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:
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 !
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.
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.