used in the amplifiers on several floors because its current of entry depends very little on its output voltage.

Another application of this assembly is the “buffer” of current, the current of I_E entry being almost equal to current I_C the output.

Characteristics in small signals

; Note: The parallel lines indicate that the components are laid out in parallel.

Profit in tension

{V_ \ mathrm {out} \ over V_ \ mathrm {in}} = g_m (R_ \ mathrm {C} \| R_ \ mathrm {load}) \,

Resistance of entry

r_ \ mathrm {in} = R_ \ mathrm {E} \ left \| {r_ \ pi + R_1 \| R_2 \ over 1 + \ beta_0} \ right.

Profit while running

A_ \ mathrm {VM} = {r_ \ mathrm {in} \ over R_ \ mathrm {load}}

Resistance of exit

r_ \ mathrm {out} = R_ \ mathrm {C} \,

The variables not listed on the diagram are:

g_m : the Transconductance in mho, calculated thanks to $g_m = {I_ \ mathrm {C} \ over V_ \ mathrm {T}} \,$ , with:

$I_ \ mathrm {C} \,$ : the current of polarization of the collector,
$V_ \ mathrm {T} = {kT \ over Q} \,$ is the thermal tension . It depends on the Boltzmann constant K , of the Elementary charge Q , and of the temperature T of the transistor in Kelvin S. With room temperature it is of 25 mV (cf Google calculator).

$\ beta_0 = {I_ \ mathrm {C} \ over I_ \ mathrm {B}} \,$ is the profit while running at low frequencies (commonly called h_FE). It is a parameter specific to each transistor. It is indicated in its data sheet.
$r_ \ pi = {\ beta_0 \ over g_m} = {V_ \ mathrm {T} \ over I_ \ mathrm {B}} \,$

See too

Internal bonds

Amplifying electronics
amplifying Assemblies:

For bipolar Transistor:

common Base;
Collecting commun run;
Transmitting commun run.

For Field-effect transistor:

common Drain;
common Grid;
common Source.

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