usekgb
Well-known member
I love the old school, drawn with a pen, self etch! Very diy.
"C" should be easy enough to find.ForthMonkey said:What about transistors? I will use BC550C & BC560C.But if i can't find,can i use BC550B & BC560B? Only hfe different,i guess.B smaller than C.
BC550C-Forward current transfer ratio (hFE), min: 420
BC550B-Forward current transfer ratio (hFE), min: 200
BC560C-Forward current transfer ratio (hFE), min: 420
BC560B-Forward current transfer ratio (hFE), min: 240
What about it?
abbey road d enfer said:"C" should be easy enough to find.ForthMonkey said:What about transistors? I will use BC550C & BC560C.But if i can't find,can i use BC550B & BC560B? Only hfe different,i guess.B smaller than C.
BC550C-Forward current transfer ratio (hFE), min: 420
BC550B-Forward current transfer ratio (hFE), min: 200
BC560C-Forward current transfer ratio (hFE), min: 420
BC560B-Forward current transfer ratio (hFE), min: 240
What about it?
Using "B" instead will not change much in performance, because the circuit is more important than the components in a heavily feed-backe'd application.
BUT, noise performance will be impaired.
Roughly, for the same operating conditions, the base current of the input transitors will double, which will produce a 3dB increase in Input Noise Current, which may be significant, or not, depending on the source impedance. In the particular case of a mic preamp, the optimum source impedance is about 5-10k. Increasing noise current may shift it down to 3-5k. You use a 1:10 input xfmr, which makes the source impedance 20k, which is probably already a little too high for the "C" version. Using "B" may make the Input Noise Current dominant over the Input Noise Voltage. (Optimum source Z is when Input Noise Voltage is equal to Input Noise Current multiplied by source Z). Anyway, in the end it's only a couple dB over the absolute minimum; in actual operation, it shouldn't make a big difference. In most cases, using the right mic and positioning it properly is more significant than the preamp's noise performance.
VictorQ said:What was the gain for the noise sim?
It's showing about 80 nV/sqrt(Hz). If the gain is 4, that's 20 nV/sqrt(Hz) input noise at the opamp input.
Now the voltage source you are using in your sim has 0 output impedance. That means that the noise analysis only reflects the voltage noise component of the opamp noise. Use a realistic impedance like 200 ohm for a microphone (times 100 since you were using a 1:10 input transformer, right? So 20 k source impedance seen by the opamp.)
To optimize noise for a given source impedance, you should balance the current noise and voltage noise contributions. You do that by changing the current in the input stage: increasing the current gives you higher gain (transconductance or gm), which translates into lower voltage noise, but more current in the emitter also means in the base, and thus more current noise.
You should either edit V1 and specify internal resistance 20k or add a 20k resistor in series with V1.ForthMonkey said:VictorQ said:What was the gain for the noise sim?
It's showing about 80 nV/sqrt(Hz). If the gain is 4, that's 20 nV/sqrt(Hz) input noise at the opamp input.
Now the voltage source you are using in your sim has 0 output impedance. That means that the noise analysis only reflects the voltage noise component of the opamp noise. Use a realistic impedance like 200 ohm for a microphone (times 100 since you were using a 1:10 input transformer, right? So 20 k source impedance seen by the opamp.)
To optimize noise for a given source impedance, you should balance the current noise and voltage noise contributions. You do that by changing the current in the input stage: increasing the current gives you higher gain (transconductance or gm), which translates into lower voltage noise, but more current in the emitter also means in the base, and thus more current noise.
Thanks for explanation.
How can i add impedance? Should i specify Rser=200? Or what?
Explain what? If your question is how to run a noise analysis in LTspice, I really recommend you post on the LTspice forum. In the meantime I suggest you look at some of the examples in the LTspice website and that you google wiki ltspice; you will find several tutorials.ForthMonkey said:Thanks for advices! I edited first post and added noise.Now can someone explain?
The noise has increased. A lot. That means that the current noise is dominating the total noise.ForthMonkey said:Thanks for advices! I edited first post and added noise.Now can someone explain?
Explain what? If your question is how to run a noise analysis in LTspice, I really recommend you post on the LTspice forum. In the meantime I suggest you look at some of the examples in the LTspice website and that you google wiki ltspice; you will find several tutorials.
OK. I'll explain, but it's not the most useful figure, because it depends on the level of wanted signal. I'll tell you first about EIN (Equivalent Input Noise).ForthMonkey said:I've learned how to analysis noise in LTspice.But i need info about calculating signal to noise ratio.
No. Read carefully the graph and particularly the units on the vertical axis. It is in uV.Hz(exp1/2) or, in more practical language, inI analysed highest gain.And it's result.
Noise about 1.882uV.
Once you know the actual EIN, calculating S/N ratio is very simple.Now how can i calculate signal to noise ratio?
So you can see by entering 200r that the intrinsic noise level of a 200r resistor is -129.7dBu. This is a figure that you should know by heart, like pi=3.14, and remember that it follows the sq root of the value in ohms and also follows the sq root of the frequency range.
When you design a circuit with LTspice, it is saved as a file with the suffix ".asc". In fact it can be opened with a simple text editor and you will see the name of the circuit, the list of components with their value and the nodes to which they are connected and also the characteristics of the analysis (AC, transient, DC...). If you post this file in the Files-> Temp directory on the LTspice, the members of the list can open the file and run the simulation and help you solve your problem.ForthMonkey said:Thanks for explanation abbey road d enfer
I'm still trying to understand that you wrote.Cause English is not my main language.
It's .asc...
https://dl.dropboxusercontent.com/u/91809016/urei.asc
abbey road d enfer said:When you design a circuit with LTspice, it is saved as a file with the suffix ".asc". In fact it can be opened with a simple text editor and you will see the name of the circuit, the list of components with their value and the nodes to which they are connected and also the characteristics of the analysis (AC, transient, DC...). If you post this file in the Files-> Temp directory on the LTspice, the members of the list can open the file and run the simulation and help you solve your problem.ForthMonkey said:Thanks for explanation abbey road d enfer
I'm still trying to understand that you wrote.Cause English is not my main language.
It's .asc...
https://dl.dropboxusercontent.com/u/91809016/urei.asc
OK. So now with gain at about 40dB, the output noise voltage computes at 0.26uV or -68.4 dBu, which in turn computes at 108.4dBu EIN or -128.4dBu reflected at the xfmr primary. This is not a bad figure at all, particularly if you can achieve it in practice. All sorts of gremlins will conspire to make this figure unachievable in real life.ForthMonkey said:Uploaded. "urei.asc"
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