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Thermometer Circuit Design with Op Amp and BJT transistor
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- čas přidán 19. 08. 2024
- How to design a thermometer circuit with operational amplifier and Bipolar Junction Transistor (BJT) is discussed and analyzed in this circuit design example. For more examples see • Electrical Engineering... . This Thermometer is designed with one Op Amp and six PNP BJT transistors using current mirror design. It is important to make sure transistors are matched (especially the two bottom transistors have to be on the same substrate and in the same package to make sure they have the same junction properties). One the P-type BJT transistors set the reference emitter current and the rest of transistors effectively mirror the value of that current at their emitters. The Op Amp combines a non-inverting amplifier and an inverting amplifier to form a difference amplifier with proper selection of the value of resistors in this circuit. A combination of Kirchhoff's circuit laws KVL and KCL and op amp virtual short is applied to do the circuit analysis and to prove that circuit output voltage is linearly proportional to the junction temperature of the transistors. With the proper selection of values of resistors we can set the temperature coefficient of this thermometer circuit.
Please note that Boltzmann constant (k) is ~1.3806×10−23 & Electron Charge (q) is 1.6x10-19 (while I have used the correct values in my calculations so the computed results are correct, I incorrectly said values of 1.606×10−23 and 1.9x10-19 in the video). For more examples see: Thermometer Sensor Circuit Explained with Op Amp and NPN Transistor czcams.com/video/5jmbZ9ak6EI/video.html
Op Amp Amplifier with Electronic Gain Control czcams.com/video/NoNgQpbj77Y/video.html
Op Amp Analog Computer Differential Equation Solver czcams.com/video/ENq39EesfPw/video.html
Push-Pull Power Amplifier with Darlington Transistors czcams.com/video/866MYibo8yE/video.html
How to find Bode Plot, Freq Response, Transfer Function of Analog Filters czcams.com/video/vZFkPeDa1H8/video.html
Universal Analog Filter Design czcams.com/video/2J-0msXZE2o/video.html
Laplace Transform Example and S-domain circuit analysis: czcams.com/video/ps8N5TPM_qU/video.html
Op Amp circuit Bode Frequency plot czcams.com/video/BLVzuuqAlZs/video.html
Analog Logarithm Computer czcams.com/video/RpKEq5WyoLg/video.html
Lowpass Butterworth Filter: czcams.com/video/UzCjkwqy-9w/video.html
Analog Computer to Raise Signal to power n czcams.com/video/IUTlBH1UraE/video.html
Triangle Oscillator Op Amp circuit czcams.com/video/JF5Up_cuL9k/video.html
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Complex Sinusoid Oscillator czcams.com/video/GXRhmwmS5Zk/video.html
Sawtooth Oscillator Design czcams.com/video/2eUsGPfqbW4/video.html
Full-Wave Rectifier circuit example czcams.com/video/DJJMNU-CYcg/video.html
Sawtooth Waveform Generator design with OpAmp, JFET, BJT czcams.com/video/5zHXTx-Vl20/video.html
op amps Circuit with feedback loops to design an analog computer that solves a second order differential equation czcams.com/video/HeZRtnRXpEI/video.html
For more analog circuits and signal processing examples see: czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope these Circuit design and analysis videos are helpful. 🙋♂
If you're going to calculate pn junction voltages as a function of temperature, I would recommend learning that k/q is about 86μV/K. It's a single value and within the grasp of most electrical engineers to remember. There's little point in trying to be more exact because of the effect of η, even if it is usually close to 1 for silicon. I can't think of a single case where an electrical engineer needs the individual values of k and q, and as you found out, they are rather harder to recollect accurately.
@@RexxSchneider Thanks Rexx for watching and sharing your thoughts and feedback. You have a good point and I will try to keep that in mind. For more circuit examples please see czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope these Circuit design and analysis videos are interesting as well. Thanks again. 🙏
I love this stuff, I'm 62 and still am fascinated with analog design. I ended up writing embedded C code most of my career but I'm an analog guy at heart. Nice video.
