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Re: [funwithtransistors] flipping leads


Jun 28, 2010

 


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#221 Jun 28, 2010

Follow-on to the FET lead story on "another group".



When I was small I received a "building block" electronics project kit.�� Various components were mounted on plastic blocks, and you snapped them together and made connections with spring clips.



Well, the various projects presented worked, but the ones using the transistor were noisy and not too useful.



Later when I was smarter (much later, and not that much smarter . . .) I noticed the transistor was installed on the block backwards --�� C and E were reversed!�� If you remember the usual theoretical "sandwich" used to describe transistors, with one P layer and two N layers (or v.v.) you might think the C and E connections are interchangeable.�� After a fashion it seems they are, and indeed the transistor did sorta work.�� Not sure what damage the reverse polarity did though.



Seems like FET are the same way.��



BTW I've always found the FET schematic symbol confusing since D and S are drawn the same.�� A source of frustration (HA!)



Regards,

Mark KB9VKE



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#222 Jun 28, 2010

Like my Uncle Pete was forever telling me....."Bobby, you may not be very bright but you sure are stupid!"�� I believe in Uncle Pete's perspicacity....bob p

--- On Mon, 6/28/10, msalbert@... msalbert@...> wrote:

From: msalbert@... msalbert@...>Subject: [funwithtransistors] flipping leadsTo: funwithtransistors@yahoogroups.comReceived: Monday, June 28, 2010, 12:49 PM



Follow-on to the FET lead story on "another group".

When I was small I received a "building block" electronics project kit.�� Various components were mounted on plastic blocks, and you snapped them together and made connections with spring clips.

Well, the various projects presented worked, but the ones using the transistor were noisy and not too useful.

Later when I was smarter (much later, and not that much smarter . . .) I noticed the transistor was installed on the block backwards --�� C and E were reversed!�� If you remember the usual theoretical "sandwich" used to describe transistors, with one P layer and two N layers (or v.v.) you might think the C and E connections are interchangeable.�� After a fashion it seems they are, and indeed the transistor did sorta work.�� Not sure what damage the reverse polarity didthough.

Seems like FET are the same way.��

BTW I've always found the FET schematic symbol confusing since D and S are drawn the same.�� A source of frustration (HA!)

Regards,Mark KB9VKE



----------------------------

#223 Jun 28, 2010

msalbert@... wrote: > Follow-on to the FET lead story on "another group".

>

> When I was small I received a "building block" electronics project kit.

> Various components were mounted on plastic blocks, and you snapped them

> together and made connections with spring clips.

>

> Well, the various projects presented worked, but the ones using the

> transistor were noisy and not too useful.

>

> Later when I was smarter (much later, and not that much smarter . . .) I

> noticed the transistor was installed on the block backwards -- C and E were

> reversed! If you remember the usual theoretical "sandwich" used to describe

> transistors, with one P layer and two N layers (or v.v.) you might think the

> C and E connections are interchangeable. After a fashion it seems they

> are, and indeed the transistor did sorta work. Not sure what damage the

> reverse polarity did though.



There are differences between the two junctions, they are not the same.

The B-E junction is more abrupt (read: steeper dopant gradient). This

has several consequences



1. The majority carriers are injected into the base from

the emitter with a higher velocity than they would be if

the C-E leads are reversed. This means lower transit times

hence lower recombination rates and (much) higher gain.





2. The B-E breakdown voltage is much less than the C-B

breakdown voltage, typically around 5V. So, in the "reverse"

connection, the effective BVcbo is only about 5V, and the

"reverse transistor" can't be used much above that.



> Seems like FET are the same way.



Many FETs are completely reversible, in respect to the D and S. Not all,

but many. Especially this is true of MOSFETs.

> BTW I've always found the FET schematic symbol confusing since D and S are

> drawn the same. A source of frustration (HA!)



The "bar" is effectively ohmic (or should be). The only issue involved

with JFETs (AIUI) is just where along the bar the "junction" is made.

If it's made centrally, that is with the junction symmetrically placed

wrt the S and D, then S and D are actually interchangeable. For this

reason, FETs are sometimes used in feedback networks as part of the

feedback fraction determining circuitry, with the gate lead connected

to some sort of rectified version of the output. In this manner, it

may be used for automatic level control. Sometimes, just remote gain

control, even in AC circuits, since many are "symmetric".



For an example of this, see Figure 26 on page 11 of



www.national.com/an/AN/AN-20.pdf



in which an FET takes the place of the traditional lamp (as described

by Hewlett, I believe) used in the Wien Bridge Oscillator circuit.



