LED温度系数

发光二极管PN结正向压降和温度的关系如下:

led1

 

 

 

其中Vj为PN结电压降,e为电子电量、Eg为PN结禁带宽度(能量)k为波尔兹曼常数

而测得的二极管外部压降和温度关系为:

led2

 

 

 

Ea为串联电阻的掺杂激活能量,S为与迁移率相关的量,Rs为串联电阻的阻值。

直接用公式来计算发光二极管正向压降和温度间的关系是可行的,不过在应用中常采用测量的方法。

在室温到120°之间,二极管正向压降和温度间的关系近似为线性。

对于红色LED,温度系数约为 -1.5mV/摄氏度

绿色LED,约为 -3.8mV/摄氏度

蓝色LED,约为-5mV/摄氏度

紫外LED,约为-2.3mV/摄氏度

————–

从概率论我们知道,如果一个参数的值受很多独立因素影响,那么它的值就服从正态分布。

对于电子元器件的各种参数值也不例外,譬如标称10K电阻的阻值,那么它的标称阻值是正态分布的期望值。

阻值的最大/小偏差则取决于正态分布的均方差,一般为4倍均方差值。

运放的输入偏移电压是一个例外,它的典型值并不等于期望值,因为期望是零,所以典型值一栏写的是均方差值,最大值则随厂家而变化,有的是四倍均方差,有的是六倍均方差。

Extreme Low Noise Preamplifier

0,4 nV/Sqr(Hz) and less – isn’t that low?

My original intention (but who cares about that, anyway?)

Originally I looked for a very low noise balanced preamplifierfor dynamic microphones, but with a gain of 20 dB only. Very lownoise was meant to be 1.5 nV/Sqr(Hz) or less. I thought thisshould be easy as usual ICs like the INA103 provide as low as1.0 nV/Sqr(Hz) voltage noise. But I was wrong.

Why do low-gain amplifiers produce more noise than a high-gainones?This is for two reasons: The current noise of the input stageand the voltage noise of the feedback resister network. Have alook at the following circuit diagram:

 

low1

 

 

 

 

 

 

The overall amplifier noise is summed up from the followingsources:

–    The input voltage noise of the Amplifier(UNoiseIn)
–    The positive input current noise multipliedby the impedance of the signal source (RSource x INoisePosIn)
–    The negative input current noise multipliedby the source resistance of the feedback resistor divider ((R1parallel to R2) x INoiseNegIn)
–    The resistor noise of of the feedback resistordivider (R1 parallel to R2)

Of course the resistance of signal source, e. g. the microphone,adds another noise, but we can’t influence that with the designof the amplifier so I don’t include it here. My goal is the designof a preamplifier that adds so few noise to the signal that thenoise caused by the preamplifier compared to the signal source’snoise is really negligible. You should know that summing noisephysically is and mathematically must be done by summing the squareof the individual noise voltages and afterwards extracting theroot from this sum:

UNoiseSum = Sqr(UNoise12 + UNoise22 + UNoise32 +… + UNoiseN2)

The noise of a 200 Ω resistor amounts to 1.82 nV/Sqr(Hz).Should the preamplifier produce another 1.0 nV/Sqr(Hz), the sumwould become 2.08 nV/Sqr(Hz), i. e. approx. 1 dB morethan the source. My goal of 1.5 nV/Sqr(Hz) was “moderate”.

When you look at the INA103 you’ll find the current noise specifiedfor its signal inputs (INoisePosIn) there, but notfor the feedback inputs (INoiseNegIn). In fact, asthe feedback inputs are the emitters of the input transistors,the input current noise there is significantly higher that thaton the signal inputs, which are the transistor’s bases. The problem,particularly for low gains, occurs at these feedback inputs.

The INA103 provides feedback resistors of 3 kΩ ateach side (R1 in the circuit above). You cannot reducethem, the op-amp won’t work, either because the driver capacitanceis too low or the system gets instable. You have to use them.For a gain of 10 you need 2 x 333 Ω (or 1 x 333 Ω,R2 in the circuit above) to set a gain of 10. Thisresistor network produces 3.15 nV/Sqr(Hz) of voltage noiseand even more caused by the (unspecified) current noise of theINA103. The sum is 5 nV/Sqr(Hz) approx. – far, far away from mygoal and drastically reducing the system’s noise performance.

So what can be done to reduce the input noise?The feedback resistor divider’s output resistance must drasticallybe reduced. It is, by the way, drastically lower when the gainis high, i. e. it is 6 Ω only in case the gain is 1000(60 dB). But for low gains it is difficult to reduce. Justimagine a gain of 2 and a feedback resistance of 6 Ω:The divider in the circuit above ought to built from two 12 Ωresistors! The input power for this divider would be 4 Win case of an output voltage of 10 Vrms, whichis not an unusual voltage in average amplifier stages.

I did not want to go that far, but a power output stage sufficientto drive 100 Ω dividers (90 + 10 Ω)should be aimed.

I finally ended in a fully discrete circuit with a very highopen-loop gain and an appropriate power output stage. I made lotsof experiments, experiences and measurements for lowest noisetransistors. Particularly measuring these small voltages and currentsreliably and to make these measurements reproducible is reallydifficult and sometimes almost drove me mad.

My ultra-low noise preamplifier circuitThe circuit diagram below shows my prototype, an unbalancedamplifier with a gain of 1000 built for experiments and for noisemeasurements. It is, so to speak, one half of the INA103 inputstage. I was disappointed to realize that it was not possibleto reduce it’s gain down to 10. I had expected that I only hadto increase C4 sufficiently, but the system shows relaxationoscillations. I am convinced that the huge open loop gain hasto be reduced to avoid that.

low2

 

 

 

 

 

Transistor SelectionThe noise performance, on the other hand is much better thanI originally aimed. Currently the input voltage noise isas low as 0.45 nV/Sqr(Hz), or, for a balanced version, itwould be 0.64 nV/Sqr(Hz). I tested a couple of transistors.The best ones all were high-voltage power transistors. My favoritesup now are BF459 or MJE13007. BF459 is better when current noisematters, i. e. impedances like dynamic microphones and low gains.MJE13007 produces less voltage noise, but much more current noiseand is better for low-impedance sources like ribbon microphonesand high gains.

I cannot explain why these transistors are good and othersnot. Some say it’s the base resistance that matters, but I believethis is not the whole truth. (Electronics, to my point of view,should not be a matter of believing, but I’m not a semiconductorphysician and what shall I do as long as I do not know. I wouldhighly appreciate everybody giving me an exact explanation.) Highercollector currents, up to a certain limit, reduce the input voltagenoise, but increase the input current noise(s). Depending on thesource resistance a specific collector current is optimal.

The voltage noises, measured at a collector current of 4 mAapprox. and source and feed-back resistances of 1 Ωeach are:

 

Single BF459 0.54 nV/Sqr(Hz) Dual BF459 0.45 nV/Sqr(Hz) Single MJE13007 0.38 nV/Sqr(Hz)

Theoretically an improvement of 3 dB could be expectedusing two transistors in parallel, each with the same collectorcurrent as the single one, but practically, if at all, it is muchless. This obviously is caused by comparably high noise currentsthrough the source and feed-back resistances. There was no improvementwith two MJE13007 in parallel. Instead, at low frequencies thenoise was slightly increased, a typical effect of current noise.I did not investigate that further. Instead, I would rather testseveral more transistors, as those few I happened to have wereso different from each other that it is very likely to find significantlybetter ones.

As an example, here are the noise measured spectra of a fewmeasurements:

low3

 

 

 

 

 

 

 

 

 

 

 

 

 

From bottom to top:

Brown: MJE13007
Red: Two BF459 in parallel
Pink: BF459
Green: BC549C, a “low-noise”, low power transistor,for comparison
Blue: Two BF459 in parallel, 1 kΩ as source resistance

In the latter spectrum the 1/f base-current noise is obvious.It can be calculated as 1.5 pA/Sqr(Hz) approx. @ 2 mA collectorcurrent through each transistor.

Further transistors I tested:

BD237, BD437, TIP152, BF471, MJE340 and several low and mediumpower transistors. Next best to the BF459 is the MJE340, whichwas my favorite until I tested the BF459.

Better do not build this circuit…… unless you are not able to discriminate by smell if a transistoror a resistor is about to get hot!

low4

 

 

 

 

 

 

 

 

 

 

This is an experimental circuit. It rather should not be usedin practice as it shows some problems that should not occur. E.g., small input voltages of some 10 or few 100 mV are sufficientto destroy the circuit because the input transistor’s collectorcurrent can cause high reverse currents through the emitter ofT4 and destroy it. I did not test the zener diode ZDabove proposed in the circuit diagram above which might help.Also, I sometimes observed the circuit falling into an oscillatingstate. I did not investigate this further, it did not happen often.Both problems should not occur when a source is fixed (not plugged)to the input. But as I said, the more harm- than useful huge openloop gain should be reduced anyway.

For a more precise gain and lower low frequency limit I recommendto use an OSCON 2700 µF/2.5 V from Sanyoas electrolytic capacitor C5, which is as small asthe standard one I used in my prototype but has 1/10thof its ESR (10 instead of 100 mΩ).

This project is experimental. Should you work on somethingwhere it might be interesting, i. e. on microphones, or shouldit be interesting for you anyhow else, I’d appreciate your feed-back.So you are very welcome to emailme!

