Transistor Feedack

[Melfior_Ra]

Hi everyone!

In circuit attached how can I calculate the voltage in point A (Va)? So far I calculated the voltages pointed in circuit, which I think are ok.

Thank you.

 

[ericgibbs]

hi,
One point, just looking at the resistance values, which indicates very low transistor currents, your choice of 0.7Vbe is too high.

Recalc the voltage points with a Vbe of say 0.60V.

Then consider the same current that flows thru R1 and into Q2 base is the same as the current flowing thru R4. Allow transistor gains of say 200 at these low collector currents.

 

[Melfior_Ra]

But q2 is a pnp transistor and the current is flowing out from his base into q1 collector
So the q1 total collector current is the current that flows through r1 + the current that flows out from q2 base. In these conditions it's ok to approximate the current that flows through r4 equal with the current that flows through r1 ?

 

[ericgibbs]

But q2 is a pnp transistor and the current is flowing out from his base into q1 collector
So the q1 total collector current is the current that flows through r1 + the current that flows out from q2 base. In these conditions it's ok to approximate the current that flows through r4 equal with the current that flows through r1 ?

Thats a quick way of estimating the voltage at Va.. you could start analysing, but unless you know the exact gain of the transistors you will have a problem being precise.

Whats the question for.???

 

[Winterstone]

But q2 is a pnp transistor and the current is flowing out from his base into q1 collector
So the q1 total collector current is the current that flows through r1 + the current that flows out from q2 base. In these conditions it's ok to approximate the current that flows through r4 equal with the current that flows through r1 ?

Yes, in first order approximation I would assume this.

Hi melfior,
I didn't recheck the given voltages.
But based on these figures - I think it is easy to calculate the wanted dc voltage.

*Collector current in T1: Ic1=0.7/12k=0.058 mA.
*This current flowing through R4=10 k causes a voltage drop of 0.058mA*10k=0.583 volts.
* Thus: VA=4.2-0.583=3.616 volts.
* With R2=10k this voltage equals 10k*(Ic1+Ic2)=3.616 volts.
* Ic2=0.3 mA.

In this calculations I have neglected all base currents.

 

[Melfior_Ra]

thank you all.

So, for rough aproximation it is ok neglect base currents.

 

[ericgibbs]

thank you all.

So, for rough aproximation it is ok neglect base currents.

hi Mel,
Yes, in this example.
Generally I would apply different solving methods for different circuits.

It really depends upon the degree of accuracy required.

E.