Basic BJT amplifiers

Thread Starter

full

Joined May 3, 2014
225
Hello everybody,

I study basic BJT amplifiers , and I can change the DC Bipolar Junction Transistor to AC Bipolar Junction Transistor......

but there is problem I don't know how I can do loop in AC Bipolar Junction Transistor....

please can you help me, how I can do loop in this circuit AC?

thanks,
 

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alfacliff

Joined Dec 13, 2013
2,458
there is no ac bjt, BJT refers to bipolar junction transistor, not bipolar. have you read your thread on bjt posted on monday? the bipoolar refers to n type and p type materials, not an ac operated transistor. transistors operate off dc, not ac.
 

Thread Starter

full

Joined May 3, 2014
225
thanks sir,
if I change the (npn common emitter) Bipolar Junction Transistor figure 6.28 to figure 6.29 , what is mean?
 

Jony130

Joined Feb 17, 2009
5,487
In figure 6.29 you have a small signal mode . A DC circuit that represent/simulates the behavior of the transistor for AC signals.
So simply use a Dc network analysis techniques. For example a KVL ; nodal or mesh and solve it.
 

Thread Starter

full

Joined May 3, 2014
225
In figure 6.29 you have a small signal mode . A DC circuit that represent/simulates the behavior of the transistor for AC signals.
So simply use a Dc network analysis techniques. For example a KVL ; nodal or mesh and solve it.
thanks sir,
please can you do KVL for figure 6.29...
because I when understand how I can do in another time ......
thanks ,
 

Thread Starter

full

Joined May 3, 2014
225
if I change the (npn common emitter) Bipolar Junction Transistor figure 6.28 to figure 6.29 , what is mean?
what happen if I change from figure 6.28 to figure 6.29?
why I change the circuit from figure 6.28 to figure 6.29?

thanks,
 

Thread Starter

full

Joined May 3, 2014
225
Hello full, have you got your maths problem solved now?



Are you perhaps referring to different circuit analyses for transistors?

We perform a static or DC analysis for the purpose of setting up the transistor biasing to the correct mean operating or Q point.

We can then perform an AC signal analysis on the transistor in its biased circuit configuration.

There are two forms of such analysis

Small signal analysis

and , yes you've guessed it,

Large signal analysis.

The reason that we have two is that transistors are not linear devices, wheich means that their properties (characteristics or parameters) change with the signal.
In a small signal analysis we assume this change is negligable.
In a large signal analysis we take it into account.
now I understand :)
thank you
 

Thread Starter

full

Joined May 3, 2014
225
the AC analysis circuit in figure 6.33 very difficult :confused:

please can you help me how I can see the circuit and analysis?

ScreenHunter_m.jpg
 

Thread Starter

full

Joined May 3, 2014
225
I have small question, what different between (gm v-bi) &(B ib )?
what meant by transconductance ?
when I use (gm v-bi) and (B ib ) in circuit?
is mean the (gm v-bi) = (B ib ) same in calculation?
thanks:)
 

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Jony130

Joined Feb 17, 2009
5,487
Hi, do you know anything about dependent sources ? Because we can model/represented the BJT in two equivalent ways.
Ordinary we treat BJT as a current controlled current source. The collector current is beta times the base current Ic = β * Ib.
And the current gain is β = Iout/Iin = Ic/Ib
http://forum.allaboutcircuits.com/threads/common-emitter-amp-doubts-and-questions.91084/#post-663195
But sometimes we use alternative equivalent model but this time transistor is represented as a voltage controlled (Vbe) current source (Ic).
So the gain is equal to Iout/Vin = Ic/Vbe = transconductance = gm in unit, Siemens (S)
gm = Ic/Vbe and also notice that Vbe = Ib * rpi.
gm = Ic/Vbe = Ic/(Ib * rpi) = β/rpi
http://www.ittc.ku.edu/~jstiles/412/handouts/5.6 Small Signal Operation and Models/BJT Small Signal Parameters lecture.pdf
http://www.ittc.ku.edu/~jstiles/412/handouts/5.6 Small Signal Operation and Models/Steps for Small Signal Analysis lecture.pdf
 
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