# Multistage BJT amplifier

Discussion in 'Homework Help' started by Ksin301, Jun 25, 2014.

1. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Hi all professionals engineers
I'm new member in this forum and I having a problem on understand the cascading amplifier. My coursework is request me to amplify a 10mV input with audio frequency to 2V peak-to-peak output that driving 10kohm resistive load. In this design, we require to build the buffer stage, gain stage and an output stage to drive the output loading.
I did a circuit with calculation that based on my knowledge that learn in the class but I keep getting the results that no amplifying the input but decade the input voltage to nano voltage :X

So the problem I want to know is:
1. What is buffer stage, gain stage and output stage is really doing?
2. Any more idea to improve my circuit on the attachment? Give me some advice.
3. I using UCE x2 and EFCx1 in this circuit. UCE is the gain stage I think to amplify the signal input and EFC is output stage to show the output.

Thank you for the expert help

File size:
23.4 KB
Views:
125
2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
There are Q1, Q2, Q3.
Which one is amplifier?
Which one is buffer?

You say you have 10 kOhm load.
Where is it?

You oscilloscope channel missing a ground connection.

Ksin301 likes this.
3. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
I think the Q1 is acting as the buffer, then Q2 acting as amplifier.
Sorry man, I have no idea where I should put my 10 kOhm load :x
opps, thank for the notice man.. I did not notice that was working on this circuit whole day and having no idea

shteii01, what acttually the buffer stage and gain stage is doing?

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,605
396
hi 301,

All 3 transistors are incorrectly biased, re-calc R1/R2,, R4/R5,, R9/R10.

E

Ksin301 likes this.
5. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
thank you ericgibbs ,
I will try to recalculate and upload the new circuit ASAP

6. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Is it Zout at output stage = Resistive load that drive output stage?

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
Last edited: Jun 25, 2014
8. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Thank you shteii01,
I would improve that
Buffer is Unity gain op amp that type right? Is it mean Av =1 at the buffer stage?

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
Yes. X goes in, X comes out. Basically you have two circuits and you want to buffer one circuit from the other so you put a third circuit (buffer circuit) between them. You are not trying to alter the signal between the two circuits (that is why buffer circuit has Gain of 1, you are not trying to amplify or attenuate or whatever the signal), you just want to "protect" one circuit from the other so you put buffer circuit between them.

If you can use op amp, then use it, it is much easier to use. But op amp is not required, a bjt transistor can be used to make buffer circuit.

Ksin301 likes this.
10. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998

That's only 1 definition for a buffer. There are lots of 'kinds' of buffers. Yours should match the source impeadance to the input impeadance of the gain stage. The buffer can have gain or not, it's not important. What is important is how the buffer and gain stages work together to efficiently and effectively product the required gain. A buffer doesn't need to be a common collector configuration.

Ksin301 likes this.
11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
All true.
I am just trying to keep it simple.

Also. Bownout has not come out and said it, but... When doing work on multistage circuit, you actually work backwards. You should have started at the speaker and speaker is connected to the output stage, so the first step in the design would have been to design output stage.

The output stage would have been the load for the amplifier circuit, armed with this info you would have designed the amplifier stage.

The amplifier stage would have been the load for the buffer stage, again, armed with this info you would have designed the buffer stage. Since you know the source, you know the source resistance. This way you would know source impedance and input impedance of the gain stage and properly design the buffer stage.

12. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Thank you BrownOut and Shteii01 helping
I should focus more researching to get more clear function of the stage that I should using the circuit, too bad I only can use bjt in the circuit to made amplifier. But BrownOut is that possible to amplify the 10mV input to 2V like 200 times larger than the original input, if like that the amplify could be very big, what if I put 1V I could support a house voltage usage 200v in Malaysia :x
Is the current gain could reduce the size of the amplifier require?

13. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Cool shteii01, I dont even know that the multistage circuit should start design from behind, I should try right now .. btw thank alot guys , I got alot of confusion when I doing Analog circuit design, my lecturer slides or notes dont even cover the brief of multistage circuit that why I got quite fail in my design :X, I would improve that ASAP

14. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
10 mV to 2 V is not the problem.
Why?
Because your "rails" are 0 volts and 12 volts. As long as your output (in your case 2 volts peak to peak) is inside the "rails", you have no problem.

However, remember that we are not working with "ideal" devices. In real life you are unlikely to actually reach the rails. I would not be surprised if the best you can get is output of say 8 or 9 volts. So you would input a wave 10 mV peak to peak and get out 8 volts peak to peak.

Also. In case you have not noticed yet and Eric pointed out, your rails are 0 volts and 12 volts, you need to bias your transistor so that the signal is oscillating about the 6 volts DC level. To put it into number:
You input is 10 mV peak to peak.
You output from the buffer should be a wave from 5.995 volts to 6.005 volts.
This wave then goes into amplifier, what comes out of amplifier is a wave from 5 volts to 7 volts.
Your lowest rails is 0 volts.
Your highest rail is 12 volts.
You lowest output is 5 volts.
Your highest output is 7 volts.
You never reach the rails, the limits.

Ksin301 likes this.
15. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,518
515
You could cut out the output stage and work from speaker-amplifier-buffer. I know you need 3 stages, but with this two stages you can actually get hands on design experience and then work on the 3 stage circuit.

Ksin301 likes this.
16. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Now that you know what your buffer is supposed to do, do you see a key requirement missing from your assignment?

Ksin301 likes this.
17. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Yea I think I could point the missing part
1. The buffer amplifier.
2. bias the transistor so that the circuit can oscillating about 6 DC volts and put in the number to recalculate the resistor values.
3. Re-analysis the circuit from backward.

I will try it and get an improve circuit and got your guys opinion and advice soon
Thank for the help guys

18. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Not quite. Remember I said the buffer matches something to something. One of those something's is missing from your requirements.

19. ### Ksin301 Thread Starter New Member

Jun 11, 2014
11
0
Oh ya, the source impedance and the input impedance of the gain stage!

20. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
The input impedance of the gain stage is something you will decide. This will typically be a reiterative process as you match interstage impedances. However, you are right about source impedance. Real sources have impedances, which is why you need a buffer stage. So, always make sure your requirements are fully specified before you start designing.