# design assignment two stage bipolar transistor... having issues with calculations but have created a schematic

#### Ajdawson94

Joined Nov 19, 2018
5
im currently having to do a project with 0 university assistance thanks to covid.
the aim is to design a two stage bipolar transistor and calculate the components required for the specifications.
i have calculated several values. heres the schematic.

with and input of 15 v and VE1 having to be

i chose a VE1 value of 3v

the transistor used is the BC846B
the supply voltage is 15 v
the collector currents 1 (ICq1) = 1.5mA
the collector current 2 (ICq2) = 7.5 mA
output voltage swing= max
frequency response = 50- 20 khz
i must calculate the required specifications for each component
i have so far calculated
VE1= 3v
VE2= 8.4v
RE1 at 2k ohms VE1/ICQ1
RC1 at 4k ohms VRC1/ICQ1
RE2 = 1120k ohms VE2/ICQ2

however im unsure on how to calculate the other areas. this is all ive been given with no lectures or contact since early april.

any assistance or formula's would be greatly appreciated im not asking for answers just help with actually understanding what the equations mean and how i can calculate whats required.

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#### MrAl

Joined Jun 17, 2014
7,895
Hi,

What does C3 do?

#### Audioguru again

Joined Oct 21, 2019
2,449
He said that C3 rolls off the high frequencies.

#### WBahn

Joined Mar 31, 2012
26,398
Part of the problem you are having is that your schematic doesn't line up with the instructions you are given. For instance, the instructions talk about R4. Your schematic doesn't have an R4. Since they don't match in this regard, there is no reason to assume that the C3 in the instructions is the C3 in the schematic, or even that the circuit in the schematic bears any resemblance to the circuit being discussed in the schematic. So take a bit of time and make the schematic and the instructions match. It will pay of considerably.

#### Audioguru again

Joined Oct 21, 2019
2,449
R4 is obviously RC1 on the schematic since the instructions say that C3 and R4 set the maximum operating frequency.

#### WBahn

Joined Mar 31, 2012
26,398
R4 is obviously RC1 on the schematic since the instructions say that C3 and R4 set the maximum operating frequency.
I agree, but I still would recommend that the TS align their schematic with the instructions. Not only does it reduce the chance of miscommunication here, but it also falls under the heading of the proper care and feeding of homework graders. Most graders only have a few minutes to grade a particular problem (or perhaps even the whole assignment) per person, so anything that makes their job go smoother will almost certainly pay dividends.

#### MrAl

Joined Jun 17, 2014
7,895
He said that C3 rolls off the high frequencies.
Ok, so then where is R4 ?

#### Audioguru again

Joined Oct 21, 2019
2,449
The schematic shows that RC1 is R4.

#### MrAl

Joined Jun 17, 2014
7,895
The schematic shows that RC1 is R4.
I would have guessed that the RC1 is really R4, but that is not on the schematic.
Where do you see that is that the schematic in the first post?

I was going to suggest that the position of C3 is not a really good choice.

#### MrAl

Joined Jun 17, 2014
7,895
Here is a new schematic with the resistor R4 labeled correctly.

I was going to suggest that maybe C3 should go to ground instead of to +Vcc that way it does not bang the output with a spike during power turn-on.

#### Audioguru again

Joined Oct 21, 2019
2,449
I agree that C3 should go to ground.