Effect of curve family on amp design?

[Conrad Hoffman]

If you look at a family of curves for a BJT, some will exhibit more-or-less constant current for a given base current as Vce changes. Nice horizontal lines. Others will show the current increasing, sometimes wildly, as the Vce increases. How does this affect amplifier designs/performance and part selections?

 

[DF96]

This is the Early effect. It reduces collector impedance, which simple BJT models regard as infinite. It can be approximately modelled by using the Early voltage, where tangents to all the curve meet.

 

[Elvee]

How does this affect amplifier designs/performance and part selections?

In theory, this Early effect should not affect the linearity of an amplifier (it is linear, as a first approximation), just the ultimate amplification possible (like in µ-amplifiers). But in practice, it has some non-linear components, which means getting rid of it (with cascoding or similar schemes) generally improves the linearity too

PS
Now, lovers of the triode-sound may disagree with the above, but tube-lovers live in their own universe, with their specific rules.....

 

[Conrad Hoffman]

Thanks! I knew the name but didn't realize what I saw on the curve tracer was the result. Germanium devices can have a really exaggerated effect compared to silicon- is that also Early, or something else, just leakage maybe?

 

[Elvee]

Germanium devices can have a really exaggerated effect compared to silicon- is that also Early, or something else, just leakage maybe?

There could be voltage-dependent leakage currents too, but they will remain more or less the same at different levels of Ic current, unlike the real Early effect.
They would show as a slope even at zero Ib current

 

[Tube_Dude]

If you look at a family of curves for a BJT, some will exhibit more-or-less constant current for a given base current as Vce changes. Nice horizontal lines. Others will show the current increasing, sometimes wildly, as the Vce increases.

The ones exhibiting horizontal lines have higher collector output impedance and behave like a "more perfect" current source.

The good old 2N3055H (hometaxial-base) used to exhibit those nice horizontal lines...