Power Dissipation Across A Transistor In Cutoff

[(*steve*)]

See post number 59. Use ohms law.

 

[Akshatha Venkatesh]

Assume Vce(sat) is 0V and that the other two transistor's input pin is equivalent to their base resistor to ground.

Those conditions lead to a slight overestimate

This calculation of current is trivial and I would be profoundly disappointed if you can't do it.

Sry, english is not my first language. this answer is in regard to which question ?

 

[(*steve*)]

The one you asked immediately prior to me posting it, and then repeated immediately after I posted it.

 

[Akshatha Venkatesh]

The one you asked immediately prior to me posting it, and then repeated immediately after I posted it.

the current is highly unlikely to exceed 1mA for the first transistor ?

 

[Akshatha Venkatesh]

No, it's in common emitter config. It's pretty much impossible for these biased transistors to be anything else.

T201 will start turning on when TM+ exceeds Vdd5 by more than about 1.8V.

approx 0.7 drop across diode and 0.6 across the transistor . so abt 1.3 volt ,right ? why 1.8 v?

 

[Akshatha Venkatesh]

I don't know, perhaps you should just say it pulls the output low.



Vbe of 0.7 is not what you need to worry about. Remember that there are not just transistors. You need to use the minimum on voltage from the datasheet.



Vcc5 - 0.2V from memory.

Isn't it 7.3V- 0.2 V ??

 

[(*steve*)]

What is the voltage on the collector of q201a (or whatever the first transistor is)?

I'm working through the "why no more than 1mA" question.

 

[Akshatha Venkatesh]

What is the voltage on the collector of q201a (or whatever the first transistor is)?

I'm working through the "why no more than 1mA" question.

The voltage on the collector of the first transistor is (volatge at Tm+ - diode drop - transistor drop )
if Tm+ is 8v , (8-0.7-vcesat(of transistor)).
Am I wrong ?

 

[(*steve*)]

Yeah, sorry, you're right. I misremembered it.

Now you have that, assume the entire load is the resistor between the input terminals and bases (assume Vbe is 0V).

What current will flow?

 

[Akshatha Venkatesh]

Yeah, sorry, you're right. I misremembered it.

Now you have that, assume the entire load is the resistor between the input terminals and bases (assume Vbe is 0V).

What current will flow?

the current then will be the voltage at collector divided by the resistance. oh ok , I guess I had a confusion regarding the Vbe . So I have to assume the Vbe to be 0 ? right ?

 

[(*steve*)]

Yep. It will overestimate the current a bit, but that's fine.

 

[Akshatha Venkatesh]

Yep. It will overestimate the current a bit, but that's fine.

Ok , thank you steve

 

[Akshatha Venkatesh]

Can I trouble you with one more doubt Steve ?
when the voltage at TM+ is 6V , i understand that the first transistor doesn't turn on . now the current is given in the datasheet as Iceo (collector to emitter cutoff current) , but what is the voltage at the collector (P203) ?

 

[(*steve*)]

Can you see a path to any rail?

 

[Akshatha Venkatesh]

Can you see a path to any rail?

there are two paths , to two base resistors of different transistors. Should I consider those two resistors parallel in connection and use ohm's law ?

 

[(*steve*)]

You can use ohms law if you want to, but you can often consider the current through the transistor to be zero (I think they specify a maximum value of some small fraction of a milliamp, but I can't remember it off the top of my head)

 

[Akshatha Venkatesh]

You can use ohms law if you want to, but you can often consider the current through the transistor to be zero (I think they specify a maximum value of some small fraction of a milliamp, but I can't remember it off the top of my head)

Would I be right in considering the resistors as in parallel ? (the base resistors of transistors)

 

[(*steve*)]

Yes, but in this case you need to consider both the input resistor and the base/emitter resistor in series. (And then both of them (one for each transistor) in parallel).

What current are you assuming?

 

[Akshatha Venkatesh]

You can use ohms law if you want to, but you can often consider the current through the transistor to be zero (I think they specify a maximum value of some small fraction of a milliamp, but I can't remember it off the top of my head)

How do I calculate the voltage at the collector of the first transistor ?

 

[(*steve*)]

See post 75