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m718
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What are some ways to improve signal to noise ratio for a microphone amplifier, beyond getting low noise components. My amplifier is a single opamp.
m718 said:What are some ways to improve signal to noise ratio for a microphone amplifier, beyond getting low noise components. My amplifier is a single opamp.
berkeman said:What are your thoughts? Do you have a particular problem with noise now, or are you just planning the circuit, and want to make it the best it can be given some constraints?
Is the microphone preamp right at the microphone? Or is the microphone passive, and the first amplifier is a cable length away? What kind of noise do you anticipate? Just thermal noise in the circuitry, or 50/60Hz hum too?
Have you looked into the general concepts of low-noise circuit design techniques and shielding? (Was there a recent thread here about that? ... I need to look now...)
I'm using OP-27G input noise voltage 3.8 nV at 1khz. I don't know how to measure the impedance of the mic but the resistance with a multimeter is 1KOhm.Mike_In_Plano said:Hello, Mike here,
First, ascertain the impedance of your source, this will dictate the type of amplifier you wish to use. Each amplifier is defined by an equivalent input noise voltage, e_n, and an equivalent input noise current, i_n. There's also a low frequency noise, 1/f noise. I haven't worked much with this lower frequency noise because I seldom design anything of great sensitivity below 100Hz, so I can't help you with it.
In any case, I can help you with en, in, and selecting a good amp.
Generally, lower noise op amps will have an e_n of .9 to about 10 nanovolts per sqrt(Hz). The current noise varies more greatly, with an i_n of 1 to 10,000 nano amps per sqrt(Hz).
Here's where your input impedance comes to play, i_n is being created by the op amp and is flowing through your mic to create a noise voltage across your mic. So,
v_noise_mic = z_mic x i_n
The resulting input noise will then be:
v_noise_input = sqrt (v_noise_mic^2 + e_n^2)
Ideally, you'd find the op amp that keeps both of these numbers to a minimum and approximately the same. For low Z microphones, with an output of 500 ohms or less, there are specialized bipolar amps with extra large input transistors. Linear Technology has a wide selection of these, some with en less than 1 nv / rtHz.
For mid z microphones, with an output of 1-5 k ohms, bipolar amps are still appropriate, but the ones with en < 2 nV/rtHz usually have too much i_n, so you have to dig around for an en less than 5 nv/rtHz and an i_n less than 2pA/rt Hz.
For high z microphones, with an output above 10 k ohms, it's usually best to go with a low noise FET amp. FET amps have negligible current noise, but suffer from a step up in voltage noise. e_n from 6 to 12 nv / rtHz is common. Curiously, FET amps give you even better noise performance as your impedance continues to increase, because the mic's output voltage is increasing and the FET amp still has negligible noise current.
FET input amps can also be ganged together in parallel to give you better noise performance. The reason is that each one is an uncorrelated noise source, the noise each is making is off doing it's own thing and isn't additive. The equation is:
e_n / sqrt (N), where N is the number of parallel amps.
Hence, four cheap amps in parallel will halve the noise voltage and quarter the noise power! Just remember that each must have it's own feedback circuit, etc, and the outputs must be tied together through resistors, otherwise, they'll fight.
Some good parts are:
For up to 1k ohm, LT1128
x
For over 10k ohm, OPA604
Mike_In_Plano said:I don't know about the AM radio thing. I'm doubtful that anything other than good engineering practice will get you out of the noise situation.
I accidentally posted the last message before I finished. Just a couple more notes:
I'd reduce the gain on that first stage. Why? Because the op amp isn't a perfect device, particularly as you go up in frequency. With a gain of 1500, you're probably loosing gain and delivering distortion in your upper frequencies simply because the amp doesn't have enough open loop gain.
For example, suppose your amp had an open loop gain of 2 million and a gain bandwidth of 10MHz. That means that the gain is 2 million only for a DC signal at 10MHz, the gain is 1. For every decade of frequency you drop, the gain goes up by a factor of 10, so you have:
gain = 10 @ 1MHz
gain = 100 @ 100kHz
gain = 1000 @ 10kHz Ooops, people can hear 10kHz, and we don't have enough gain!
A gain of 100 would be far more reasonable for a single stage - assuming you have a gain bandwidth of at least 10MHz.
Also, if you're oscillating out of the audible band, that can make noise in-band.
Some good parts are:
For up to 1k ohm, LT1128
For up to 15k ohm, LT1007 and LT1677
For over 15k ohm, OPA604 (my favorite!)
Or, if your super cheap, TL084, with 4 devices paralleled for about 9nV/rtHz.
In all these cases though, a well designed discrete transistor amp will run circles around an op amp, but that's another story.
- Mike
Signal-to-noise ratio (SNR) is a measure of the strength of the desired signal compared to the background noise present in the signal. In the case of microphone amplifiers, a high SNR is important because it ensures that the amplified signal from the microphone is clear and free from unwanted noise. This is especially crucial for recording audio, as a low SNR can result in a poor quality sound and make it difficult to distinguish the desired signal from the noise.
One way to improve the SNR of a microphone amplifier is to use a high-quality opamp (operational amplifier). Opamps with a low noise floor and high gain are ideal for achieving a high SNR. Additionally, proper circuit design and shielding techniques can also help reduce noise and improve SNR.
One common challenge in improving SNR for microphone amplifiers is the presence of external noise sources. These can include electromagnetic interference (EMI), ground loops, and thermal noise. Careful consideration of the circuit design and use of noise-reducing components can help mitigate these challenges.
Yes, there are a few specific opamp tips for improving SNR in microphone amplifiers. First, selecting an opamp with a low noise figure is crucial. Second, using a high-gain opamp can help boost the desired signal and minimize the impact of noise. Additionally, attention should be given to the opamp's power supply and grounding to reduce noise and improve SNR.
Yes, there are some trade-offs to consider when trying to improve SNR in microphone amplifiers. For example, using a high-gain opamp can increase the risk of introducing noise and distortion. Additionally, increasing the opamp's gain may also decrease its bandwidth. Careful consideration and testing should be done to find the optimal balance between SNR and other performance factors.