Phototransistor with three pins

Keyser Soze wrote:

> "Tim Williams" <(E-Mail Removed)> wrote in message
> news:dCI8f.3990$(E-Mail Removed)...
>
>>"Robert Baer" <(E-Mail Removed)> wrote in message
>>news:_yE8f.1180$(E-Mail Removed) link.net...
>>
>>> In effect, charge is being injected in the base.
>>> You now may open the eyes all the way and finish stepping forward.
>>
>>Ok, so you get a variable knee (i.e., variable Vbe) diode? Kinda spoils
>>the
>>useful switching character of a transistor.
>>
>>Tim
>>
>
> There are a few things that have not yet been mentioned.
>
> Most phototransistors are junction transistor, not field effect transistors.
>
> This means that they operate on current flow not voltage levels as are
> common with FETs.
>
> The rise and fall times of junction phototransistors are determined by the
> rate of change of the current through the base-emitter junction.
>
> The collector current of phototransistors is usually very low, in the range
> of 400 to 800 microamps with full illumination. Darlington phototransistors
> can switch more collector current but rise and fall times increase from a
> few microseconds to several milliseconds.
>
> The larger the area of the base is the more sensitive the phototransistors
> is. A lager base area means a large base-emitter capacitance. This sets up
> two competing goals. Where one would like fast turn on and off times the
> emitter impedance should be low, but to get a large output voltage swing
> with only 400 microamps of collector current the emitter impedance needs to
> be quite large.
>
> One way to manage all these thing is to use a three terminal phototransistor
> and establish the base impedance independent of the emitter circuit.
>
> Keeping the voltage drop across the phototransistor (Vce) low can also
> improve rise and fall times and sensitivity to light.
>
>
>
....for example, keep the Vcb zero with an opamp would improve the
risetime by a significant amount.

Robert Baer wrote:
> Keyser Soze wrote:
>
>> "Tim Williams" <(E-Mail Removed)> wrote in message
>> news:dCI8f.3990$(E-Mail Removed)...
>>
>>> "Robert Baer" <(E-Mail Removed)> wrote in message
>>> news:_yE8f.1180$(E-Mail Removed) nk.net...
>>>
>>>> In effect, charge is being injected in the base.
>>>> You now may open the eyes all the way and finish stepping forward.
>>>
>>>
>>> Ok, so you get a variable knee (i.e., variable Vbe) diode? Kinda
>>> spoils the
>>> useful switching character of a transistor.
>>>
>>> Tim
>>>
>>
>> There are a few things that have not yet been mentioned.
>>
>> Most phototransistors are junction transistor, not field effect
>> transistors.
>>
>> This means that they operate on current flow not voltage levels as are
>> common with FETs.
>>
>> The rise and fall times of junction phototransistors are determined by
>> the rate of change of the current through the base-emitter junction.
>>
>> The collector current of phototransistors is usually very low, in the
>> range of 400 to 800 microamps with full illumination. Darlington
>> phototransistors can switch more collector current but rise and fall
>> times increase from a few microseconds to several milliseconds.
>>
>> The larger the area of the base is the more sensitive the
>> phototransistors is. A lager base area means a large base-emitter
>> capacitance. This sets up two competing goals. Where one would like
>> fast turn on and off times the emitter impedance should be low, but to
>> get a large output voltage swing with only 400 microamps of collector
>> current the emitter impedance needs to be quite large.
>>
>> One way to manage all these thing is to use a three terminal
>> phototransistor and establish the base impedance independent of the
>> emitter circuit.
>>
>> Keeping the voltage drop across the phototransistor (Vce) low can also
>> improve rise and fall times and sensitivity to light.
>>
>>
>>
> ...for example, keep the Vcb zero with an opamp would improve the
> risetime by a significant amount.

Would there be any advantage to keeping the Vcb high, smaller Ccb
but let the dVcb/dT be zero??
mike

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"Keyser Soze" <(E-Mail Removed)> wrote in message
news:_aW8f.6904$(E-Mail Removed) m...
> The collector current of phototransistors is usually very low, in the
> range of 400 to 800 microamps with full illumination. Darlington
> phototransistors can switch more collector current but rise and fall
> times increase from a few microseconds to several milliseconds.

