# Design a 2 stage BJT amplifier - need some guidance

Discussion in 'Homework Help' started by mihachris, Jan 31, 2015.

1. ### mihachris Thread Starter New Member

Jan 31, 2015
2
0
Hello everyone!

So I have to design a 2 stage amplifier given this circuit of a BJT so that the Amplification of voltages equals to 1500. First of all, I have no idea whether the second BJT should be the same as the given one or I should remove the R1 and R2 resistances and keep only Rc and Re.

Here is the given BJT circuit:

Some guidance would be much appreciated!

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,444
1,075
What is the theoretical voltage gain of the circuit you posted? Suppose it is k.

To get a voltage gain of 1500, you would need 1500/k = number of stages...

So what must be done to the circuit you posted if you have any hope of satisfying the 2 stage constraint?

Last edited: Jan 31, 2015
3. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,667
745
That is parallel resistors biasing circuit for bjt transistor amplifier. It puts the transistor into specific region of operation and make it very stable in that region. The reason that is important is that you want your transistor to operate in PREDICTABLE and CONSISTENT manner.

You are missing everything else.

My suggestion is: use fewer words, use more numbers.

The only useful thing you said so far is: "I need two stage bjt amplifier with gain of 1500." See? I compressed your several dozen words into 10 words.

4. ### mihachris Thread Starter New Member

Jan 31, 2015
2
0
What I know so far, is that the to total voltage gain equals the product of voltage gains on each stage: Av = Av1 * Av2
Av = 1500
Av1 -> I believe it can be calculated since I have the numbers for the first stage

From that I can find Av2. Am I on the right path?

5. ### MikeML AAC Fanatic!

Oct 2, 2009
5,444
1,075
Yes. Now compute the gain of the circuit you posted.

6. ### shteii01 AAC Fanatic!

Feb 19, 2010
4,667
745
My transistor textbook used 2N222 bjt npn transistor. This transistor has max gain of 300. How do you get gain of 1500 from 2N2222? You build multistage amp circuit.

For example get the first stage to have gain of 40, get second stage to have gain of 37-38, like you found out the total gain is the product of the two stages, Gain=40*38=1520.

Last edited: Jan 31, 2015
7. ### Efron Active Member

Oct 10, 2010
81
15
Hi,

0/ You didn't mention what is the purpose of the amplification. Is it DC amplification or AC (small signal - e.g. audio).
1/ A good initial approach in this case is to divide the total gain by 2 >> approx Av1 = Av2 = 40 .
2/ However, for Av1 calculation, you need to know the input impendance of stage 2. Do you know what the input impendance is? Why is it so important?
For the ac small signal, the Rc in your circuit will be seen as to be in parallel with the input impendance of stage 2, which means that the real Av1 will decrease with respect to with an amplifier without stage 2.

3/ Therefore, you should design stage 2 so as to have an input impendance much bigger than 4K (your Rc), for example 40K or above so that the gain of stage 1 is not impacted by stage 2. Then you can effectivelly do Av1 * Av2.

8. ### #12 Expert

Nov 30, 2010
18,076
9,691
Make that: square root of 1500 = 38.73
Still, there will be a loss between stage one and stage two as noted by Efron. This will throw your numbers off a bit.

9. ### Veracohr Distinguished Member

Jan 3, 2011
685
104
It's a possibility, but you'll probably find it easier to just use a similar circuit for the second one (perhaps with different value resistors) and connect them with a coupling capacitor.