# BJT Common Base Configuration Analysis

#### dacarvalho

Joined Oct 27, 2017
12
First of all, sorry for bad english, and if i do something wrong! It's my first thread here... i will do my best to explain my doubts!

My teacher send this homework.

We need to prepare in order to compare the theoretical values with practical values. We are learning about BJT configurations and analysis, but i can't even start, im so confused!!!

The questions are:
1. Calculate all parameters of the circuit with colector current equal to 5mA.
2. Gain Vo/Vi2
3. Calculate de input resistance

Joined Mar 10, 2018
4,057
Always start by observing/listing what you know ?

In this case we know what Ic is, so by inference if we look at datasheet we know
~ what Ib is because....what is the relationship between Ic and Ib.

Then we know what Vi2 is because we know what Vi1 is (thats under your control)
and it flows thru a R divider.

Then we know what the relationship between Ic and Ie.....so a signal impressed
on Vi2 creates a change in Ie which does what to Ic........?

Then you have to work out the bias Rs......find out what you know at this point
and the rest falls into place.

Regards, Dana.

#### LvW

Joined Jun 13, 2013
1,337
Just some remarks as a starting aid:

* Be aware that there are some parts which you can/must select (R1, R2, Rc). Of course, Rc is important for the gain - on the other hand, it determines the DC operating point (DC voltage Vce between C and E).
In this context: You need the available supply voltage Vcc. It must be known!
Select a proper and suitable value for Vce.

* The niveau of the base voltage divider (R1,R2) can also be chosen. It should be designed so that the DC voltage at the base is rather "stiff" (which means: Rather insensitive with respect to the unknown base current into the B node).
In practice, the current through R1 very often is chosen to be at least 10 times the estimated base current. The DC voltage provided by this divider should be app. 0.7V larger than the DC voltage at the emitter (which can be calculated easily).

* Are you familiar with gain expressions? Do you know the meannig of the transconductance gm and its relation to Ic ?
The shown circuit is not the classical common-base stage (which can be found in all textbooks) because the input signal is not applied directly to the emitter (via a capacitor).

#### dacarvalho

Joined Oct 27, 2017
12
Just some remarks as a starting aid:

* Be aware that there are some parts which you can/must select (R1, R2, Rc). Of course, Rc is important for the gain - on the other hand, it determines the DC operating point (DC voltage Vce between C and E).
In this context: You need the available supply voltage Vcc. It must be known!
Select a proper and suitable value for Vce.

* The niveau of the base voltage divider (R1,R2) can also be chosen. It should be designed so that the DC voltage at the base is rather "stiff" (which means: Rather insensitive with respect to the unknown base current into the B node).
In practice, the current through R1 very often is chosen to be at least 10 times the estimated base current. The DC voltage provided by this divider should be app. 0.7V larger than the DC voltage at the emitter (which can be calculated easily).

* Are you familiar with gain expressions? Do you know the meannig of the transconductance gm and its relation to Ic ?
The shown circuit is not the classical common-base stage (which can be found in all textbooks) because the input signal is not applied directly to the emitter (via a capacitor).
Hi LvW, thank you for your help!

I will try to follow your instructions... that said, when you talk about 'proper' value for Vce, what is 'proper' for you? Can i have some points in order to choose the proper Vce?
We talk about gain expressions, and i must understand them, but to be humble... this is so difficult for me to understand that bottom part of circuit and how can i do the AC analysis?

I will post my resolution soon, so we can talk about my problems ok?

Many many thanks!!!

#### dacarvalho

Joined Oct 27, 2017
12
Always start by observing/listing what you know ?

In this case we know what Ic is, so by inference if we look at datasheet we know
~ what Ib is because....what is the relationship between Ic and Ib.

Then we know what Vi2 is because we know what Vi1 is (thats under your control)
and it flows thru a R divider.

Then we know what the relationship between Ic and Ie.....so a signal impressed
on Vi2 creates a change in Ie which does what to Ic........?

Then you have to work out the bias Rs......find out what you know at this point
and the rest falls into place.

Regards, Dana.
Hi Dana, many thanks!

I will try to follow your instructions, soon i will update with my resolution and the possible errors, this way we can find and fix my doubts!

Many many thanks again!

#### LvW

Joined Jun 13, 2013
1,337
....when you talk about 'proper' value for Vce, what is 'proper' for you? Can i have some points in order to choose the proper Vce?
Well, it will be your goal to have a DC voltage at the collector (DC bias point) which - during amplification of the input signal - allows an output voltage that is able to swing around the bias point in both directions. A good starting point is to allocate app. 50% of the supply voltage to the collector resistor Rc.