Strange transistors configuration...

[Regnum]

I have attached a circuit, and here you'll find some info about:
**broken link removed**

The question is:

What's the behavior of both intentionally "reversed" phototransistors ?

Thanks.

 

[flatulent]

It looks like they connect the input of the op amp active filter circuit to either the power supply or ground.

 

[djalli]

Not forgetting what are LED? diodes capable of emmision. Even regular diodes emit something and phototransistors can measure light, and temperature. Something very clever.

Title is "16-Bit PWM Optoisolated DAC Is PC-Controlled"

flatulent gave a corrent answer before me.

 

[Regnum]

Dear all,

Thank you very much for your help, I agree about the function of the circuit. Glancing the schematics and the article is enough to fully understand its functionality, where both isolators are coupling a PWM signal that is converted to its DC voltage later.

However, my question was specifically focussing on the pair of phototransistor which are working in "inverse mode" ( BOTH TRANSISTORS ARE NPN, WHOSE COLLECTORS DEAL WITH LOWER VOLTAGE THAN EMITTERS! )

The article says that you may use transistors in "inverse mode" in order to get a sub-millivolt VCEsat voltage, but It's really the first time I hear about reversing transistors -unless as an efficient way to burn them...- so I'd like to know about the transistor behavior in this configuration.

What's the BJT model when you swap emitter with collector ?

Thanks

 

[jorgito]

Hello Regnum,
Maybe I'm much older than you, I've seen this trick before, if memory serves me well, to interface discrete signals to TTL circuitry. BTW, Stephen Woodward is a very smart guy!
Best Regards

 

[House_Cat]

However, my question was specifically focussing on the pair of phototransistor which are working in "inverse mode" ( BOTH TRANSISTORS ARE PNP, WHOSE COLLECTORS DEAL WITH LOWER VOLTAGE THAN EMITTERS! )

Look again - the phototransistors are NPN. Inverted mode refers to using the Base-Collector junction as the controlling junction, rather than the Base-Emitter junction. Because of doping differences in the emitter and collector regions, the gain is generally less in the inverted bias condition. Junction transistors will work with the functions of collector and emitter swapped; however, the current gain will be considerably less in the inverted configuration.l

 

[flatulent]

As far as inverting the transistors, this is a classical way to make Vcesat smaller. It is still an NPN when you invert it, but the new emitter (old collector) is doped lower than the base. The beta is very small.

 

[Regnum]

Once again, thank you for your prompt and useful help.

House_Cat: I was wrong when I wrote PNP, my mind thought of NPN but my fingers wrote the opposite... yeah, you're right. Sorry.

Now, the questions are:

1-Does any usual BJT model already include "inverted mode" behavior?
2-May I derive an "inverted-mode BJT model" from the usual mode BJT model ?
3-What are the rupture voltages in this case ?

Thanks

 

[hock]

this configuration is known as invers transistor configuration.

"the gain is generally less in the inverted bias condition. Junction transistors will work with the functions of collector and emitter swapped; however, the current gain will be considerably less in the inverted configuration."

at the same time transistor will operate at very high speed.
i have used this type of configuration in many places where i need very fast speed from a slow reansistor.

hock

 

[House_Cat]

1-Does any usual BJT model already include "inverted mode" behavior?
2-May I derive an "inverted-mode BJT model" from the usual mode BJT model ?
3-What are the rupture voltages in this case ?

1 - Someone must have modeled this in the past; however, I've never seen a specific model for inverted use. You should be able to use a standard normal active mode model.
2 - You should be able to use a standard Ebers-Moll model to accurately describe the inverted behavior (provided the device data sheet contains enough information - such as the inverted mode beta). Junction capacitances, resistances, diode characteristics, etc. remain the same.
3 - Voltage and current ratings don't change just because you use the device inverted. The same Vce, Vbe, Vbc, Ic, etc. ... apply.