We don't use those curves when designing transistor circuits. It's really not like designing tube amps. Transistor behaves more like a very perfect penthode with collector curves very straight and almost horizontal. Never use load line to design. I think the really old books might have talk about load lines in the 70s that I saw following from the tube days, later on, never seen those load lines.
Transistor has so much gain we never truly ground the emitter and let the transistor running high gain. eg if you ground the emitter and run 1mA through the transistor, the emitter resistor is equal to 26ohm, if you have a 10K load resistor at the collector, you have gain of 10,000/25=400!!! If you run 5mA, you have gain of 2000!! Just from 1 stage. You don't want to run transistor like this. You put an emitter resistor to lower the gain.
Say you run 1mA, you put a 300ohm resistor in series with the emitter and use a 10K resistor as collector load, you get gain of -10000/326=-30.7. Almost like -10000/300=33.3. By putting a 300ohm resistor, the 26ohm internal resistor of the transistor almost got drown out and the little change of the internal resistor does not affect the final gain much. In another words, gm of transistor circuit really does not depend on the transistor, it would be a bad design if so.
The philosophy of design transistor circuit is to take the transistor OUT of the picture and use resistors to determine the gain. So we don't rely on the characteristic of the transistor for the gain. The result is transistor circuit can easily be designed with much much lower distortion, much more stable in characteristics.
This sense, transistors are much more simpler to design.....BUT transistors have both NPN and PNP( N and P FETs). So one open to a whole new world of possibility like complementary design, active current mirrors and all sort of goodies.
Last edited:
