A typical transistor amplifier is shown in the Figure. Find the amplifier gain G.
Circuit diagram: http://i.imgur.com/Rzv4JR7.png
Vs = 280mV
R1 = 100Ω
R2 = 7kΩ
R3 = 100Ω
Vd = 4*10^5 Ib (Ib is current through R3)
R4 = 7kΩ
R5 = 900Ω
Vo = ? (Voltage drop of R5)
Ohms Law, current division, voltage division
The Attempt at a Solution
1) To find the current through R3, I have to find the voltage drop first. So, I wanted to use current division from the start, but I don't know the current coming out of Vs, the 280mV source. To do so, I combined R2 & R3 which are in parallel: 1/(1/7000+1/100)=98.59Ω
2) Combine my Req from step 1 with the R1, which are in series: 98.59Ω + 100Ω = 198.59Ω
3) Now I have 1 resistor in a loop with my voltage source. Applying Ohm's law: .28/198.59 = 1.4mA, which is the current coming out of the source.
4) Now I can apply current division to find the current Ib in the figure: 7000/(7000+100)*0.0014=1.38mA
5) Since Vd is defined as 4*10^5*Ib(0.00138)= 555.6v (<-- My first indication that something went wrong)
6) Applying voltage division on R5, the 900Ω resistor: 900/(900+7000)*555.6 = Vo = 63.29v. After dividing the Vo and Vs, I get a 226.05 gain, which is incorrect. Where did I go wrong?
Thanks for your help