Amplifier

Discussion in 'Homework Help' started by Merk Morondos, Sep 26, 2016.

1. Merk Morondos Thread Starter New Member

Sep 20, 2016
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May I ask on how to edit this schematic to have a following:
Gain of 8
Vi=0.5Vrms
Power=2 watts
Vcc= 9V

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2. Jony130 AAC Fanatic!

Feb 17, 2009
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Try to play with this version

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3. Merk Morondos Thread Starter New Member

Sep 20, 2016
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but i need a 0.5 vrms in input

4. Jony130 AAC Fanatic!

Feb 17, 2009
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But 0.5 Vrms = 0.7Vpeak

5. Merk Morondos Thread Starter New Member

Sep 20, 2016
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sorry, you are right i forgot the conversion, may i ask how about the circuit with the following specs;
Vcc=12V
Vin=1vrms
power= 3 watts
gain of 8

thank you again

Apr 26, 2005
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7. Merk Morondos Thread Starter New Member

Sep 20, 2016
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3 ohms, joejester when i try the first circuit that you give it only has 4.8 gain

Sep 20, 2016
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3 ohms

9. Merk Morondos Thread Starter New Member

Sep 20, 2016
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jony130 when i try your circuit it has only 4.8 gain , what components should i adjust

10. Jony130 AAC Fanatic!

Feb 17, 2009
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To get 2W at the output load (2.66Ω) we need VLoad_RMS = √(P*RL) = √(2W*2.66Ω) = 2.3V RMS --->3.25V peak. So the gain should be around 2.3V/0.5V ≈ 4.6[V/V] If the voltage gain need to be equal to 8[V/V] Vin cannot be larger than 2.3/8 = 0.29V RMS.
So to have a gain of 8 and Vin = 0.5V you need change the value of a R1 resistor and add a voltage divider.

11. Merk Morondos Thread Starter New Member

Sep 20, 2016
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how about changing the load ? is it possible to have a gain of 8?

12. JoeJester AAC Fanatic!

Apr 26, 2005
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Is 3 ohms the same load as in the first question?

Vs = 9V
Gain = 8
Vi = 0.5V (0.707 pk or 1.414 pk-pk) Vo would equal 4.0 V rms (5.656 V pk or 11.312V pk-pk)
Power = 2W

A 9V supply will not swing the output signal 11.312 volts peak to peak.

You could, as Jony stated, change R1 to reduce Vi to allow 2W at the load you have posted in the schematic.

Last edited: Sep 26, 2016
13. Merk Morondos Thread Starter New Member

Sep 20, 2016
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First question the specs are:
Gain of 8
0.5 Vrms
2 Watts
9 Vcc
no clipping

Second question specs
Gain of 8
1 Vrms
3 Watts
12 Vcc
no clipping

14. JoeJester AAC Fanatic!

Apr 26, 2005
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Second circuit:

Gain of 8
Vi = 1V rms (1.414 V pk or 2.828V pk -pk) Vo would equal 8V rms (11.312V pk or 22.624V pk -pk)
Power 3W
Vs = 12V

A 12V supply will NOT swing the output voltage 22.624V peak to peak.

You will need to design a gain of 8 amplifier, then place a voltage divider (potentiometer) before the input capacitor to achieve your stated power goals. You can compute what you need at the load ... probably a speaker impedance.

It would have been nice if you specified all the design specifications in your opening post.

Sep 20, 2016
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16. JoeJester AAC Fanatic!

Apr 26, 2005
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A 12 V supply can swing a maximum of 12V. Zero to 12. You are designing an amplifier that takes a sine wave input and produced a higher power sine wave to the load with little distortion (no visible clipping). Look at your output circuit. Even if you put the junction of the two emitter resistors at 6.0 volts, then one half cycle can go from about (0.5V + Vce) to 6 volts and the other half cycle can go from 6V to (12V - (Vce + 0.5V)) at 1 ampere. Outside of that, you will see clipping.

You can make an amplifier with a gain of 8 with a 0.5 V rms input, but not with a Vcc of 9 Vdc.
You can make an amplifier with a gain of 8 with a 1.0 V rms input, but not with a Vcc of 12 Vdc.
You can make an amplifier with a gain of 8 with an output of 2 W using a common load, satisfying all your conditions except Vcc.
You can make an amplifier with a gain of 8 with an output of 3 W using a common load, satisfying all your conditions except Vcc.

Do the math and figure out what the output voltage will be using your gain, the input voltage. From there you can find the load that will draw 2 W or 3 W.

Last edited: Sep 27, 2016
17. Merk Morondos Thread Starter New Member

Sep 20, 2016
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Ok I get it thanks for your help. One question, is my schematic right? having a .3Vrms, gain 8, 12 Vs, 3 Watts

18. JoeJester AAC Fanatic!

Apr 26, 2005
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Do you mean the one in post number 1?

Have you consulted with your professor to see what he thinks of the inconsistency in the assignment?

Sep 20, 2016
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20. JoeJester AAC Fanatic!

Apr 26, 2005
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What does the measurements from your simulation tell you?

What is the peak to peak of Vi * 8? ... is it less than the Vs you selected?

What is the equivalent resistance of a 3 W load for Vi * 8?

I don't see your edited schematic posted. I don't use LTSpice.

Last edited: Sep 28, 2016