# Urgent >> I need your help

Discussion in 'Homework Help' started by Nice, Jun 13, 2009.

1. ### Nice Thread Starter Member

Jun 13, 2009
19
0
I have a project due tomorrow

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It is requested to design an audio amplifier which will be inserted between a TV receiver and a stereo system. The designed voltage amplifier should exhibit the following specification:
- Small signal overall voltage gain >50 V/V.
- Input resistance Ri > 40kΩ.
- Output resistance Ro < 1kΩ.

- Verify the computed nodal voltages, currents (for small signal & DC) and gain of the designed amplifier with the results obtained form the related PSPICE program.
(a)Load resistance RL= 1kΩ.
(b)Single supply voltage of VCC= 12 V.
(c)Input voltage (vsig) and the internal resistance Rsig
vsig =0.01 sin(2π 1000t) and Rsig = 2kΩ.
(d)Check for the mode of operation.
(e)What is the maximum allowed input voltage with no distortion or clipping at the output voltage.
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I used two stage amplifier (CE & CC) but it didn't work in the PSpice

please till me what to do , This course is my first one !!

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
Can you post up your results?

3. ### Nice Thread Starter Member

Jun 13, 2009
19
0
Can you post up your results?
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for CE:

Rb1 = 160 & Rb2= 80 kΩ
Rc = 8.7 kΩ , Re = 6.6kΩ

for CC:

Re= 2kΩ

4. ### Nice Thread Starter Member

Jun 13, 2009
19
0
also I assume the value of the capacitors to be 22u

the Questions are:

How I start the problem ??
should I assume any thing ??
should I begin with dc analysis or small signal analysis ??

I am so confused

5. ### Audioguru Expert

Dec 20, 2007
10,799
1,218
Why didn't you post your schematic?

6. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
This schem. will help you get started in some direction,
forum rules state that we are not allowed to give the answers,
so use this schem. as a way to help get started, in your design, add to it or deviate from it,
do what ever is nessary for you to learn to design a transistor circuit. That's why the values for the bias resistors are left blank.

Data concluded on this circuit. All aproximations.

Rin = 100K +

Rout = 100 ohms

Av. = 100

-------------------

When
Rout = 33 ohms

Av. = 50

Here it is..

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7. ### Nice Thread Starter Member

Jun 13, 2009
19
0

Thank you so much

there are 4 BJTs in the circuit...can I use two rather than four??

8. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
Yes, it could be designed with 2 transistors,

I used 2 for starters, CC,CE, and the Rout was around 820 ohms, but my Rin was a 56K and 68K in parrallel which reduced the Rin to around 30K which was to low for the parameters set, but that's because I used a CC amp for my input, and 2 bias resistors. That could be reworked to increase Rin...

I added the CE amp to the input to raise the Rin, to a much higher value,

depending on which amp config. you use for input and output, would determine the amount of transistors needed.

9. ### Nice Thread Starter Member

Jun 13, 2009
19
0
I will try to use CE-CC or CE-CE-CC if needed

thanks bro

10. ### Nice Thread Starter Member

Jun 13, 2009
19
0
one question to ask...

what is the better way to start my analysis ??

the small signal or the dc analysis..what assumption should I made ??

11. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
I used more DC analysis when I started this circuit design, because of the Resistance parameters that had to be met, so once I met those parameters, then I applied the input signal and verified if this circuit was getting the small signal gain required.

12. ### Nice Thread Starter Member

Jun 13, 2009
19
0
I think I must assume B for the BJT and Ie for Q1

13. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,667
1,311
Why you don't want to try the amplifier with overall negative feedback.
Here is a classic example

And β - you will find in a datasheet

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14. ### Nice Thread Starter Member

Jun 13, 2009
19
0

Thank you .. this is my first course in electronics & I want to make every thing as simple as possible

Also we didn't take the -ive feedback yet

15. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
It seems like your trying too hard to take all these variables at one time,
B,IE,IC,small sig. dc analysis,,ect....

Try to take a design in bite size pieces. Otherwise your spinning your wheels.

I'm assuming you were taught how to bias a CE, amp using the voltage divider and emitter resistor, as well as emitter bypass capacitor.
That is pretty much a standard design.

First start by drawung a schem. of a CE amp.

Now look at your parameters for the output impedance.
First your load is 1K ohm. Now your given that your Rout < 1K.

You have learned that Rout of a CE. approx. = to RC.
So choose a value of RC that is lower than RL.

Once that is chosen you know for a linear amp. Q point should be around 1/2 VCC. Now solve for your IC. Now there are many ways to do this, but since your given a gain parameter, try to choose a RE that would come close to your gain by (RC/RE).

Now you have established RC and RE, and IC.

Now solve for VE and add 0.7 to it to give you the required VB.

Now choose a value of RB1 (base res. to ground), that gets you close to your desired Rin, remembering if you go to high, it will upset your bias voltages at the collector due to base current loading on RB2.

Solve for divider current by taking your VB/RB1, and then using that value to solve for the resistor remaining that goes from base to positive supply.

Build test (simulate) anf check for proper bias voltages, and if voltages are off by a good amount adjust values on the resistors till you get your targeted values in key areas, mainly the VC which is 1/2 VCC.

Now from there you probably won't have the proper Rin needed, so that's where you take the next stage development.

Either choose another CE, or go fet, or CC, but what I'm showing you is design one step at a time simulate it, adjust values redesign simulate adjust values until your able to get the proper results.

When you get a schematic drawn and simulated it post it and we would be happy to help you along with any questions concerning your design.

Last edited: Jun 14, 2009
16. ### Nice Thread Starter Member

Jun 13, 2009
19
0
^^^

Thank you man

That is what I need>> the starting point