Tricky BJT Circuit

.

Please post your best effort at solving the problem, so that we can guide you where necessary.

newbe_always said:

problem attached.It is very confusing to me

Click to expand...

The circuit is not "tricky".
Why is it confusing to you?
It should not be if you understand how BJTs work.

NPN or PNP?

The circuit is not "tricky" - it is crazy (with regard to the shown values and polarities)

I say that the circuit is "tricky", as in, there are several things which don't make sense.

The transistor is drawn as a PNP type.
A PNP transistor has the emitter connected to the +ve side of the supply and the collector to the -ve side.

The "supply" is drawn as a battery (a single cell actually), the long stroke of the battery symbol is usually the +ve side.
But here we have the +ve side connected to the collector and the -ve side connected to the emitter.
This is the wrong way around for the PNP transistor.

We are told that Icq is 3mA, I assume this means the quiescent collector current is 3mA.
I assume that the transistor has high gain so the base current is very small compared with the collector current, so we can say that the emitter current = collector current = 3mA.

We can calculate the voltage across the emitter resistor using V = I x R
so
V = 3ma X 500 Ohm = 1.5 volts.

We can calculate the base voltage, measured with respect to the common point of the circuit (the earth symbol).
So,
Base Voltage = Emitter voltage + Emitter- Base voltage

For a silicon transistor, base-emitter voltage will be about 0.5 to 0.6 volts when the transistor is biased into conduction, so we have:
Base voltage = 1.5v + 0.5v = 2.0 volts. (Negative with respect to the common point).

Looking at the circuit in the question, the base voltage is shown as -8volts (with respect to the common point.

So what is it 2v or 8v on the base?
Damned if I know!

As I have been typing, LvW has described the circuit as "Crazy", I think that he is correct.

JimB

Presumably it's one of the many completely wrong exam questions which have been doing the rounds for decades (long before the internet). In the final of the Sharp Electronics Engineer of the Year competition one year they had a similar completely wrong question, except the circuit was OK, but none of the four options were correct. I, and many others, added a fifth option with the correct answer - and when we came out of the exam room we all harassed the Sharp staff running it.

In a fine example of 'passing the buck' they all threw Jim under the bus - then ran away as we all surrounded Jim.

He admitted setting the exam paper, but had just culled questions from old exam papers, and never actually checked the answers were correct.

He then said he'd remove that question from the exam, where we then said what about those who added the correct answer?.

I can't remember what he said to that, but it doesn't matter, I won the Competition that year anyway