Find Vs and the voltage gain

1. Mar 4, 2016

kelvin56484984

1. The problem statement, all variables and given/known data
assume β=200,Vt=26mV,Vbe(on)=0.7V
Find the voltage gain Vo/Vs

How to find the Rth , Vs ,then find the voltage gain?
I try to draw the circuit diagram below
2. Relevant equations

3. The attempt at a solution
Vth=[R2/(R1+R2)]*15
Vth=1.63V

Which method should I use to find Rth?
Rth=R2//R1
=5.53k ohm
or
I see this formula Rth=0.1Re(1+β) on my book
Rth=0.1Re(1+β)
=10.3k ohm

I assume Rth=R2//R1
Vth=Rth*Ib+Ie*Re+Vbe
Vth=Rth*Ib+(1+β)*Ib*Re+0.7
Ib=8.57μA

Rπ=Vt/Ib
Rπ=26m/8.57μ
Rπ=3.03k ohm

Assume my small signal model diagram is correct
Vb=ib*Rπ
Vo=-βib*Rc

But how to find the Vs
then I can find the voltage gain(Vo/Vs)?
=[Vo/Vb]*[Vb/Vs]

File size:
7.9 KB
Views:
66
File size:
12.9 KB
Views:
73
File size:
9.9 KB
Views:
68
2. Mar 5, 2016

Staff: Mentor

Vs is arbitrary, representing some small signal. The gain will be a ratio, so the particular value of Vs is not important. What may be important, though, is the frequency of Vs.

Notice where Vo is on your circuit diagram: It's not the voltage at the collector! What effect do you think the 20 μF emitter bypass capacitor is going to have? You may need to take a closer look at how the capacitors in the circuit are going to affect things, and perhaps examine the emitter circuit in more detail for your small signal equivalent.

Your circuit diagram shows the source resistance labelled with a capacitance value. Presumably that was meant to be associated with CC1. Is there a resistance value for the source resistor?

3. Mar 5, 2016

kelvin56484984

Thank you
the resistot with wrong label is the internal resistor of the signal generator
I think it can be neglected during the calculation since the value is not given

4. Mar 6, 2016

5. Mar 6, 2016

Staff: Mentor

Is the given circuit diagram not correct? It shows Vo connected to the emitter.

6. Mar 6, 2016

kelvin56484984

Yes , I make some mistakes when drawing the circuit
Vo should be at the collector