hi
i need a help in the common base configuration
i have this circuit below

i need to draw its ac equivalent model but i have a problem with inserting the hybrid pi model or any model because of rb1 and rb2
they are supposed to be in parallel but where to put them in the equivalent circuit i don't know
in fact the hole equivalent circuit isn't so clear to me i find it difficult compared to the common emitter equivalent circuit
i know it's wrong but will it look like this ??

 

At signal frequencies, what does Cb look like?

 

At signal frequencies, what does Cb look like?

what do u mean what does it look like ? if i knew i wouldn't ask i didn't know how to draw the equivalent circuit
do u mean what is the circuit's behavior or something like that ?

 

At frequency when Xc << (Rb1||Rb2) the capacitor reactance short the Q1 base to GND.

 

At frequency when Xc << (Rb1||Rb2) the capacitor reactance short the Q1 base to GND.

yes i know

 

WAIT
that's right i almost forgot they are gonna be shorted because they are connected to grounded base and also the other end of rb is connected to shorted vcc

 

So why didn't you include this on your small-signal diagram?

 

i get it now thank u so rb is canceled and now i can use the equivalent model with no troubles

 

what do u mean what does it look like ? if i knew i wouldn't ask i didn't know how to draw the equivalent circuit
do u mean what is the circuit's behavior or something like that ?

It's going to always be very hard to draw AC equivalent circuits (better known as small-signal equivalents) if you don't understand the major functions of the parts of the circuit, if for no other reason than you don't know which components do and don't get included in the equivalent circuit, let alone why they do or don't.

What's the purpose of Rb1 and Rb2? It's to set the DC bias point of the transistor.

What's the purpose of all of the capacitors? It's to separate the AC operation of the circuit from the DC operating point. As such, they are chosen so that their impedances at the frequencies of interest are low relative to the other impedances.

In particular, Zcb << Rb2. So what does that imply about the base of the transistor at signal frequencies?

 

In particular, Zcb << Rb2. So what does that imply about the base of the transistor at signal frequencies?

this means that the resistors connected to the base won't affect the small signal and it won't pass through them so in this case they won't get included in the small signal equivalent circuit

 

this means that the resistors connected to the base won't affect the small signal and it won't pass through them so in this case they won't get included in the small signal equivalent circuit

Hello,

Unless you need to analyze for frequency response.

 

Hello,

Unless you need to analyze for frequency response.

no i don't but why would it change for frequency response ?
resistors are elements that don't affect the frequency

 

no i don't but why would it change for frequency response ?
resistors are elements that don't affect the frequency

But what if you would like to know the bandwidth of your circuit?

 

But what if you would like to know the bandwidth of your circuit?

u are right i forgot

 

no i don't but why would it change for frequency response ?
resistors are elements that don't affect the frequency

Hi,

Oh, i was talking about the capacitor which changes the frequency response as i think you already understand now. Probably not too significant for your exercise though, but only you can decide that.

 

Hi,

Oh, i was talking about the capacitor which changes the frequency response as i think you already understand now. Probably not too significant for your exercise though, but only you can decide that.

i thought u were talking about rb1 and rb2

 

i thought u were talking about rb1 and rb2

Hi,

Well depending on what model you are using (if it contains any capacitance) all of the components may alter the frequency response. I was talking about the most significant.
As i said though it may not be relevant to your problem.