# audio amplifier

Discussion in 'Homework Help' started by e-le, Mar 2, 2016.

1. ### e-leNew Member

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Design an audio amplifier that can deliver an average power of 280mW to a 64 ohm speaker.
Signal which is used to amplify is 2000Hz sinewave with a peak-to-peak amplitude 0.9V.

provided with LM741 op-amp, resistors, capacitors, inductors and 2N2222 transistors.

I need guidance to design a circuit .

Last edited: Mar 7, 2016
2. ### cowboybobWell-Known MemberMost Helpful Member

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Welcome to ETO, e-le!

Is this a class assignment?

You might consider googling "Low power LM741 audio amplifier schematic".

3. ### e-leNew Member

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yes, it is an assigment.

I am unable to find a suitable schematics.
kindly, could anyone guide me with the circuit schematics?

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5. ### e-leNew Member

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especially with LM741 op-amp..

6. ### ronsimpsonWell-Known MemberMost Helpful Member

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1) 280mW average -----you need to find the voltage that puts 280mW into a 64 ohm resistor.
2)With the average voltage you need to convert to peak voltage.
3)The output of you amplifier needs to swing from +peak to -peak voltage.
4)The 741 can not out put a voltage that is the same as its power supply voltage. So give you amp +/- supply that is 4 volts more than the output you need.
5)The gain =Peak to peak output voltage/peak to peak input voltage (0.9V)
6) Find the peak out put current. Can the 741 do that? Probably you will need to make a current amplifier using the 2N2222.
7) Can you use a 2N2222 and a 2N2907? We need a PNP and a NPN transistors.

This will get you started.

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7. ### cowboybobWell-Known MemberMost Helpful Member

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OK. But I'm not going to do your homework for you.

I found this with 2 clicks and some scrolling (using the above suggested google query phrase):

This is, at the least, a starting point and uses the devices you listed. Component values are for you to figure out. Also, this uses an electret condenser microphone for an input. Just eliminate from the circuit and substitute with your 200Hz sinewave.

And no inductors needed.

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8. ### AnalogKidWell-Known Member

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The schematic in post #6 is close, but has several significant problems. Since you are a student with very little experience base, I recommend breaking up the task into two separate, smaller problems:

1. Work through the basic circuit parameters as in post #5. Work out the input and output voltages and currents.
2. Ignoring the high output current requirement for now, design an opamp circuit to do the amplification. Use a non-inverting design.
3. Read the 741 datasheet, paying attention to the input common mode range and the output voltage swing range, and adjust your design's power and biases to make sure the signals are within the 741's operating limits.
4. Only then, start looking into ways to modify your circuit to boost the output current.

The problem can be solved with the components you are restricted to using, although the 2N2222(s) will get very hot.

ak

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9. ### audioguruWell-Known MemberMost Helpful Member

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A 741 was designed to use a 30V supply so it will not work from a 5V supply. Some 741 opamps barely work from a 12V supply. Besides, 5V is much too low for an amplifier to provide 280mW into 64 ohms. The peak-to-peak power of the opamp and transistor circuit is 5V squared/64= 390mW and the peak power is half at 196mW and the RMS power is half again at 97.5mW.
Will a little speaker survive with continuous DC in it? Therefore shouldn't the circuit provide only AC to the speaker without any DC?
Without DC in the speaker and without an added PNP transistor to make a push-pull AC amplifier then do I smell a hot class-A amplifier?

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10. ### AnalogKidWell-Known Member

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Dude - homework problem...

11. ### audioguruWell-Known MemberMost Helpful Member

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Why didn't the teacher teach the kids how to design the amplifier??

12. ### AnalogKidWell-Known Member

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That's what I was doing before you gave away half the answers. It's called "Homework H.e.l.p" right there in the title.

ak

13. ### audioguruWell-Known MemberMost Helpful Member

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I believe that "learning something" should be done by being taught how to do it correctly, not by randomly trying many errors (trial and error?) until it works.

14. ### e-leNew Member

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hi everyone,
thanks for your guidance.
I am provided with 2N4921 npn power transistors, 64 ohm speaker (rated o.5W).

I have attached a circuit for the design problem stated.
Kindly, could you help check the schematics, are all resistors values correct and what capacitance value I should use?

Thank you.

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15. ### e-leNew Member

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Sorry , I am provided with bipolar npn power transistors.
hi everyone,
thanks for your guidance.
I am provided with 2N4921 npn power transistors, 64 ohm speaker (rated o.5W).

I have attached a circuit for the design problem stated.
Kindly, could you help check the schematics, are all resistors values correct and what capacitance value I should use?

I really need help as I have to submit tomorrow. please!!

Thank you.

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774.6 KB
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16. ### ronsimpsonWell-Known MemberMost Helpful Member

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No time now.
+10V supply and -10V supply
Feedback is OK but better after the transistors. (please tell me why)
Current mirrors do not work as well in real life with real transistors. So I added emitter resistors that should have about 1V across them. This will help reduce errors because the two transistors are not exactly the same. (please think about this. Why emitter resistors help)
I did not check if you have enough current. 78.3 ohms is strange value. Probably not critical as long as you more than enough current. (peak current)
You have about 10V across the 78.3 ohms. So the emitter resistors could be 1/10 that value.

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17. ### e-leNew Member

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I am using three bipolar 2N4921 npn power transistors. with a signal of 2kHz sinewave with a peak-t0-peak amplitude of 0.9v.

feedback after the transistor as to ensure the voltage is higher than 0.
emitter resistance are placed to prevent the transistors from saturation.
Am I right?

Can I use emitter resistors of 10 ohm?
R value of 82ohm or 100ohm instead of 78.3 ohm?
what capacitance value I have to use?

18. ### audioguruWell-Known MemberMost Helpful Member

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Why does the teacher ask the students to design a power-wasting class-A amplifier?
I noted a few problems with it and suggestions to make it better:

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19. ### canadaelkActive Member

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I believe that "learning something" should be done by being taught how to do it correctly, not by randomly trying many errors (trial and error?) until it works.
Quote by AG, post 11.
By that thinking Edison would be a failure and we would still be without light-bulbs! E

20. ### specWell-Known MemberMost Helpful Member

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Hi e-le,

This is the simplest circuit I could come up with to meet you projects objectives (errors and omissions excepted).

spec

ERRATA
(1) The transistor should be named Q1 and labeled as type 2N2222

NOTES
(1) R10 has no functional purpose. It is just there to give a degree of isolation of the 741 opamp from the input of the emitter follower transistor.
(2) Decoupling capacitors on the opamp power supply lines have not been shown. On a practical circuit these would be required (100nF minimum from each opamp supply pin to the 0V supply line)
(3) The input impedance of the amplifier is 1K Ohms. This can be changed if required.
(4) The voltage gain of the amplifier is 1+ (R12/R14)
(5) This is a class A amplifier. Even the 741, which normally operates in class B, will be operating in class A. Because of this the amp should sound quite good. To improve the quality more a 10K Ohm resistor could be connected between the opamp output pin and the 10V supply line.

DATA SHEETS
(1) LM741 Opamp
(2) (P)2N2222 NPN Bipolar Junction Transistor (BJT)

Last edited: Mar 16, 2016
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