7. RF Transistor Curve Selection

Describes the RF transistors curve selection of the diode bridge switch.

I got 8 HF transistors.
Two were used and soldered out somewhere, six are new.

Many times I use a curve tracer, this insturments shows faults which you don't see with a DMM diode tester.

For all charts:
vertical         0.5mA/DIV
horizontal     0.5 volts/DIV
base current 5µA/step

No.1  (used transistor)         OK                             No.2 (used) has a fault, a DMM could not find this faulty transistor, negative resistance !

No.3                                                                                                                            No.4

No.5                                                                                                                              No.6

No.7                                                                                                                             No.8

All transistors are different in gain and curve characteristics, choose two transistors with similar curves. 

I choosed No.4 and No.6

Transistor No.4 and No.6 are overlayed with a photo software.
The diode switch runs as comparator, hence matching of both transistors is not so important like with amplifiers.
Accurate selection of transistors is a hard job, need hours. 


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