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flippityflop

the finer points of electronics - II

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alright, continuing the series...

another question i would like to as is on NPN BJTs, would it be better to put the load on the collector side or the emitter side?? in schematics, i almost always see it on the collector side (Ic = hfe * Ib), but the emitter side also works and when doing plain switching (in saturation) the emitter side works at least as well (Ie = Ib * (1 + hfe))... so would there be any difference?

the same with PNPs, when it comes to switching, would the collector side (Ic = Ib * (1 + hfe)) work better than the emitter?

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i'm asking as i've been working on a design that to my understanding should've worked. i have a generic NPN restoring the supply voltage and feeding it's emitter output to a 2N2222 parallel to a charging capacitor for a delayed avalanche. somehow the avalanching only works when i swap the 2N2222 + charging capacitor sub-circuit to the collector side.

it surprised me as when it was connected to emitter, it was also directly connected to ground. and when it was on the the collector side it was directly connected to the supply. so it was not isolated and would be biased to a common ground. although, as a side note, when it was on the emitter side, the charging capacitor might have caused a reverse current through the generic NPN and leaking to other parts of the circuit. so i probably should've put a diode in there. well i didn't get the chance to as i blew my 2N2222, so now i have to go to my local supplier to have more.

so i'm not completely sure what to make of it...

EDIT:
the generic NPN is "on" strictly less than the time needed by the 2N2222-capacitor sub-circuit to avalanche.

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How about providing a circuit.
One picture is worth a thousand words!


i'll solve the specific circuit problem on my own... although, i would like an answer on the first question... when switching, is it better to go with the emitter on NPNs and the collector on PNPs?? do their electrical characteristics differ more than slightly higher currents?

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i'm asking as i've been working on a design that to my understanding should've worked. i have a generic NPN restoring the supply voltage and feeding it's emitter output to a 2N2222 parallel to a charging capacitor for a delayed avalanche. somehow the avalanching only works when i swap the 2N2222 + charging capacitor sub-circuit to the collector side.

it surprised me as when it was connected to emitter, it was also directly connected to ground. and when it was on the the collector side it was directly connected to the supply. so it was not isolated and would be biased to a common ground. although, as a side note, when it was on the emitter side, the charging capacitor might have caused a reverse current through the generic NPN and leaking to other parts of the circuit. so i probably should've put a diode in there. well i didn't get the chance to as i blew my 2N2222, so now i have to go to my local supplier to have more.
charging
so i'm not completely sure what to make of it...

EDIT:
the generic NPN is "on" strictly less than the time needed by the 2N2222-capacitor sub-circuit to avalanche.



answering my own question again:

it just struck me, but maybe the capacitor was not charging not because of properties of currents in collector and emitter for any NPN, but because i've been thinking in terms of conventional current...


Let:
    Q be a sub-circuit of an avalanching transistor and capacitor in parallel. in my case, the avalanching transistor is 2N2222;
    the "generic NPN" is not the 2N2222 mentioned.

whenever Q was in the emitter side of the generic NPN, the negative terminal of the Q's capacitor is already in the highest negative charge. the momentary opening of the generic NPN's collector applies a "more electropositive voltage" to the capacitor's positive terminal, but there are no charge carriers migrating to said terminal. hence when the generic NPN stops conducting, the "more electropositive charge" is not held.

if we were to put Q on the collector side, the positive terminal of its capacitor is always what it is supplied to it. when the generic NPN starts conducting, electrons rush to the negative terminal of Q's capacitor and when NPN stops conducting, the electrons stay there. hence, we have negative charge there.

is this correct? i'd appreciate it if somebody would confirm this.

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i'll solve the specific circuit problem on my own... although, i would like an answer on the first question... when switching, is it better to go with the emitter on NPNs and the collector on PNPs?? do their electrical characteristics differ more than slightly higher currents?


If you are using the transistor as a switch, it makes no difference provided you can apply enough base bias to get the transistor into its saturated state.

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If you are using the transistor as a switch, it makes no difference provided you can apply enough base bias to get the transistor into its saturated state.


well,  emitter in saturations is still slightly bit higher... (Ie_sat = Ib_sat * (1 + hfe))

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A transistor with the load at its emitter does not saturate and is not a switch. It is an emitter-follower. When it is turned on then its emitter voltage is a Vbe voltage drop less than its base voltage and its base voltage is probably not as high as its supply voltage.

A transistor used as a saturated switch has the load connected between its collector and the power supply voltage. The base current must be at least 1/10th the collector current for most little transistors or 1/3rd the collector current for a power transistor like a 2N3055 regardless of its hFE (beta) number. The hFE or beta number is its DC current gain when it is a linear amplifier with plenty of collector to emitter voltage so it is not saturated.

The definition of a saturated transistor is when its collector-base diode is forward biased and conducts so the transistor cannot turn on any more.
An NPN transistor usually saturates better than a PNP.

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A transistor with the load at its emitter does not saturate and is not a switch. It is an emitter-follower. When it is turned on then its emitter voltage is a Vbe voltage drop less than its base voltage and its base voltage is probably not as high as its supply voltage.


so it's Ve = Vc - Vbe? ok... i missed that from just reading from wikipedia... i read the voltage on the load is *mostly* is constant for most of the operating current range... then again the examples from wikipedia has the load on the collector side...

collector side, then...

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Are you suggesting that a circuit would not be better understood this convoluted description?

Quote "i'm asking as i've been working on a design that to my understanding should've worked. i have a generic NPN restoring the supply voltage and feeding it's emitter output to a 2N2222 parallel to a charging capacitor for a delayed avalanche. somehow the avalanching only works when i swap the 2N2222 + charging capacitor sub-circuit to the collector side.

it surprised me as when it was connected to emitter, it was also directly connected to ground. and when it was on the the collector side it was directly connected to the supply. so it was not isolated and would be biased to a common ground. although, as a side note, when it was on the emitter side, the charging capacitor might have caused a reverse current through the generic NPN and leaking to other parts of the circuit. so i probably should've put a diode in there. well i didn't get the chance to as i blew my 2N2222, so now i have to go to my local supplier to have more"

 

QUOTE "i'm asking as i've been working on a design that to my understanding should've worked. i have a generic NPN restoring the supply voltage and feeding it's emitter output to a 2N2222 parallel to a charging capacitor for a delayed avalanche. somehow the avalanching only works when i swap the 2N2222 + charging capacitor sub-circuit to the collector side.

it surprised me as when it was connected to emitter, it was also directly connected to ground. and when it was on the the collector side it was directly connected to the supply. so it was not isolated and would be biased to a common ground. although, as a side note, when it was on the emitter side, the charging capacitor might have caused a reverse current through the generic NPN and leaking to other parts of the circuit. so i probably should've put a diode in there. well i didn't get the chance to as i blew my 2N2222, so now i have to go to my local supplier to have more.
charging".

Are you suggesting that a circuit would not be better understood this convoluted description?

 

(Note to Admin.  Although I can get into the editing section, there is no way I can edit or cancel my post, so I am restating it. Also, you have no 'preview' button).

 

Edited by pebe
my reply in wroong place - no preview is available

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