# 2-Stage volt amplifier.

Discussion in 'Homework Help' started by Richjtf, Oct 12, 2013.

1. ### Richjtf Thread Starter New Member

Oct 12, 2013
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Hey there, if anyone would kindly help me, i would very much appreciate it! Any kind of help works, tips, advises.. I'm retaking a course and it has been so long that i don't remember much from before i left!

I'm asked to design a 2-Stage voltage amplifier and i have no idea where to start from..

I have to use a common emitter amplifer in the first stage, and a common collector one for the second stage, this has to supply a load of 100-Ohms.

Power supply=20V
Suggested transistors=123AP

The circuit must have a voltage gain of Av=-15 and a Ai=-80

This design must not have an emitter capacitor.

2. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
519
Well you could start by saying if you know what the common emitter (CE) and common collector (CC) look like?

Sketch them out and see if you can fit them together.
What is the difference and what does that mean for you amp?
Why do you think they want one of each in your design?

Why might you want an 'emitter capacitor, and where would you put it?

It's your hoemwork we can offer help (which can be substantial) but we do not do your homework for you.

3. ### Richjtf Thread Starter New Member

Oct 12, 2013
15
0
Yes mate, i know.. And i want to design it on my own, but first i need advise or tips on how to do it because i've tried but didn't succeed, i forgot the procedures/formulas from which to work from. As i remember in a common emitter, the signal input is the base, the output is in the collector, and the emitter goes to GND, also has no voltage gain but does have current gain, as i remember..

In a common collector the signal input is in the base, output in the emitter and the collector goes to GND, that is what i remember, has both voltage, and current gain?

I have read that the total gain will be equal to the product of each individual gain, so since this is a 2-staged amplifier and i need a total gain of -15V, i should be looking for individual gains of 3 volts and 5 volts, i suppose.. Then again i will also have to get a total gain of Ai=-80

About your last question, i don't know, the problem just states so, it must not have one.. I will work on a design and will get back to post the results later, then i could get some more tips that could help me fix it, right?

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
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Look at the tabs at the top of this page, VOL III - SEMICONDUCTORS for some info.

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,922
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I am either missing something or misreading something.

Av is voltage gain?
Ai is current gain?

Am I understanding correctly that you will input 1 volt, your two stage amplifier will amplify it 15 times, your output will be 15 volts?

6. ### WBahn Moderator

Mar 31, 2012
20,069
5,665
Since it has been so long since you took the prerequisite courses, you can expect that you will have to do a lot more than rely on what you remember. Pull out your text for the prereq course (or get another one dirt cheap from Amazon or find material on the web) and bring yourself back up to speed on the stuff that you were expected to have fairly fresh in your brain when you started the class. The necessary review will do you wonders.

If you've tried but didn't succeed, then post your attempt. If nothing else, post the basis configuration that you think the circuit will take on to meet the constraints.

7. ### Richjtf Thread Starter New Member

Oct 12, 2013
15
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Yes, Av and Ai are voltage and current gain, and, the input 20 volts, the total voltage gain in the 2-stage amplifier must be -15V

Yes, i know man, but i'm asked to do this by monday, and i have plenty of other things to do as well for other subjects, so i'll get to that afterwards..

Also, just one question, what exactly is R-load?

I will post the circuit i'm doing as soon as i get it to atleast work

8. ### #12 Expert

Nov 30, 2010
17,840
9,178
Some basics: The voltage gain is -15 so you must have one stage invert the signal, not zero stages or 2 stages. The usual method is to get most of the voltage gain in one stage and most of the current gain in the other stage.

Work backwards. Do the output stage first and it will tell you what the input stage has to give it to make it work right.

anhnha likes this.
9. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Gain does not have units. Either you need gain of 15 or you need to produce 15 volts. Pick one or the other, you can not have both.

If you need to produce 15 volts, then you need to find out the input voltage, Gain==(Output Voltage)/(Input Voltage).

You power supply is 20 volts, power supply and input voltage are different things. So again. Either you have power supply that is 20 volts, or you have input voltage that is 20 volts, not both.

See this pdf. Example 5 and 6 are the type of circuit you need. First stage common emitter, second stage common collector (emitter follower). And they don't have bypass capacitors at the emitters, so that fits your design requirements.
Unfortunately the examples use already designed, but maybe you can work backwards and figure it out:

10. ### Richjtf Thread Starter New Member

Oct 12, 2013
15
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I'm sorry bro, that's what the problem states, i got no idea. Look it says -"Design a 2-Stages amplifier, using a CE (Stage one) and a CC(Stage two). This must supply a load of 100Ohms. You dispose a 20V source, 123AP transistors are suggested. The circuit should have a tension gain of Av=-15 and Ai=-80. This design must not have emmiter capacitor."-

I feel ashamed, i've been woking for like two hours already and i can't find the way to make it work, i mean, this probably is pretty darn easy, but i just don't remember(I have to state that i learnt how to design mono-stage amplifires on the last course, not multi)

These two circuits are all i got so far, not working properly at all though..

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11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,922
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Nothing to be ashamed of. If I remember right, the class that taught this material, I barely got through it with a B, might have been C.

