# 2 stage amp help

Discussion in 'The Projects Forum' started by zionarbadon, Jun 10, 2011.

Jun 10, 2011
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Hello,
I tried the search function but most of what I found was op amp

I have a project to make a 2 stage (or multistage) amp to take a 20mVpp 2500Hz 600 ohm source resistance ac input and get a 2V pk-pk output on an 8 ohm speaker.

I also want to try and do a 15 volt vcc and a -15V on the ground side.

The hard part is that we are not in a design class and we have to design a circuit. So we always are given the resistor values and in this we have to figure them out. Also there are really odd requirements (must use an npn, pnp, and 2 of the folowing: common emmiter, common collector, common base)

Are there any formulas that will help me figure out what resistance values to use? I may be able to proceed from there.

Thank you,
Brandon

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,386
1,605
You need a transistor circuit with a gain of 2V / .02V = 100.

By saying "must use an npn, pnp, and 2 of the folowing: common emmiter, common collector, common base" he's actually giving you the hint this needs to be at least a 2 stage amp.

Is the +/- 15 volt power your idea or his?

Did you learn any basic amplifier stages yet?

(It is much better for you that we talk you thru this rather then giving the complete answer.)

3. ### wayneh Expert

Sep 9, 2010
12,087
3,026
Current gain is another clue to what is needed. Input is 33.3µA at peak, output is 0.25A.

Jun 10, 2011
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Yeah I realized I need 100 gain across 2 stages, which is one of the things that make it hard because i'm not sure how much gain to do at each one. I guess thats kind of up to me.

as for the +15 and -15, the other part of the project is to is to build a bridge rectifier. The one i was going to attempt was going to have a pos 15 off of the right side of the bridge and a neg 15 off the other side, but I guess I could be making it too complicated?

We learned how to solve both dc/ac sides of basic CC, CE, and CB circuits. We havent really done any multistage and we wont have anymore lecture before the end of the quarter, just one more lab.

I didn't think about the current gain, I wonder if I can use that to figure out β but again, having more than one makes things confusing.

As for the attachment: This is the set-up that I was attempting but the resistors are really just place holders as I'm not sure how to figure out the values, or even if that particular set-up will work. I wish we had time to go over the multi-stage stuff in class. xD

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5. ### #12 Expert

Nov 30, 2010
16,248
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As a matter of opinion:
split fifteens are a good thing. They make the math easier and the ground convenient, especially when you are going to need to drive 8 ohms with +/- .25 amp.
One way of figuring gain across multiple stages is to use the square root of the needed gain and have each of 2 stages do the square root of the total amount, or cube root for three stages.
This is crying out for an emitter follower for the output stage, and they don't have voltage gain, so that square root method won't work here.

Can you do a voltage gain of 100 in a single stage transistor amp?
Can you figure a Class A output for that load?
Can you figure a push-pull output for that load?

Just some ideas.

Jun 10, 2011
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I'm not sure what an emitter follow is, we must not have gone over that one. I think learning about the difference between classes (A,B, etc) is the next electronics classes. We also haven't learned what push pull means.

I feel so unprepared. . .

Also, as a note, I had asked the teacher if I could use darlington and he said if we do that it only counts as one of our two stages :-/

I may be able to figure out the gain of 100 for a single load though, let me try to figure something out.

As i said, I'm used to working with resistor values given so staring with nothing is so overwhelming. I will work on that and let you know what i figure out though!

Jun 10, 2011
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I think I can figure out the beta if it's a single stage, so I know that the Ic would be .25 and that beta is usually about 100 for the transistor so Ib should be about .0025

Let me know if I'm doing it wrong xD I will continue to work on it

Jun 10, 2011
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I can also get Ie by adding Ib and Ic together. so .2525A

Jun 10, 2011
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Now I'm not sure where to go because I feel blocked by not having any resistances? Anything to jog my thought process?

10. ### CDRIVE Senior Member

Jul 1, 2008
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Why are you using an 8Ω speaker when a small 30Ω speaker would make much more sense for a little amp like this?

11. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,386
1,605
Because that was a given of the assignment?

12. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,386
1,605
That's a good start, one stage for voltage gain and a stage for current gain.

The Q2 stage (common emitter I believe) is what I was thinking of too. A hint to get the resistors (to bias it): try starting with all the resistors at 5K, and get the voltage at the emitter to 5V, and the collector to 10V. That gives you 1/3 1/3 1/3 of the 15V across R5 Q2 and R3.

Also your capacitors need to be much larger, do you know how to compute how big?

I'm guessing Q3 is your emitter follower (common collector) for current amplification.. You need to shift the load (the speaker) into the emitter, and then connect the other end of the speaker to +15; it looks weird but that is what the pnp wants and the speaker doesn't care what you call the voltages it connects to.

Hey, the -15 didn't get used! (I didn't think it would be.)

Are you going to actually build this circuit?

13. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
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A plain 500 or 1K ohm to 8 ohm transformer might simplify the output alf alllow the gain to be spread out a little.

Whit that you've got drawn R6 should be lightly less than R8 and you wouldn't need C5.
The ratio of R1 to R2 should be much closer or Q2 will never come on, also and R4 isn't necessary.

Jun 10, 2011
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Yeah, that was a silly mistake of me on the PNP. Common collectors have the output on the emitter. As for capacitors our teacher literally told us this "uh, you can just use 1uF or .1uF for all of your capacitors, it should be fine." So that what I was doing.

I may be building it, but not likely as 1 out of 4 parts at our lab center are bad and most of the equipment (scopes generators etc) have quirks at each station. But if I have time I may try to as it is extra credit.

So, I changed all resistors to 5k (except Rs and RL) and I am checking on the voltage drops for the emitter of Q2 and Collector of Q3. Now, on the emitter of Q2 I have a 14.166V. So does that mean my collector would have A voltage of 0.834 or 15.834. Or do you mean AC? Also, should I not even bother with the -15V then? I often make things harder than they have to be on accident.

The really odd thing is that the DC voltage across R6 was constantly dropping. I have no idea why it is doing this.

How did you decide to start with 5k then work off of that? is that just a generic starting value you like to use or was there some theory behind it? Thanks for your help by the way!

@Marshal in all of our examples from class the R1 was always significantly larger than R2 (at least 3 or 4 times as large) and thanks for noting that I did not need that cap, changed the circuit like 5x and forgot to take it out.

15. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Your first post stated that you have to deliver 2VPP across an 8Ω speaker load. Your last post stated that your instructor said that you can use .1μF or 1μF caps throughout your circuit. FYI... a 1μF speaker coupling cap will require at least 16VPP drive voltage on the input side of the cap for an 8Ω speaker. You would be well advised to use a 100μF cap for this.

Jun 10, 2011
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Yeah, that was just something he said as an aside (about the caps). He's not considered to be one of the good teachers xD. I will try that.

Jun 10, 2011
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Okay, what i ended up doing was looking up the Qpoint on the data sheet and caculating the max current and then I used the total resistance from that max current and split it between R3 and R5. Any suggestions on how to do the next stage with no gain cause I got it about perfect on the first stage. The teacher said that the second stage doesn't really have to do anything as long as we have it in there and meet the requirements.

18. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
What the heck does that mean? Is your teacher awake when in class???