Audio Amp with negative feedback

In summary, the conversation discusses converting from decibels to linear terms and the equations used to do so for voltage and power gain. It is mentioned that the equations may differ if the input and output resistances are not the same. It is also noted that the formula for power gain is 20 log10(Vout/Vin) while the formula for voltage gain is 10 log10(Vout/Vin).
  • #1
306
6

Homework Statement


Screen Shot 2016-04-23 at 2.29.20 PM.png


Homework Equations


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The Attempt at a Solution


I am not really sure how to do this problem. How can I convert from db to linear terms just given db? Don't I need more information? For do I have to do a system of equations or something using the Power gain = 20log(vo/vi)? I'm not even sure if these are the right equations to use.
 
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  • #2
You aren't missing any problem information but you might be missing concepts. We can convert ratios to there corresponding db this way.
$$ \text{any ratio of quantity Q in (db)} = 10 \log_{10} \left(\frac{Q_1}{Q_2}\right) $$
This tells us
$$\text{Voltage Gain (db)} = 10 \log_{10} \frac{V_{out}}{V_{in}}$$
##\text{Power} = V^2/R## connects the concepts of voltage and power gain. For this, ##R_{out} = R_{in}## which explains the bottom left formula you posted (a long with a log property).
$$\text{Power Gain (db)} = 20 \log_{10} \frac{V_{out}}{V_{in}}$$
This should be enough to relate different gains and obtain the linear voltage gain ## \frac{V_{out}}{V_{in}}##
 
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  • #3
Voltage gain is by convention 20 log10(Vout/Vin).
 
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  • #4
rude man said:
Voltage gain is by convention 20 log1010(Vout/Vin).
That's the formula supplied for power gain by the OP. I don't think they are the same.
 
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  • #5
MisterX said:
That's the formula supplied for power gain by the OP. I don't think they are the same.
It is if input and output resistances are the same (e.g. 50 ohms is typical for high-freq equipmt.)
But otherwise, no. Power is 10 log10(Pout/Pin).
 
  • #6
rude man said:
20 log10(Vout/Vin).

rude man said:
Power is 10 log10(Pout/Pin)

They are both correct for Power Gain when impedances are the same for input and output.
10 V, 50 Ohms = 100/50 = 2W (P1)
3.16V, 50 Ohms = 10/50 = 0.2W (P2)

10*Log10(P1/P2) ; == ; 10*Log10(2/0.2) = 10dB
20*Log10(V1/V2) ; == ; 20*Log10(10/3.16) = 10dB

This is because in the first case you are calculating power gain using ratios of powers, all the same units.
In the second case, you are calculating power gain using ratios of voltages. Mixed units. Power being proportional to the square of voltage, and we aren't squaring the voltage as part of the ratio, we instead multiply the log by 2; effectively squaring the ratio.

Following the above verbiage, the voltage gain would be:
10*Log10(V1/V2) ; == ; 10*Log10(10/3.16) = 5dB
 
  • #7
Tom.G said:
Following the above verbiage, the voltage gain would be:
10*Log10(V1/V2) ; == ; 10*Log10(10/3.16) = 5dB
Well I wouldn't follow that verbiage!
Voltage gain = 20 log(10/3.16) = 10 dB.
 
  • #8
rude man said:
Voltage gain = 20 log(10/3.16) = 10 dB.
Well, after more research than it should have taken, I agree with you. o:)
 

1. What is an audio amplifier with negative feedback?

An audio amplifier with negative feedback is an electronic circuit that uses negative feedback to control the gain, or amplification, of an audio signal. This means that a portion of the output signal is fed back and subtracted from the input signal, resulting in a more stable and accurate amplification of the audio signal.

2. What are the benefits of using negative feedback in an audio amplifier?

Negative feedback in an audio amplifier can improve several aspects of its performance, including reducing distortion, increasing bandwidth, and improving stability. It also allows for more precise control over the amplification of the audio signal.

3. How does negative feedback reduce distortion in an audio amplifier?

Negative feedback reduces distortion in an audio amplifier by correcting any errors in the amplification process. As the feedback signal is subtracted from the input signal, any non-linearities or distortions in the amplification are also reduced, resulting in a cleaner and more accurate output signal.

4. Can too much negative feedback be harmful for an audio amplifier?

Yes, too much negative feedback can be harmful for an audio amplifier. It can reduce the overall gain and sensitivity of the amplifier, and can also increase the output impedance, making it less efficient at driving speakers. It is important to find the right balance of negative feedback for optimal performance.

5. How is the amount of negative feedback determined in an audio amplifier?

The amount of negative feedback in an audio amplifier is determined by the feedback network, which consists of resistors, capacitors, and/or inductors. The values of these components can be adjusted to change the amount of negative feedback and therefore the overall performance of the amplifier.

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