#12 made a good point. Since you have minus signs, one of these stages must be inverting. Also from reading just now. Common Emitter inverts the voltage, so if you apply positive voltage at the input, you will get negative voltage at the output of CE amplifier. Also, CC is the way to get current gain, like #12 said. Also, like #12 said, do Stage 2 (CC) first, then do Stage 1 (CE)

Vcc (the supply on the right) = 20 V

Your Vin for CC (Vin Stage 2) is going to be Vout for CE (Vout Stage 1) which is 15*Vin for CE (15*Vin Stage 1).
So lets say your input voltage to Stage 1 is 1 V at 1 KHz, your output from Stage 1 will be 15 V at 1KHz. The output will be 180 degrees out of phase when compared to input, so that is the minus sign. So you put in 1 V, you got out -15 V, you meet your Av requirement for Av=-15. And now you use that 15 V at 1 KHz as input to Stage 2. In Stage 2 voltage does not change phase, so you put in +15 V, you get out +15 V. If you put in -15 V, you get out -15 V. But compared to the input of Stage 1, your output of Stage 2 will be 180 degrees off, meaning if input is positive, output is negative.

Last edited: Oct 12, 2013
12. ### studiot AAC Fanatic!

Nov 9, 2007
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Looking at you circuit diagram, I suggest you take note of the comments being offered.

You require a voltage gain of 15, inverting.

Fine a CE stage will happily produce that.
A CC stage following will have a voltage gain of nearly +1, but will supply the current gain.

Now as to the input, 1 volt was suggested, but your diagram shows 5 volts.

5 volts what? rms, peak to peak?

Either way it is too much. 5 volts times 15 = 75 volts, which cannot be accomodated within a 20 volt supply rail.

1 volt is too much.

I would suggest that your input signal should be about 0.1 volts rms.

You will then have an output of 1.5 volts rms, sensibly achievable within the design spec.

Similarly the current gain.

I suggest an input of 0.1 milliamps yields an output of 8 milliamps as achievable.

Where do you take the output from your second stage to drive the 100Ω Load? Is your 100Ω resistor meant to be the load?

Someone suggested starting at the end and working back.

So start at the load ask yourself, what is the required peak to peak voltage across it?

This will establish the required minimum DC bias for the emitter of the second transistor.

Add 0.7 volts to get the required minimum bias for its base.

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13. ### Richjtf Thread Starter New Member

Oct 12, 2013
15
0
Yes, i now get a negative voltage output, my problem now would be on how to play with the current, how does it look so far?

Actually the thing is, i had left College long ago, now i got back and could retake from where i had left but i lost practice, i'm on Electronics II now(That's the name of the subject), and this problem is what i need to design and take with me for the lab session i got on monday, the professor had to teach us atleast some quick insight on how to work around this, but he didn't and that is why i'm havin such a hard time here..

Edit: I never learnt to work with multi stage amplifiers before either..
Edit2: Hadn't read your edit nor the comment above this one before i posted this, going to try that.

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14. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Your R5 is the load resistor listed in requirements. It should be 100 Ohm. Your meter should be connected across the R5, that is where you look for the output voltage and current.

15. ### Richjtf Thread Starter New Member

Oct 12, 2013
15
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Thanks very much to both of you for the tips, i'm going to use your advises to work around this and will get back as soon as i get this done!

Edit: Or if i get to need more help!

Last edited: Oct 12, 2013
16. ### studiot AAC Fanatic!

Nov 9, 2007
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519
I need to knock off soon and would like to know if you understood what I was saying?

I have attached a quick sketch to help.

Can you see that if you have 1.5 volts rms across the emitter load (the emitter resistor in parallel with the load) you have 4.2 volts peak to peak.

So the emitter vary must be able to vary up to 4.2 volts and down to zero?

Leave the value of this resistor for the moment, because its value will be determined by the current gain as well and go on to see where that places the base of the output transistor - That leaves the first stage collector working between 5 volts and 20 volts.
This will set its base bias for you.

Because of the bias on the base of the input transistor I note you realise you will also need a capacitor between the voltage source and the base of the input transistor.

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17. ### Richjtf Thread Starter New Member

Oct 12, 2013
15
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To be honest, i don't entirely get it. Because when i try that on the simulator it does not give same results,(that and i'm sleepy as hell too, so i can't focus very much) but don't worry, with all the tips you and the others have given me so far i should be able to carry on and get this done with by tomorrow!

In case you're about to go to sleep, have a nice sleep, and thank you again mate, goodnight!

Last edited: Oct 12, 2013
18. ### WBahn Moderator

Mar 31, 2012
20,069
5,665
One thing that I haven't seen pointed out -- but I might easily have missed it -- is how to resolve meeting BOTH a voltage gain spec and a current gain spec.

You can do it, but it indirectly imposes a constraint on your first stage amplifier that you aren't taking into account because you haven't recognized it.

Consider the following questions:

Your input consists of an input voltage, Vi, and an input current Ii.

Q1) What amplifier parameter relates Vi and Ii?

The output consists of an output voltage, Vo, and an output current, Io.

Q2) What parameter that you have been given relates Vo and Io?

Q3) The voltage gain, Av, is the relationship between what two quantities above?

Q4) The current gain, Ai, is the relationship between what two quantities above?

Q5) Taking all of this together, what is the required relationship between the parameters that are the answers to Q1 and Q2?