Source Adress:http://www.beis.de/Elektronik/LNPreAmp/LNPreAmp.html

脉冲发生器

前几日,设计中突然觉着需要一个电路,在外来一个脉冲后,能输出唯一的一个宽度为系统时钟周期的脉冲,竟不知此为所谓脉冲发生器。思索几日,亦有一得如下,

原理图:

pulse_sch_revise

 

 

 

 

 

 

 

 

 

(more…)

可预置计数器/分频器

这个电路比较简单,就是一个加法计数器,在计数到最大值时候,同步载入预置值,实现可预置的计数/分频。不过还是值得我学习,代码如下:
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
entity counter is
port(clk : in std_logic;
q   : out std_logic;
set_q :in std_logic_vector(7 downto 0));
end counter;
architecture behav of counter is
signal reg : std_logic_vector(7 downto 0);
begin
process
begin
reg<=set_q;
wait until falling_edge(clk);
end process;
process
variable num : integer range 255 downto 0;
begin
if num=255 then
num:=conv_integer(reg);
q<=’1′;
else
num:=num+1;
q<=’0′;
end if;
wait until rising_edge(clk);
end process;
end behav; (more…)

"Le Monstre" – The Monster Class-A 8W Amplifier

 

After reading Jean Hiraga’s article Le Monstre I was interested to hear for myself how this simple 8W class A amplifier would sound.

I used the original board layout, transistors and JFETs, and made some modifications. Heat sinking was increased to approximately triple the amount recommended. Instead of using the standard bridge rectifier, capacitor bank and battery setup, I opted for a fully regulated supply with a total of 127,0000 uF capacitance per channel and a 500 VA toroid transformer (rather than a 160 VA EI or C core transformer as per the article). (more…)

线性稳压器DIY

简单翻译下这篇文章:http://tangentsoft.net/elec/opamp-linreg.html

基于运算放大器的线性稳压器

为什么DIY?

原因有二,第一是市面上的三端稳压器不满足要求,要么性能不好,要么功率不够。第二,市面上也有一枝独秀,性能优异的稳压器,不过它们价格不菲,譬如凌力尔特的LT1581,13美元/片,购买一堆元器件了。或者它们封装独特,不能和普通的三端IC简单互换。

线性稳压器如何工作?

这个看模电书去吧,这里就不翻译了。总之就是一个负反馈环路,采样输出端电压变化,和参考电压比较,误差信号放大后去控制调整管的电流,进而调整其上的压降以达到稳压的目的。 (more…)

动圈式唱头放大器

原文地址:http://users.ece.gatech.edu/mleach/headamp/ ………..间有省略………

作者:W. Marshall Leach Jr.

引子

基于各种实用性原因,碟式电唱机已经被CD播放机取代。虽然,还存在一众LP唱机的追随者。早在1978年,我在Audio杂志上发表了一篇文章,提出了一个用于动圈式拾音器的前置放大器,或者说是唱头放大器。从收到的反馈来看,电路挺受欢迎的。先前我用的标题为”Build a Head Amp for Moving Coil Cartridges.” 不过,Audio杂志的编辑,Gene Pitts将标题改为 “Build a Pre Preamp for Moving Coil Cartridges.” 他告诉我说James Bongiorno,Ampzilla的设计者,最先将术语”head amp” 用于某一款产品,并告诉我说我们不应该在我的文章中使用那个术语。在接下来的行文中,我将交替使用”Head amp”和”Pre preamp”.

在最初的设计版本中,我使用的对管为2N5210,2N5287.它们在Motorola晶体管手册中被列为低噪声音频应用晶体管。当我将样机送给Audio的Gene时,他说他找到他的一位评审员评估过。评审员联系到我并建议将晶体管改为 2N4401,2N4403,并说这是音频设计者在低噪声应用中的秘密。Motorala将它们列为通用开关晶体管。当我用它们替换掉原先的对管后,电路信噪比提高了4dB。2N4401和2N4403具备更低噪声的秘诀在于它们有着更低的基区扩散电阻。这个参数极少在晶体管参数指标中提到。 (more…)

在MultiSim中导入Spice模型

任何以“.model” 开头的SPICE 模型是一种core 模型,Multisim SPICE引擎中已经包含了其管脚定义。Multisim SPICE引擎中默认的MOSFET型号带有四个引脚。如果你使用了3个管脚的型号,Multisim在仿真中会报错误。例如:你希望导入器件的模型与这个模型相类似:

.MODEL B4 NMOS VTO=1.7 KP=322E-6 LAMBDA=0.005
+CGSO=2.5E-9 CGDO=2.5E-9

SPICE MOSFET 通常的模型格式为:

Mxxxx D G S B model_name

在一个3个引脚的型号中使用上述模型,你必须使用“.SUBCKT” 声明,并且将 “S” 和 “B” 引脚内部相连。这是一个等效的模型:

.SUBCKT MOS D G S
M1 D G S S B4
.MODEL B4 NMOS VTO=1.7 KP=322E-6 LAMBDA=0.005
+CGSO=2.5E-9 CGDO=2.5E-9
.ENDS

注意到在 “M1” 这行上有2个“S”节点。这就是将 “S” 和 “B” 这两个引脚连接在一起了。

如果你不想修改模型,你必须使用4个引脚的型号。

源自NI官方网站:http://digital.ni.com/public.nsf/allkb/E1DA418DD7A5E4A1862574B800219513