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PICList Thread
'[EE]:Load at Collector or Emitter?'
2002\07\23@032955 by Pang

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Hi all,

I remember reading some advice on piclist that mention the issue of putting
the load at either the collector or the emmiter end. Does anyone remember
the title of the thread?

Rgds,
Pang

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2002\07\23@041047 by Vasile Surducan

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A load in collector allows the transistor to saturate which is good if
you are dealing with switching modes or large loads. A load in emitor must
take care about the command parameters ( Ib or Vbe ), a large load in emiter
will need a large command voltage or a way to pass the command to a high
+V voltage. There are so many ways to solve the requirements as the birds
on the sky. ( including driving loads under constant current )

regards,
Vasile
http://www.geocities.com/vsurducan


On Tue, 23 Jul 2002, Pang wrote:

{Quote hidden}

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2002\07\23@044750 by ards, Justin P

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I thought it was dependant on the polarity.  I am unsure of the thread
title.

I have a 50% chance here...

Collector to load for NPN, Emitter to load for PNP if using them to
simply switch the load.

Warning: Before use allow 24 hours for this reply to set especially as
Vasile has already provided a response I would be sceptical of mine.

Justin
{Original Message removed}

2002\07\23@051743 by Vasile Surducan

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On Tue, 23 Jul 2002, Richards, Justin P wrote:

> I thought it was dependant on the polarity.  I am unsure of the thread
> title.
>
> I have a 50% chance here...
>
> Collector to load for NPN, Emitter to load for PNP if using them to
> simply switch the load.


*****************
>
> Warning: Before use allow 24 hours for this reply to set especially as
> Vasile has already provided a response I would be sceptical of mine.
>
*****************

Shut me if I understood this one !
Does it's an english mother's tongue, please ?

BTW, load to colector for a PNP too is very usual... as load to emiter for
a NPN. Depends on load and on designer brains, especially the last one ;o)

best, Vasile


> Justin
> {Original Message removed}

2002\07\23@094404 by Tal Dayan

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(assuming NPN transistors, otherwise, reverse polarities)

Typically you want your transistor to conduct at its maximum in the 'on'
state. This provides almost
full voltage to the load and minimizes the heat of the transistor. This
state of the transistor is called 'saturation'. In saturation, the collector
is about 0.2V more positive than the emitter and the base is about 0.7 volt
more positive than the emitter. This means that you need your base to be
more positive than the collector. If your load is connected to the emitter
and the collector is connected to V+, you will need a signal higher than V+
to drive the base which is not always available.

This is why it is easier to connect the emitter to the ground and the load
to the collector so you can use standard signals in the range 0 to V+ to
drive the base (typically through a current limiting transistor).

As for PNP, you just reverse the polarities. That is, the base and the
collector are more negative than the emitter.

Check also
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html

Tal

> {Original Message removed}

2002\07\24@043015 by Michael Rigby-Jones

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> -----Original Message-----
> From: Richards, Justin P [SMTP:spam_OUTJustin.RichardsTakeThisOuTspamTEAM.TELSTRA.COM]
> Sent: Tuesday, July 23, 2002 9:46 AM
> To:   .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject:      Re: [EE]:Load at Collector or Emitter?
>
> I thought it was dependant on the polarity.  I am unsure of the thread
> title.
>
> I have a 50% chance here...
>
> Collector to load for NPN, Emitter to load for PNP if using them to
> simply switch the load.
>
> Warning: Before use allow 24 hours for this reply to set especially as
> Vasile has already provided a response I would be sceptical of mine.
>
> Justin
>
In common emmiter configuration, with the load connector to the collector,
the transistor provides voltage and current gain.  In emitter follower
configuration (load connector to emitter) the transistor provides only
current gain, voltage gain is slightly less than unity.  This applies
irrespective of transistor polarity (PNP or NPN).

Which configuration you use depends very much on what you want to do.  For
switching loads on and off quickly, you would usualy connect the load to the
collector.  For driving heavy loads where only current gain is important, an
emitter follower would be used.

Regards

Mike

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