John Curl's Blowtorch preamplifier part II

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#60,828
keantoken said:
Previously, glaring at the Early effect equation did not leave me any the wiser about the relationship between Vce and Vbe. Things are making more sense now however, so I pulled the Early effect equation inside out in order to plot theoretical Vbe vs Vce at constant Ic. I used parameters from my BC337-40 model.

X axis is Vce. This chart shows that at 1Vce, Vbe changes by about 246uV per volt Vce. This corresponds to a PSRR of about 72db which lines up with my experience.
Click to expand...
In fact, the Vbe variation is a secondary consequential effect of the prime cause, which is the Hfe variation.

This can be demonstrated in this simple "thought experiment": in this circuit, the collector current is kept constant to ~10e-3.
The transistor belongs the C group, meaning highest Hfe for minimal current error, and also highest Early effect.
For a 1 to 31V Vce variation, the base current changes from 2.2µA to 1.4µA, while at the same time Vbe changes from 621.5 to 610mV.
26*ln(2.2/1.4)=621.5-610, thus everything falls nicely in place 🙂
 

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#60,832
MiiB said:
Why NOT just buy a Flir for your iPhone, Then you have really good thermal shots
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Quite expensive for now (€ 285,00) but it’s just the beginning
FLIR thermography camera, , 80 x 60 pix bolometer matrix from Conrad.com

"With a slim case which attaches to an iPhone 5 or 5s, FLIR ONE turns a smartphone into a thermal infrared camera, giving the ability to “see” and measure minute temperature variations on the phone’s screen. Fitted with two cameras, including a long-wave infrared Lepton sensor, the FLIR ONE uses MSX Technology to provide thermal imaging capability and sharpness by blending the thermal and visible spectrum. This technology allows the FLIR ONE to detect temperature differences as small as 0.1 °C across a range of 0°C to 100°C."



FdW said:
Why not? Using IRFP240's as MC amplifier gives also real good results (only tied in sim until now). One large (MOS)FET can be seen as many... many devices in parallel. SO the question is: Why not?
Click to expand...

After all, power transistors at the input of low level, low noise preamplifiers have been suggested again in the past due to their claimed low base resistance
Example: Extreme Low Noise Preampifier

George
 
#60,834
FdW said:
Why not? Using IRFP240's as MC amplifier gives also real good results (only tied in sim until now). One large (MOS)FET can be seen as many... many devices in parallel. SO the question is: Why not?
Click to expand...

Yes power bipolars if selected, they never specify noise in any way, have been known to do .3nV and they don't heat up. They have big Cj's but far less than JFET's .

EDIT- missed the link is already posted above.
 
#60,836
RNMarsh said:
😱 Now I'm up against device low Vp. What to do next.... need minimum 3v ?


THx-RNMarsh
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Hi Richard
In the old days we used an lm394 diff amp for a thermocouple amp. Inside it was a large number of parallel transistors which gave it the low noise and "matched" properties. I looked this morning and see it is discontinued, my trusty row of old "Blue national books" obsolete.

If i could ask (not recall seeing it) what is the magnitude, bandwidth and source impedance of your input signal?
Best,
Tom

This is what drove the earlier question about temperature and noise.

Understanding CCD Read Noise
 
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