You are welcome. Thanks for watching & your interest. Glad that you like this Thermometer design video. For more Analog Circuit Examples please see: Op Amp Amplifier with Electronic Gain Control czcams.com/video/NoNgQpbj77Y/video.html
Op Amp Analog Computer Differential Equation Solver czcams.com/video/ENq39EesfPw/video.html
Push-Pull Power Amplifier with Darlington Transistors czcams.com/video/866MYibo8yE/video.html
How to find Bode Plot, Freq Response, Transfer Function of Analog Filters czcams.com/video/vZFkPeDa1H8/video.html
Universal Analog Filter Design czcams.com/video/2J-0msXZE2o/video.html
For more examples see the Analog Playlist: czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope these Circuit design and analysis videos are useful and interesting. 🙋♂️
I'm watching as many as I can - thanks@@STEMprof
@@rockpadstudios Thanks & Hope you enjoy the videos. 🙏
@@rockpadstudios I'm 30 years younger and the same as you.
@rockpadstudios Thank you! 🙂 I hope you also enjoy the rest of videos in this channel.
Not just temperature, but absolute temperature.
And Shockley's is exactly how a transistor works, over a very wide range, so this works over the operable range of the transistor, and is even self-calibrating (So long as you have good matches, at least, and have K and q tabulated well).
Main remaining points so far as accuracy, would be do with the junction heating from the bias current.
For bonus accuracy, modulate ir or Vsupply, and then run the output through a lock-in amp. That should help clear up the noise substantially, depending how you tune the lock-in amp.
Thanks for watching and sharing your thoughts & suggestions regarding practical considerations. For more JFET, BJT, Op Amp Circuit examples please see:
Op Amp Amplifier with JFET gain control czcams.com/video/NoNgQpbj77Y/video.html
Analog Logarithm Computer with BJT czcams.com/video/RpKEq5WyoLg/video.html
Sawtooth Waveform Generator design with OpAmp, JFET, BJT czcams.com/video/5zHXTx-Vl20/video.html
And more examples in my Analog Circuits Video playlist czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Thanks again 🙏
Great circuit and explanation.
And lots of interesting videos in your channel, thanks for sharing all this knowledge!
You're very welcome. Thanks for watching and your encouraging comment. Glad that you liked my circuit channel. I will post more Op Amp and Transistor circuit examples soon. In the meantime you might also like the following BJT, JFET Circuit videos:
Sawtooth Oscillator with Op Amp, JFET and BJT Transistors czcams.com/video/5zHXTx-Vl20/video.html
Voltage Regulator design with BJT JFET & Op Amp czcams.com/video/CJl-urzeiTo/video.html
Thanks again. 🙏
Great, thank you for this analysis. Are quadruple and double, matched transistors packages still on the market? I haven't checked DGKeys and others for the availability of those components within the last few years.😏
Thanks for watching and your good question. Yes, Matched Monolithic Quad Transistor and dual Transistors packages are available (Mouser.com), for example Analog Devices MAT14ARZ-R7 is a quad monolithic NPN transistor that offers decent parametric matching for precision amplifier. And OnSemi (Fairchild) MMPQ2907A Quad PNP and FMB2907A Dual matched PNP transistors packages are also available to order from Mouser.com. For more examples please see czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Thanks again for your interest and comment.
Thanks for the detailed explanation.
But please start with a schematic with the junction dots in place, otherwise it can get confusing. And like somebody else already said: you should have just called the differential amplifier what it is and maybe put the derivation of its formulas in another video. I think it is unnessessary in this video.
But it is a great video nonetheless!
You're welcome. Thanks for watching & your feedback and good suggestions. Glad that this Thermometer design video is useful. Sure, I will try to have junction dots in future videos. For more Analog Circuit Examples please see: Op Amp Amplifier with JFET Electronic Gain Control czcams.com/video/NoNgQpbj77Y/video.html
Op Amp Analog Computer Differential Equation Solver czcams.com/video/ENq39EesfPw/video.html
Push-Pull Power Amplifier with Darlington Transistors czcams.com/video/866MYibo8yE/video.html
And Analog Circuits Video Playlist: czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Thanks again. 🙋♂️
That 's really good
Thanks for watching. Glad that you liked this Thermometer BJT Op Amp video. For more BJT, JFET Circuit examples please see
Sawtooth Oscillator with Op Amp, JFET and BJT Transistors czcams.com/video/5zHXTx-Vl20/video.html
Voltage Regulator design with BJT JFET & Op Amp czcams.com/video/CJl-urzeiTo/video.html
I hope these examples are interesting.
