Question

A BJT in a common-base configuration is used to amplify a signal received by a 50 Ω antenna. Assume kT/q = 25 mV. The value of the collector bias current (in mA) required to match the input impedance of the amplifier to the impedance of the antenna is________.

Answer 1

Concept:

The input impedance of a Common Base Amplifier is given as:

\({Z_i} = \frac{{{V_T}}}{{{I_E}}}\)

Also \({I_E} \approx {I_C}\)

Where I_E is emitter current, I_c is collector current & V_T is the thermal voltage, given as:

\({V_T} = \frac{{kT}}{q}\)

For impedance matching, the input impedance of the system must be matching to the output impedance of the preceding system.

Calculation:

50 ohm is the impedance of the antenna which needs to be matched to the input impedance of common base configuration.

Therefore,

\(50 = \frac{{25{\rm{*}}{{10}^{ - 3}}}}{{{{\rm{I}}_{\rm{E}}}}}\)

\({\rm{Ic}} \approx {{\rm{I}}_{\rm{E}}} = 0.5{\rm{\;mA}}\)