Odd, my 4N35 says 20mA minimum Ic for approximately 20mA in the LED.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Tim Williams wrote:

> "Keyser Soze" <(E-Mail Removed)> wrote in message
> news:_aW8f.6904$(E-Mail Removed) m...
>
>>The collector current of phototransistors is usually very low, in the
>>range of 400 to 800 microamps with full illumination. Darlington
>>phototransistors can switch more collector current but rise and fall
>>times increase from a few microseconds to several milliseconds.
>
>
> Odd, my 4N35 says 20mA minimum Ic for approximately 20mA in the LED.
>
> Tim
>
> --
> Deep Fryer: a very philosophical monk.
> Website: http://webpages.charter.net/dawill/tmoranwms
>
>
Hmm, i always thought raise/fall (slew/skew) depending on
how you want to use the lingo? improves on more current
since the miller effects are reduced ?


--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

mike wrote:
> Robert Baer wrote:
>
>> Keyser Soze wrote:
>>
>>> "Tim Williams" <(E-Mail Removed)> wrote in message
>>> news:dCI8f.3990$(E-Mail Removed)...
>>>
>>>> "Robert Baer" <(E-Mail Removed)> wrote in message
>>>> news:_yE8f.1180$(E-Mail Removed) nk.net...
>>>>
>>>>> In effect, charge is being injected in the base.
>>>>> You now may open the eyes all the way and finish stepping forward.
>>>>
>>>>
>>>>
>>>> Ok, so you get a variable knee (i.e., variable Vbe) diode? Kinda
>>>> spoils the
>>>> useful switching character of a transistor.
>>>>
>>>> Tim
>>>>
>>>
>>> There are a few things that have not yet been mentioned.
>>>
>>> Most phototransistors are junction transistor, not field effect
>>> transistors.
>>>
>>> This means that they operate on current flow not voltage levels as
>>> are common with FETs.
>>>
>>> The rise and fall times of junction phototransistors are determined
>>> by the rate of change of the current through the base-emitter junction.
>>>
>>> The collector current of phototransistors is usually very low, in the
>>> range of 400 to 800 microamps with full illumination. Darlington
>>> phototransistors can switch more collector current but rise and fall
>>> times increase from a few microseconds to several milliseconds.
>>>
>>> The larger the area of the base is the more sensitive the
>>> phototransistors is. A lager base area means a large base-emitter
>>> capacitance. This sets up two competing goals. Where one would like
>>> fast turn on and off times the emitter impedance should be low, but
>>> to get a large output voltage swing with only 400 microamps of
>>> collector current the emitter impedance needs to be quite large.
>>>
>>> One way to manage all these thing is to use a three terminal
>>> phototransistor and establish the base impedance independent of the
>>> emitter circuit.
>>>
>>> Keeping the voltage drop across the phototransistor (Vce) low can
>>> also improve rise and fall times and sensitivity to light.
>>>
>>>
>>>
>> ...for example, keep the Vcb zero with an opamp would improve the
>> risetime by a significant amount.
>
>
> Would there be any advantage to keeping the Vcb high, smaller Ccb
> but let the dVcb/dT be zero??
> mike
>
Zero Vcb tends to zero out a lot of terms; noise, miller effect,
leakage to name a few (i forgot all of them, but i think about 6 moer or
less).

"Tim Williams" <(E-Mail Removed)> wrote in message news:KV39f.32591$(E-Mail Removed)...
> "Keyser Soze" <(E-Mail Removed)> wrote in message
> news:_aW8f.6904$(E-Mail Removed) m...
>> The collector current of phototransistors is usually very low, in the
>> range of 400 to 800 microamps with full illumination. Darlington
>> phototransistors can switch more collector current but rise and fall
>> times increase from a few microseconds to several milliseconds.
>
> Odd, my 4N35 says 20mA minimum Ic for approximately 20mA in the LED.
>
> Tim
>
The 4N35 is an optocoupler not just a phototransistor.

See: http://www.fairchildsemi.com/ds/4N/4N35.pdf for data sheet.

You may want to note that while the current transfer ratio for the 4N35 is specified to be 100% at 25C it drops to 40% at the high
and low temperature extremes.

The CTR performance of related devices (4N25-4N28) is so poor (10-20%) that they have no specification for temperature extremes at
all.

Optocouplers tend to have the optical design optimized for best performance.

When using phototransistors on the other hand the optical path does not lend itself to convent optimizations.

Manufactures of phototransistors seem to try very hard to obscure the specification for photon sensitivity of their products. This
makes it quite difficult for a designer to compare products from various vendors.

