BJT help....

[elec123]

Hi,

Please find circuit diagram attached,

I need to find the quiescent operating point and the mid band voltage gain (Vo/Vi) given that:

Vo=0.7V
Ri = 1kohm
Bdc=200
Bac=200

I haven't studied this for years and can't find my notes on it, Would be great if someone could spare a few mins and help me out on this....

Thanks in advance,

 

[Roff]

Hi,

Please find circuit diagram attached,

I need to find the quiescent operating point and the mid band voltage gain (Vo/Vi) given that:

Vo=0.7V
Ri = 1kohm
Bdc=200
Bac=200

I haven't studied this for years and can't find my notes on it, Would be great if someone could spare a few mins and help me out on this....

Thanks in advance,

That sure looks like homework. If you haven't studied it in years, now would be the time to start... :roll:

 

[audioguru]

It is a lousy circuit!
1) You can't buy a transistor with a current gain of exactly 200. It will be in a range of current gain from about 70 to maybe 500 or more. The transistors with a low and with a high current gain won't work in that circuit.
2) Without negative feedback the output of the circuit will be extremely distorted if the output level is a half-decent amount. The output will resemble a rectifier's output and its level will be distorted so much that the voltage gain will be impossible to measure.

 

[Roff]

It is a lousy circuit!
1) You can't buy a transistor with a current gain of exactly 200. It will be in a range of current gain from about 70 to maybe 500 or more. The transistors with a low and with a high current gain won't work in that circuit.
2) Without negative feedback the output of the circuit will be extremely distorted if the output level is a half-decent amount. The output will resemble a rectifier's output and its level will be distorted so much that the voltage gain will be impossible to measure.

True, but as the circuit is defined, there are still answers to the questions.
It's a good exercise to see if a student understands BJT DC and AC fundamentals.

 

[elec123]

It's a good exercise to see if a student understands BJT DC and AC fundamentals.

And that's the whole reason of my post, because i don't understand it!!

I don't what the whole full solution, just an explanation or formula showing the route to the answer....

I know this isn't a place for students to get there homework done, this is actually extra work that will help me with my future studies. I thought this was a place for people to broaden there understanding and knowledge of electronics, hence my post.

Any further assistance would be greatly appreciated.

Thank you.

 

[rancid rock]

"google it"

 

[akg]

Any further assistance would be greatly appreciated.

Thank you.

apply voltage laws along the Rb , B-E section to find Ib->Ic, Ie etc.
then u can find the Vce and other values.

 

[Roff]

OK, knowing the following, you should be able to get the answers. Post them and we'll critique them.

1. Vbe~0.7V
2. Ic=beta*Ib
3. Ie~Ic (follows from 2, because beta>>1)
4. Emitter resistance re~.026/Ie
5. Gain ~ -Rc/re, where Rc is parallel combination of all resistors connected to collector (including through capacitors, if capacitive reactance is low compared to resistance). This is relative to voltage on the base. You will have to use the following equation to calculate the attenuation due to the resistance of the source (I assume that's Ri, although it is labeled rs in the schematic).
6. Rin~beta*re

You will, of course, need to use Ohm's law in addition to these equations.

PS I think Vo=0.7V is a red herring, or maybe it's just there to indicate that this is meant to be a small-signal analysis.