# Voltage amplifier Design

Discussion in 'Electronics Homework Help' started by fastcharlie2, Nov 11, 2014.

1. ### fastcharlie2

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Oct 16, 2014
Any help is welcome.

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2. ### Harald KappModeratorModerator

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Nov 17, 2011
Not so fast, fastcharlie2
O.K., show us what you have done so far:
• Have you researched into the general design of voltage amplifiers?
• Have you decided on a basic design?
• Which components are to be used (discrete design or are integrated circuits aka operational amplifiers allowed)?
• You shall have to simulate the amplifier. Which simulator are you going to use?

We will help you finding the answers once we recognize that you're actually doing your homework. We will not let you hang when you're stuck, but we will not do your homework.

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3. ### fastcharlie2

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Oct 16, 2014
I was planning of using a combination of several stages of bjt(2N3904) but I don't know how to start a design problem. At school we do analysis of problems but not a design from scratch.

4. ### Harald KappModeratorModerator

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Nov 17, 2011
Is that a requirement? Do you know operational amplifiers and how to use them? Would you be allowed to use one? If yes, this is the easy way to go.
If no, what kinds of amplifiers using bjts do you know? From those you know, which ones do you think could be used for this purpose?

5. ### Colin Mitchell

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Aug 31, 2014
You will need 2 stage of amplification as a single stage generally provides a gain of 70 to 100.
You don't know the input voltage. It may be 1mV or 5v p-p.
It is going to very hard to accommodate the wide range with a general-purpose circuit, but here's an answer:
You could try driving a LED via an electrolytic with a 100R across the LED.
This would satisfy the output requirement.
From 1mV to 1,500mV is a voltage gain of 1,500.

6. ### Harald KappModeratorModerator

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Nov 17, 2011
At a max. output voltage of 1.5V and a gain of 1000...1500, the input voltage cannot be higher than 1.5mV (at gain=1000).

Colin, are you mixing up answers to diffferent posts? What's the LED supposed to provide in terms of gain for an amplifier?

7. ### Colin Mitchell

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Aug 31, 2014
The LED is to limit the output to 1.5v or you can put 2 diodes in the output.

8. ### Harald KappModeratorModerator

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Nov 17, 2011
I don't think this is what is meant by the task description. If so, however, an LED is imho not the correct component.
1. The knee voltage of an LED is not well defined and very temperature dependent.
2. The non-linear knee of the LEDs characteristic will introducce distortion in the analog signal (as long as the outpuut impedance of the amplifier is noticeably > 0Ω)
3. Two LEDs in antiparallel circuit are required for bipolar (AC) operation.
4. 1.5V rms = 2.12V peak, much more than a typical LED will limit.

9. ### Colin Mitchell

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Aug 31, 2014
"bipolar (AC) operation." No-where does it specify bipolar output.
The output can be 20% distortion. There is no required specification on the distortion.
"1.5V rms = 2.12V peak, much more than a typical LED will limit." Green LEDs go to 2.3v but two diodes will go to 1.5v The only specification is a MAX of 1.5v rms.
No-one else has come up with a way to limit the output to 1.5v. You have to come up with something simple.

10. ### Harald KappModeratorModerator

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Nov 17, 2011
Well, it states "... output at 1kHz...", that sure is AC for me. And I consider AC to be bipolar.

Right, there is no such requirement. On the other hand there is the sub-task "simulate and measure the harmonic distortion of your design..." I'd consider an amplifier with 20% distortion in the pass band a very bad design.

Thats because I honestly think that no limiting is required. As I understand the requirement "...maximum output is 1.5 V rms" this means that the amplifier is not required to deliver more than 1.5V rms. In other words forinput voltages > 1.5V / gain, the amplifier may go into saturation.

If the task required a voltage limited output, I'd expect this to be stated clearly in the task description (e.g. "limit output voltage to not more than 1.5V rms" or similar.

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11. ### Colin Mitchell

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Aug 31, 2014
"Well, it states "... output at 1kHz...", that sure is AC for me. And I consider AC to be bipolar."
What about all the thousands of 9v transistor radios you must have fixed.
The output can be 1kHz. Where is the -9v battery ????
AC does not mean bipolar. It just means a waveform similar to a sinewave. It can be 0v to 9v.
"Right, there is no such requirement. On the other hand there is the sub-task "simulate and measure the harmonic distortion of your design..." I'd consider an amplifier with 20% distortion in the pass band a very bad design."
Who says it is an audio amplifier ???
"If the task required a voltage limited output, I'd expect this to be stated clearly in the task description (e.g. "limit output voltage to not more than 1.5V rms" or similar."
That's EXACTLY what is says. The maximum output is 1.5v rms.

12. ### Harald KappModeratorModerator

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Nov 17, 2011
The supply voltage is DC, right, but the output to the speaker is AC.

That would be AC superimposed on DC.

I never did.

Sorry, that is your interpretation. I understand this part of the task description differently. It says "...max. output is...", not "...output shall be limited to...". For me this is a difference, but without further clarification by the op this is an open issue.

13. ### Colin Mitchell

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Aug 31, 2014
"That would be AC superimposed on DC."
Not necessarily. There is no DC on a speaker. It would shift the cone.

14. ### Harald KappModeratorModerator

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Nov 17, 2011
Right, that's what a decoupling capacitor is for: blocking the DC component so the speaker sees pure AC - and back we are where we started:

15. ### Colin Mitchell

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Aug 31, 2014
"And I consider AC to be bipolar."
It depends on if you expect the signal to have a negative excursion.

16. ### Harald KappModeratorModerator

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Nov 17, 2011
It will have, after you have it decoupled by a capacitor, see posts #13 and #14 . Without a negative excursion you have AC superimposed on DC and, picking up your example, a speakers cone would be statically shifted.

17. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Actually, it does!
No, it doesn't have to be similar to a sinewave. It's generally a periodic waveform, if that's what you meant.
The assignment specified the -3 dB bandwidth as 10 Hz to 10 kHz. That is the audio range. The assignment mentioned distortion figures. This makes it pretty clear to me that it is supposed to be an audio amplifier, even though the assignment doesn't say that explicitly.
I'm with Harald on this one too. The assignment doesn't say that the output must be limited to 1.5V RMS. I see this as a guideline so the student has some idea of the signal level expected at the output.