# Bjt

#### beenuseren

Joined Sep 4, 2006
6
How can we represent the BJT in the model.

I mean what is the explanation behind that model. I have read in the books about this model but could not figure how we can arrive at that model.

In simple I didnot understand the re model. Please help me with this.

If I understand the re model then only I can understand the signal and frequency analysis of the transistor.

thanks.

#### Dave

Joined Nov 17, 2003
6,969
I'm a touch confused, are we talking about Spice models here?

You may want to be little clearer about your query.

Dave

#### beenuseren

Joined Sep 4, 2006
6
Oh I forgot to mention re model.
Actually I donot understand the small signal model of the transistor.
There are re model, hybrid model. I donot understand any model.
However I am trying hard to understand re model. I donot understand how we can represent the BJT as in re model. How we arrive at the re model parameters. In short I need the theory behind re model. Please help me with this.

thank you

#### Dave

Joined Nov 17, 2003
6,969
I'm sorry I still don't understand what the re model is, as far as I'm concerned when talking about RE with respect to BJT I think of the emitter resistance. I know what the hybrid (pi) model for a BJT is.

To start with the basics, the small signal model is used to characterise a non-linear device, eg a BJT, using small-signal analysis. The advantage of small signal analysis is that since we are looking at the operation from a small signal perspective, i.e. small alterations in the signals around the bias point, we can assume that the device/circuit behaves linearly, hence simplifying the Maths. In effect we are linearising the device around the bias point through ascertaining the partial dervatives of the characteristics with respect to the operational variables, where the partial dervatives describe the characteristics of resistance, capacitance and inductance in the device/circuit.

This link provides a concise and simple overview of how the small-signal model applies to BJT, but until I know what the re model is I can help no further.

Dave

#### Dave

Joined Nov 17, 2003
6,969

#### beenuseren

Joined Sep 4, 2006
6
Thanks Dave. Its a very informative link. Right now I am going through it.

#### beenuseren

Joined Sep 4, 2006
6
In the small signal model, we represent only the currents that develop due to the input AC signal. Am I right?

If so, the DC currents are not taken into consideration while developing the small signal model?

#### beenuseren

Joined Sep 4, 2006
6
Also what does ro signifies in the small signal model ?

#### Dave

Joined Nov 17, 2003
6,969
In the small signal model, we represent only the currents that develop due to the input AC signal. Am I right?

If so, the DC currents are not taken into consideration while developing the small signal model?
Your small signal model is analysising the small time-varying signals (i.e. AC signals) about a fixed DC bias point.

Also what does ro signifies in the small signal model ?
The output resistance.

You may be interested in the following information on BJT and small signal models: http://users.ece.gatech.edu/~mleach/ece3050/notes/bjt/bjtbasics.pdf

Dave

#### JoeJester

Joined Apr 26, 2005
4,390
I think it's time for you to find the pSpice Reference Manual online.

It will tell you what each of those signify.

re is the emitter ohmic resistance and the default value is 0.

You can also download the HSpice reference manual online.

#### beenuseren

Joined Sep 4, 2006
6
Thanks for the info.

Let us assume I am designing a audio amplifier or any signal amplifier. Is the amplifier circuit designed assuming the load is connected ? or the amplifier circuit is designed first and the range of resistive values connected to the load is determined later ?

#### mozikluv

Joined Jan 22, 2004
1,435
the design of the driver transistor circuitry is usually based on what the load is. the load can either be resistive or capacitive. in the case of an audio amplifier using discrete components, the design is usually made to drive a 32ohms down to 2ohms load.

moz

#### System_Ghost

Joined Oct 9, 2007
6
Re model and Hybrid models are essentially the same models that industries and educational institutions refer in the small signal analysis of single stage BJT's.
In hybrid model, it is obvious that the parameters involved are of hybrid nature such as hix,hox,hfx,hrx...where the x represents what configuration of BJT's you are dealing with ( Common Emitter, Common Collector, Common Base, Voltage Divider, Emitter Stablized..ets).
I did say they are essentially the same because both models are mere equivalents, the difference only is the accuracy of both model calculations at quiescent point or your BJT point of operation.
For example, hix in hybrid is equal to Bre in re model,where B is beta or your amplification factor etc.
For better understanding on the two equivalent models, I suggest you read the book ( I forgot the title , but its quite popular ) by Nachelsky and Boylestad, p.434 8th ed.
If you still dont understand it, then i maybe can explain it to you step by step in this forum #### System_Ghost

Joined Oct 9, 2007
6
Your design is determined by your load so you must know the specs of your load first for you to make an efficient design.
For example, your load might be efficient when input inpedance is greater than its output admittance, or if your load operates at currents or voltages greater than your Vi...in that case, you design the amplifier depending on this specs ( Hi input impedance Low output admitance Higher operational voltage than input specs for example means employing a voltage divider bias BJT)