Mac

--

p="p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}

Oppose globalization and One World Governments like the UN.

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----------------------------

#224 Jun 29, 2010

--- On Mon, 6/28/10, Mike McCarty Mike.McCarty@...> wrote:

> From: Mike McCarty Mike.McCarty@...>

> Subject: Re: [funwithtransistors] flipping leads

> To: funwithtransistors@yahoogroups.com

> Date: Monday, June 28, 2010, 5:11 PM

>

> There are differences between the two junctions, they are not the same.

> The B-E junction is more abrupt (read: steeper dopant gradient). This

> has several consequences

>

> ... 1. The majority carriers are injected into the base from

> ... the emitter with a higher velocity than they would be if

> ... the C-E leads are reversed. This means lower transit times

> ... hence lower recombination rates and (much) higher gain.

>

> ... 2. The B-E breakdown voltage is much less than the C-B

> ... breakdown voltage, typically around 5V. So, in the "reverse"

> ... connection, the effective BVcbo is only about 5V, and the

> ... "reverse transistor" can't be used much above that.



That's true for modern transistors, but not necessarily true for early

transistors. If you've ever disassembled/reverse-engineered an early

alloy junction Germanium transistor, you'll find the junctions are pretty much

the same, composed of a dot of Indium metal that has been diffused into each

side of the bar of Germanium that is the base. Of course, early alloy junction

transistors typically had low gains, low breakdown voltages, high leakage,

and a host of other weaknesses.

> > Seems like FET are the same way..

>

> Many FETs are completely reversible, in respect to the D and S. Not all,

> but many. Especially this is true of MOSFETs.

>

> > BTW I've always found the FET schematic symbol

> confusing since D and S are

> > drawn the same.. A source of frustration (HA!)

>

> The "bar" is effectively ohmic (or should be). The only issue involved

> with JFETs (AIUI) is just where along the bar the "junction" is made.

> If it's made centrally, that is with the junction symmetrically placed

> wrt the S and D, then S and D are actually interchangeable. For this

> reason, FETs are sometimes used in feedback networks as part of the

> feedback fraction determining circuitry, with the gate lead connected

> to some sort of rectified version of the output. In this manner, it

> may be used for automatic level control. Sometimes, just remote gain

> control, even in AC circuits, since many are "symmetric".



It's always impressed me how close a JFET is to a UJT.

> For an example of this, see Figure 26 on page 11 of

>

> www.national.com/an/AN/AN-20.pdf

>

> in which an FET takes the place of the traditional lamp (as described

> by Hewlett, I believe) used in the Wien Bridge Oscillator circuit.

>

> Mac



Dave



----------------------------

#225 Jun 29, 2010

Dave wrote: > --- On Mon, 6/28/10, Mike McCarty Mike.McCarty@...> wrote:

>

>> From: Mike McCarty Mike.McCarty@...>

>> Subject: Re: [funwithtransistors] flipping leads

>> To: funwithtransistors@yahoogroups.com

>> Date: Monday, June 28, 2010, 5:11 PM

>>

>> There are differences between the two junctions, they are not the same.

>> The B-E junction is more abrupt (read: steeper dopant gradient). This

>> has several consequences

>>



[...]



> That's true for modern transistors, but not necessarily true for early

> transistors. If you've ever disassembled/reverse-engineered an early

> alloy junction Germanium transistor, you'll find the junctions are pretty much



Even the indium alloy dot transistors had a larger collector and

deeper melt, so they show different characteristics. This resulted

in higher collector-base capacitance than emitter-base capacitance,

and higher power capability. Reversing the C-E leads resulted in

a transistor with lower power capability and lower GBW. Also, with

the deeper melt, the "mix" was different, though I don't know by

how much.

> the same, composed of a dot of Indium metal that has been diffused into each

> side of the bar of Germanium that is the base. Of course, early alloy junction

> transistors typically had low gains, low breakdown voltages, high leakage,

> and a host of other weaknesses.



Yep. The point contact transistors were, however, completely

reversible, AIUI. I've never experimented with one, however.



While we're talking about "abusing" transistors, one can sometimes

see in vintage lit to use a BJT C-E junction as an ersatz Zener

diode. This works, but they are noisier than diodes intended for

that purpose, and it ruins them for later use as a transistor.



Mac

--

p="p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}

Oppose globalization and One World Governments like the UN.

This message made from 100% recycled bits.

You have found the bank of Larn.

I speak only for myself, and I am unanimous in that!



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