Help with a simple amplifier design for an audio signal.

In summary: I always thought to put noise through the cap between the signal and ground, not between the two signals. I looked through my... electronics book and... cannot find a... single example... where a cap is... AC coupling the two inputs to the amp.. does anyone know about this?In summary, the conversation was about designing an amplifier that outputs an audio signal through an 8 ohm speaker at 1 watt. The input signal is 1Vrms and the power supply is 5V. There were discussions about the gain and whether to use RMS or peak to peak voltage, as well as concerns about power supply voltage and biasing of the input signal. There was also a question about using
  • #1
TheAnalogKid83
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Hi, I'm designing an amplifier and need help with some basic ideas (Filtering is already taken care of). My main question is bolded.

The output is an audio signal, and it drives an 8 ohm speaker. The power is 1 watt through this speaker.

The input signal is coming from another amplifier that says its full-scale output is 1Vrms.

So the output specification is 1 watt through 8 ohm speaker, so (Vo^2)/R = 1W, Vo = sqrt(8) = +or- 2.828427V. The power rail of the amp will be 5V and it can source over 2 watts, so this will be ok.





So now I know my gain is to be Vo/Vi = 2.828427V/1Vrms. . .
Do I need to convert my output audio voltage to RMS, or do I need to convert my 1Vrms input to normal V? Also, I would imagine my Vi of 1Vrms is going to actually be greater since the input impedance of my audio amplifer is going to be more than a 10k ohm load as specced for the input signal.
 
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  • #2
I've decided to convert RMS to peak to peak voltage and get 1V * sqrt(2) = 1.41421 Vpp

which is funny that it works out to an exact gain of 2V/V. I'm going to give a little extra gain to 2.1V.

Is this reasonable?
 
  • #3
One thing. You computed the 2.82v as RMS (or DC).
You might want to reconsider what this means when converted to a peak to peak sine wave in terms of your power supply voltage and output amp headroom requirements (unless its a rail to rail amp in which case headroom does not apply).

Also your gain will be wrong unless input and output are specified the same. You don't want mixed RMS and Peak.
 
  • #4
NoTime said:
One thing. You computed the 2.82v as RMS (or DC).
You might want to reconsider what this means when converted to a peak to peak sine wave in terms of your power supply voltage and output amp headroom requirements (unless its a rail to rail amp in which case headroom does not apply).

Also your gain will be wrong unless input and output are specified the same. You don't want mixed RMS and Peak.

Okei well my dilemma has been that the gain units did not match up (V and Vrms) so I did then just change my Vrms to Vpp instead, which should be my magnitude of highest signal, which will be best to use as the DC gain. Is this right to you?

Also, the power supply is 5V to 0V, and I should never go over 5V according to my power calculation giving me a voltage max below 3V. I look through the datasheet, but see nothing about thresholds or clipping, so I am optimistically assuming it is rail to rail.

What you mentioned has just put another thing of consideration into my mind though, is what the audio signal is biased at going into the amp. . it better not be biased at 0V or else I'm in trouble.

The power supply to the chip that is sourcing the audio signal into my amp is GND to 3.3V. So at best, its bias should be 1.65V so that there can be equal negative and positive swing in the signal. If this is the case, I have to worry about clipping at the negative end if my gain is too high.
 
  • #5
This question is important. On the inputs of the amplifier, I was told to put a cap between the inputs, to "keep the noise potential the same" between them. I always thought to put noise through the cap between the signal and ground, not between the two signals. I looked through my electronics book and cannot find a single example where a cap is AC coupling the two inputs to the amp.. does anyone know about this?
 
  • #6
TheAnalogKid83 said:
Okei well my dilemma has been that the gain units did not match up (V and Vrms) so I did then just change my Vrms to Vpp instead, which should be my magnitude of highest signal, which will be best to use as the DC gain. Is this right to you?
RMS and DC are equivalent representations.
That is the definition of RMS.
You had it right in the first post :smile:

TheAnalogKid83 said:
Also, the power supply is 5V to 0V, and I should never go over 5V according to my power calculation giving me a voltage max below 3V. I look through the datasheet, but see nothing about thresholds or clipping, so I am optimistically assuming it is rail to rail.
This is related to the confusion you are having in the prior question.
Perhaps it would help you to graph a 5v Peak to Peak sine wave,
the max for your 5v power supply, then compute the RMS voltage from the graph.
Tell us what you get for the RMS voltage.

TheAnalogKid83 said:
What you mentioned has just put another thing of consideration into my mind though, is what the audio signal is biased at going into the amp. . it better not be biased at 0V or else I'm in trouble.
You did say you had the datasheet for your power amp.
Most of these are just a variation on the op-amp.
The manufacturer usually includes a generalized circuit for their chip, which can be quite useful as a starting point for a design.

TheAnalogKid83 said:
The power supply to the chip that is sourcing the audio signal into my amp is GND to 3.3V. So at best, its bias should be 1.65V so that there can be equal negative and positive swing in the signal. If this is the case, I have to worry about clipping at the negative end if my gain is too high.
You could use AC coupling in which case this is not a problem.
If you really need DC coupling then you will need to do some level shifting.
 
  • #7
TheAnalogKid83 said:
This question is important. On the inputs of the amplifier, I was told to put a cap between the inputs, to "keep the noise potential the same" between them. I always thought to put noise through the cap between the signal and ground, not between the two signals. I looked through my electronics book and cannot find a single example where a cap is AC coupling the two inputs to the amp.. does anyone know about this?
Perhaps with a resistor as a low pass filter to reduce the high frequency response.
One of amp inputs would be at a ground equivilant.
It seems unlikely that you would want to do this.
Where did you get the "keep the noise potential the same".
That just sounds wrong.
 

FAQ: Help with a simple amplifier design for an audio signal.

1. How do I choose the right components for my amplifier design?

When designing an amplifier for an audio signal, it is important to consider the input voltage, output power, and impedance of your audio source. The components you choose should be able to handle these values and provide enough gain for your desired output. It is also important to consider the frequency response and distortion levels of the components to ensure high-quality sound.

2. What is the difference between a single-ended and a differential input amplifier?

A single-ended amplifier amplifies the voltage of a single input signal, while a differential input amplifier amplifies the difference between two input signals. Differential input amplifiers are commonly used in audio applications as they can reject common-mode noise and provide better noise performance.

3. How can I calculate the gain of my amplifier?

The gain of an amplifier can be calculated by dividing the output voltage by the input voltage. For example, if the output voltage is 10V and the input voltage is 1V, the gain would be 10. You can also use the formula Gain = Vout/Vin to calculate the gain.

4. What is the purpose of negative feedback in an amplifier?

Negative feedback is used in amplifiers to stabilize the gain and reduce distortion. It involves feeding a portion of the output signal back to the input in a way that cancels out any changes in the gain, resulting in a more stable and linear output.

5. How can I test and troubleshoot my amplifier design?

To test your amplifier design, you can use a multimeter to measure the input and output voltages, as well as the current flow. You can also use an oscilloscope to observe the waveform and check for any distortion or noise. If there are any issues, you can troubleshoot by checking the connections, components, and circuit design for any errors or malfunctions.

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