Question about R that sets the current iR via the transistors.
I looks like the answer is not really depending on it, but the in real life a low iR current will have some effect on the output. What will be the effect of setting iR too low or too high?
Thanks for watching and your good question. Output voltage of this circuit is practically independent of the reference current iR as long as it is set properly by selecting proper choices for supply voltage and Resistor R. If resistor R value is too low, reference current will be too high and will drive the reference current mirror PNP transistor to saturation (which we have to avoid). If resistor R is too large then the iR reference current will be too small and circuit will be more prune to noise and variations. I hope that this explanation is helpful. For more examples please see czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
Thanks again for your interest and question. 🙋♂
What you have here is essentially a classic Wheatstone Bridge setup and opamp in a differential ampilifier configuration. Would have been helpful to describe it this way since it is a very common application.
Thanks for watching and sharing your thoughts. I appreciate your follow-up comment. While I understand the similarity and your reference to Wheatstone Bridge (delta voltage between two resistance branches), I am afraid it is not an accurate representation of what matched BJT current mirror bridge is achieving in this Thermometer circuit involving Shockley PN junction exponential I-V equation. My recommendation is just to refer to them as BJT Current mirror Bridge if a reference to Bridge concept is desired. As for the OpAmp portion, yes it is effectively a Differential amplifier composed of an inverting amplifier and a non-inverting amplifier and matched resistors. For more examples please see czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope these Circuit design and analysis videos are interesting. Thanks again.
Very nicely explained. Fun to watch. Is this circuit used in real ICs? What if those transistors are not at the same substrate, will the precision degrade too much? Let's say 2 TO92 transistors touching each other?
You're welcome. Thanks for watching & your good questions. Glad that this Thermometer Circuit Design with BJT and Op Amp is interesting. Answering your questions, I wouldn't be surprised that Thermometer integrated circuits use similar core design (with additional features). The four top BJT transistors should be matched (use a quad transistor package that are easily available to purchase from say Mouser.com or DigiKey). The bottom two BJT Transistors should be matched as well (use a dual transistor package). Any mismatch can result in substantial circuit sensitivity to junction thermal variations because saturation current of transistor is a exponential function of temperature in such a way that ~10 degree C temperature change can result in 3-5x increase in saturation current. I hope this explanation is helpful. For more Analog Circuits Examples please see: czcams.com/play/PLrwXF7N522y4c7c-8KBjrwd7IyaZfWxyt.html
I hope these Circuit design and analysis videos are helpful.
My principal criticism of the circuit in practical terms is the rather small change in output voltage of 1mV per degree Celsius, despite the opamp's gain of 16.7. For cheap common opamps like the LM358, for example, its offset voltage drift referred to the output would typically be around 10%-20% of that, so somewhat spoiling the accuracy of the thermometer. A TL072 would be even worse.
Thanks again for watching and bringing up good practical points like offset. We should not use high-offset op amps like LM358 or TL072. Instead, for an accurate Thermometer there are cheap but very low-drift op amps that we should use in this design for instance TSZ151 (7 µV) or TSB182 (20 µV rail to rail) op amp from ST Microelectronics or at least MCP6497T op amp (0.6 µV/C drift) from Microchip Technology.
I hope this explanation is helpful.
@@STEMprof This is definitely helpful additional information. Intriguing circuit.
@@Guishan_Lingyou You're very welcome. Thanks for watching and comment. Glad that this circuit & explanation are helpful. For more Op Amp and Transistor circuit examples please see:
Sawtooth Oscillator with Op Amp, JFET and BJT Transistors czcams.com/video/5zHXTx-Vl20/video.html
Voltage Regulator design with BJT JFET & Op Amp czcams.com/video/CJl-urzeiTo/video.html
Thanks again for watching 🙏
What units are you using? Boltzmann's constant is 1.381E-23 J/K and the unit charge is 1.602E-19 C.
Please see the comment right below the video that states: Note that Boltzmann constant (k) is ~1.3806×10−23 & Electron Charge (q) is 1.6x10-19 (while I have used the correct values in my calculations so the computed results are correct, I incorrectly said values of 1.606×10−23 and 1.9x10-19 in the video).
Thanks for watching.