# Cascade BJT AMP two stages - HELP

Discussion in 'The Projects Forum' started by darryl capulla, Dec 16, 2014.

1. ### darryl capulla Thread Starter New Member

Dec 14, 2014
3
0
Hello Engineers,
I just want to Design a Cascade BJT Amplifier Two Stages,
but my problem is, Im only given two values
Vin = 50mVp @ 25Khz
Vo= 3Vp

Vcc = Any Value

Amplifier should work 25kHz

Any suggestion how to find the values or of resistor and capacitors??

File size:
21.9 KB
Views:
58

Dec 14, 2014
3
0
3. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,098
1,478
There are no hard and fast rules for determining the values, but I'd use the following steps for both stages, as a first approximation. Re and/or Rc might then need tweaking to get the exact voltage gain you need:-

1) Choose Vcc. Since you need Vo=3Vpk, make Vcc >> 3V. I'd choose 9V for example.
2) Choose the collector current Ic. As you are not constrained re the load the amp drives, I'd go for ~2mA (arbitrary).
3) Calculate the Rc value such that the collector current drops half the supply voltage. This will bias the collector appropriately.
4) Choose Re as about 20% of the Rc value.
5) Calculate the Re voltage drop and add 0.7V to get the base voltage Vb.
6) Calculate R1 and R3 as about 10 x Rc.
7) Calculate R2 and R4 to give the calculated Vb.
8) Calculate the total gain as 3V/50mV = 60 and assign a gain A of square root 60 = ~8 to each stage.
8) Replace each Re by two resistors Rea,Reb in series, where Rea = Rc/A and Reb = Re-Rea.
9) Connect Ce across Reb only. Calculate Ce to have a reactance of < Reb/100 at 25kHz.
10) Calculate Cb to have a reactance of < Rea at 25kHz.

BobTPH and darryl capulla like this.
4. ### #12 Expert

Nov 30, 2010
17,889
9,298
Remove Ce1 and Ce2. They make the gain unpredictable in that configuration.

5. ### darryl capulla Thread Starter New Member

Dec 14, 2014
3
0
Thanks alot Sir!!!