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Old 7th November 2012, 01:19 PM   #1
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Default Effect of curve family on amp design?

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?
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Old 7th November 2012, 06:56 PM   #2
DF96 is offline DF96  England
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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.
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Old 7th November 2012, 07:30 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Conrad Hoffman View Post
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

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Last edited by Elvee; 7th November 2012 at 07:34 PM.
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Old 9th November 2012, 03:32 AM   #4
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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?
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Old 9th November 2012, 08:23 AM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Conrad Hoffman View Post
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
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Old 10th November 2012, 02:33 AM   #6
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Quote:
Originally Posted by Conrad Hoffman View Post
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...
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