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My question regards BJT and JFET devices.

Noise figures for transistors are usually given along with specific signal source resistances and collector / drain currents.

How can one apply these figures if the device is in common base / common gate configuration? What is the signal source resistance in these cases?

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  • \$\begingroup\$ Maybe you should quote an example or link to a transistor whose data sheet specifies this. \$\endgroup\$ – Andy aka Jan 26 '17 at 11:28
  • \$\begingroup\$ Of course. Let's see 2N4403: tme.eu/es/Document/563e6f237c698909b9b993e58450b503/5326116.pdf. The noise performance of the device is given on the bottom of page 4 at certain collector currents, source resistances and frequencies. In case of the J309 and J310 JFETs, the noise performance is given as the equivalent short-circuit input noise voltage: danssmallpartsandkits.net/J309-D.pdf at the bottom of page 2. \$\endgroup\$ – netom Jan 26 '17 at 12:35
  • \$\begingroup\$ Also draw a schematic and identify the impedances on it. In common base, the signal source resistance is still the source impedance of the signal, i.e. the circuit driving the emitter, while the base is (or should be) effectively shorted to 0V (at AC). \$\endgroup\$ – Brian Drummond Jan 26 '17 at 12:37
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One model for bipolar noise is the Rbb, the resistance from Base gold bond wire down thru some doped silicon, to the base region between Emitter and Collector.

Regardless of the topology, that Rbb contributes to the noise.

Thus I think the NF is the same for CCE and CB.

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