in the above simulation, I noticed that the beta changed! why? beta_Q1 != beta_Q2 ??? but the author said they are identical.
Another question, what's the C-E saturation voltage, Vce? if the Q2's emitter was grounded. Assume those Vces = 0.3 volt for single transistor, Vbe=0.7 volt.
Thanks a lot.
Q1 is saturated, so its base current is higher. Saturation occurs when the collector-base junction becomes forward biased, ie colector voltage is about a diode drop below the base voltage.
Colin =^.^=
biased,
"At very low collector currents, hfe drops significantly. " hope this will help:
[snip]
Indeed. Due to recombination (ISE).
But the first device in a Darlington loses _effective_ beta due to
*saturation*....Jim Thompson
4e60956ffbab993
I think that post is refering to the usual base emitter resistors in most darlngton configurations, certainly integrated darlington devices.
Its surprising the collector base leakage doesnt cuase the opposite problem, but its only a simulation. In practice there will probably be a wide range from one extreme to the other given different batches of transistor at different temperatures etc.
Yeah thats what I just blindly assumed initialy without realy looking, but theres plenty of collector voltage.
Colin =^.^=
Maybe what the OP is observing (and confused by)...
If you curve trace a Darlington, you will observe low beta at low currents because... the output device is NOT conducting due to the inner base resistor shunting away any drive current to it.
...Jim Thompson
See....
Newsgroups: alt.binaries.schematics.electronic Subject: why this simulation looks like that? (Darlington Pair) - from S.E.D - DarlingtonBeta.pdf Message-ID:
...Jim Thompson
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