"Robert Baer" <(E-Mail Removed)> wrote in message
news:Ifk8f.2917$(E-Mail Removed) ink.net...
> Tim Williams wrote:
>
> > "Geir Klemetsen" <(E-Mail Removed)> wrote in message
> > news:djrd72$khn$(E-Mail Removed)...
> >
> >>But with a 3 pins phototransistor, I don't know how to assemble it
> >>to do the same as in the schematic above.
> >
> >
> > If the third pin is base, leave it floating, or connect it to
emitter. If
> > for some reason you wanted to operate the phototransistor as a
regular
> > transistor as well, that's what it's there for.
> >
> > Tim
> >
> > --
> > Deep Fryer: a very philosophical monk.
> > Website: http://webpages.charter.net/dawill/tmoranwms
> >
> >
> Poor choices.
> 1) floating base: NO GO

No go electrically, but that's not what it's used for. It's used for a
phototransistor, and normally the base is left floating.

> 2) base tied to emitter: less sensitivity than if base tied to
collector.

Then it will have no photo sensitivity. It'll just be a forward biased
diode.

"Robert Baer" <(E-Mail Removed)> wrote in message
news:_yE8f.1180$(E-Mail Removed) nk.net...
> Tim Williams wrote:
>
> > "Robert Baer" <(E-Mail Removed)> wrote in message
> > news:Odk8f.2916$(E-Mail Removed) ink.net...
> >
> >> A simple-minded way is to connect the transistor in the DCT mode,
> >>making it look like a diode, but still act like a transistor:
> >
> >
> > No?? If it's NPN, it's going to be forward-biased! I don't see how
that
> > could ever work.
> >
> > My understanding is the transistor has an additive (OR) choice
between base
> > current and light turning on the collector.
> >
> > Tim
> >
> > --
> > Deep Fryer: a very philosophical monk.
> > Website: http://webpages.charter.net/dawill/tmoranwms
> >
> >
> Start with a transistor.
> If the base is connected to the collector, it is still technically
> and pracitcally an active device, with the base current controlling
the
> collector current.
> Take one step backwards, close your eyes to that.
> Shine a light on a silicon PN junction and notice that an
electrical
> voltage is produced if open circuit (or hi Z load) and that an
> electrical current is produced if shorted circuit (or low Z load).
> Partly open eyes, take one-half step forward.
> Shine a light on a transistor die (that is how the vast majority of
> phototransistors were made; a lens that focused the light on a 2N2222
or
> equivalent die).
> In effect, charge is being injected in the base.
> You now may open the eyes all the way and finish stepping forward.

My understanding is that phototransistors are not used as photovoltaic
devices. They are used just as a PIN photodiode is used, reverse
biased, but with current amplification.

Watson A.Name - "Watt Sun, the Dark Remover" wrote:

> "Robert Baer" <(E-Mail Removed)> wrote in message
> news:_yE8f.1180$(E-Mail Removed) nk.net...
>
>>Tim Williams wrote:
>>
>>
>>>"Robert Baer" <(E-Mail Removed)> wrote in message
>>>news:Odk8f.2916$(E-Mail Removed) thlink.net...
>>>
>>>
>>>> A simple-minded way is to connect the transistor in the DCT mode,
>>>>making it look like a diode, but still act like a transistor:
>>>
>>>
>>>No?? If it's NPN, it's going to be forward-biased! I don't see how
>
> that
>
>>>could ever work.
>>>
>>>My understanding is the transistor has an additive (OR) choice
>
> between base
>
>>>current and light turning on the collector.
>>>
>>>Tim
>>>
>>>--
>>>Deep Fryer: a very philosophical monk.
>>>Website: http://webpages.charter.net/dawill/tmoranwms
>>>
>>>
>>
>> Start with a transistor.
>> If the base is connected to the collector, it is still technically
>>and pracitcally an active device, with the base current controlling
>
> the
>
>>collector current.
>> Take one step backwards, close your eyes to that.
>> Shine a light on a silicon PN junction and notice that an
>
> electrical
>
>>voltage is produced if open circuit (or hi Z load) and that an
>>electrical current is produced if shorted circuit (or low Z load).
>> Partly open eyes, take one-half step forward.
>> Shine a light on a transistor die (that is how the vast majority of
>>phototransistors were made; a lens that focused the light on a 2N2222
>
> or
>
>>equivalent die).
>> In effect, charge is being injected in the base.
>> You now may open the eyes all the way and finish stepping forward.
>
>
> My understanding is that phototransistors are not used as photovoltaic
> devices. They are used just as a PIN photodiode is used, reverse
> biased, but with current amplification.
>
>
...and tying the base to the emitter gives a sensitivity almost three
orders of magnitude less than with a floating or biased base.