# Multi stage BJT

Discussion in 'Homework Help' started by skusku, Aug 13, 2012.

1. ### skusku Thread Starter Well-Known Member

Aug 9, 2009
73
1
Hi again

http://i45.tinypic.com/1zqt2t4.jpg

Hope you guys can help me please. For those who know it should be easy.

This is the circuit in orcad, but the given values of resistors only include R7=100Ω, rs=600Ω.

Have to determine the resistor values(E-12):
Vcc=20V, Load resistance=100Ω, ouput voltage is to be +-100mV, Ie2=2mA, Ve1=5V, Vce1=3V, Ic1=1mA and R4 is equal to one tenth of R2. Vbe=0.7V.
Use approximate method.

I would like only clarification on R5 and Vb2 please. The rest of the resistors I have filled in but this might need adjustment if Vb2 is wrong.

What ive done is to say:
R5=Vo/Ie2=100mV/2mA=50Ω
Vb2=Vc1=Ve2+Vbe=0.1+0.7=0.8V.

Im assuming Vo is the output voltage but the +-100mv might mean that Vo needs to be 200mV. I cant find any other examples having a similar question to this. Using Boylestad and Floyd books. But nearly no examples to designing. And the worst part is the lecturer does not want to help me at all and unfortunately Im not understanding all of this so well (and I need all the marks I can get).

2. ### Yakima Member

Jan 23, 2012
35
2
I am going to help you buddy. But I have to copy your stuff and do the calculations. Get back with you real quick.

3. ### Yakima Member

Jan 23, 2012
35
2
You calculated the dc voltage at the base of Q1 wrong, according to my reckoning: The dc voltage at the base of the first transistor is calculated as if R1 and R2 were a voltage divider, with 20 volts being the source. This means:

Vb=20(47k)/[(47k+120k)]=5.6 volts. Are you with me?

Last edited: Aug 13, 2012
4. ### skusku Thread Starter Well-Known Member

Aug 9, 2009
73
1
Thanks Yakima,when you replied it was 2:30am in the morning where I am. So I was fast asleep..

What ive done is:
R4=Ve1/Ie1=5V/1mA=5k Ohm(E-12=4.7K Ohm)
R2=10.R4=47k Ohm
Vb1=Ve1+Vbe=5+0.7=5.7V

Basically I just rearranged the voltage divider formula where I then get
R1=(R2.Vcc/Vb)-R2=117.9k Ohm (E-12=120k Ohm)

So you say my R5 and Vb2 calculations are correct (using the 100mV)?

5. ### skusku Thread Starter Well-Known Member

Aug 9, 2009
73
1
Just for an update.The +-100mV output I realised is the AC output voltage.

The correct value to work out R5 is to first determine Vc1=Vb2=Vce1+Ve1=5+3=8V
Then R5=(Vb2-Vbe)/Ie2=(8-0.7)/2mA=3650 Ohm.

The Vs we were suppose to select as well. When simulating in Pspice we should get the 200mV peak to peak at RLoad. When setting Vampl to 0.5mV one should get the correct value.

Hopefully his helps someone in the near future or as long as this site exists.

Edit:I made a mistake for C2 the value should be something in the uF range (In my case 68uF) otherwise pspice gets all different values)

6. ### Audioguru Expert

Dec 20, 2007
10,721
1,200
Why are you using a class-A heater instead of a class-AB cool amplifier??

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
If I understand correctly, the topology was determined by the lecturer.