2N7002 as a radio frequency amplifier - Page 1
Forum section: electronics / beginners

author: AlexRG
author_rank: Regular Contributor
timestamp: June 03, 2025, 08:38:06 am
content: Good afternoon. I'm designing a AM receiver (0.5-7MHz). The main purpose of the experiment is to try to replace a two-gate transistor like "BF998" type with two single-gate + one bipolar. The scheme works good in multisim. But, I'm confused by the following point.What is the noise level of the 2N7002 transistor? For example, for Nexperia 2N7002AK-Qhttps://www.mouser.com/ProductDetail/Nexperia/2N7002AK-QR?qs=mELouGlnn3ff0hT2G%252Bqyfw%3D%3DI didn't find the noise parameter in the datasheet, so I can't calculate the transistors' own noise level, and hence the receiver's sensitivity. What should I do?Explanation of the scheme:The V2 source is a simulation of automatic gain control. At a voltage of 2.65V, the gain is maximum. At 2.75V, it decreases 20-30 times. This voltage will be supplied from the amplitude detector.Resistors R6 and R11 set the quiescent current of the transistors (19.7mA for Q2 + Q3). The resistor R7 sets the gain. In the diagram, it is now equal to 20 (~26dB). With these parameters, if a 50mV signal is applied to the input of the amplifier, the IMD3 level is -50dB.However, such a quiescent current causes increased heating of the transistors, 43/85mW. This heating, I think, will further increase the receiver's own noise level, which means it will reduce sensitivity.In general, what will be the approximate sensitivity with such transistors? I tried to use the Deepseek AI, it gave a sensitivity of about 30-40uV. I think it's wrong, it's toooo much. Or not?...

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: June 03, 2025, 10:18:54 am
content: Have you already implemented the circuit in real life and tested?

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2025, 10:30:57 am
content: The BF999 is a single gate transistor.https://www.mouser.co.uk/datasheet/2/196/bf999-56149.pdfIt has much lower capacitances than the 2N7002.IThe BF999 is designed as a linear amplifier, whilst the 2N7002 is a power switch.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 03, 2025, 10:39:23 am
content: BF999 is probably a typo for BF998https://www.nxp.com/docs/en/data-sheet/BF998.pdf

author: AlexRG
author_rank: Regular Contributor
timestamp: June 03, 2025, 10:40:24 am
content: to Zero999I was referring to the BF998 and similar ones from this series with two gates.to ahsrabrifat:No, in reality, I haven't done the circuit yet, because I need to know if it makes sense to use 2N7002 transistors, or if they will have a very high noise level.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 03, 2025, 11:01:26 am
content: When a parameter is not specified in the datasheet, it usually means average to poor performance. If you search for "noise 2N7002" the first result is discouraging, but I didn't read all that thread, not sure if the big noise remark there is about 1/f noise only:Quote

author: AlexRG
author_rank: Regular Contributor
timestamp: June 03, 2025, 11:47:19 am
content: If we can't find the intrinsic noise level for the 2N7002 transistor anywhere, can you recommend mosfet, which has the noise level indicated? Just not less than the SOT-23 case, otherwise I won't be able to solder it)p.s. And that it should be modern and freely available. That is, accessible.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2025, 02:00:17 pm
content: Perhaps you could use a J-FET such as the MMBFJ309? The biasing will need to be altered to account for the negative threshold voltage.

author: AlexRG
author_rank: Regular Contributor
timestamp: June 03, 2025, 02:15:26 pm
content: Quote from: Zero999 on June 03, 2025, 02:00:17 pmCan you draw the changes on the diagram for JFET? Then I'll try to model it. The main thing is to control the gain through the positive voltage of the automatic gain control in the same way.

author: iMo
author_rank: Super Contributor
timestamp: June 03, 2025, 02:32:56 pm
content: The QRM and QRN noise on 500kHz to 7MHz bands is huge therefore any transistor off your junkbox will do well.The "low noise" transistors are required for VHF and higher bands usually.

author: AlexRG
author_rank: Regular Contributor
timestamp: June 03, 2025, 03:36:48 pm
content: Quote from: edavid on June 03, 2025, 02:21:18 pmWell, yea, they are not linear. Watch the simulation at different frequencies. 500kHz is a 4-time gain, 4MHz is an 18-time gain, 7MHz is a 12-time gain.Of course, this is not good, but it doesn't seem to be terrible for the simple AM receiver)). The most important thing is that when I apply a two-tone signal to the input, at a level of 50mV each (this is a huge level for HF frequencies) (1.5MHz and 1.51MHz), the IMD3 level is 50dB. This is much more important.

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 04:02:17 pm
content: But watch out for 3rd-order IM on crowded bands due to device non-linearity.Of course, the only actual linear device is the vacuum triode.Good MOSFETs and JFETs have predominantly 2nd-order non-linearity, which does not produce 3rd-order IM.

author: AlexRG
author_rank: Regular Contributor
timestamp: June 03, 2025, 04:16:01 pm
content: Quote from: iMo on June 03, 2025, 02:32:56 pmHowever, I would still like to know the level of the 2N7000 or 2N7002 mosfets' own noise. This is necessary to understand the maximum sensitivity of the receiver. Unfortunately, I don't have the equipment to measure the level of this noise.. And we can't blindly trust any simulators in this matter. I don't trustmaсhineMultisim

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 04:24:59 pm
content: Noise characterization is expensive, so manufacturers rarely quantify noise for devices intended for digital operation.Note, as a rule of thumb, that the pink-noise "corner frequency", below which 1/fnoise exceeds white noise, is highest for MOSFETs, intermediate for JFETs, and lowest for BJTs.The voltage noise corner frequency for the 2N3631 MOSFET is roughly 1 MHz, maybe 1 kHz for the 2N3821 JFET, and perhaps 10 Hz for the 2N4250 audio PNP BJT.(Old data: feel free to look up newer devices.)

author: Zero999
author_rank: Super Contributor
timestamp: June 05, 2025, 10:54:47 am
content: Quote from: AlexRG on June 03, 2025, 02:15:26 pmFor future reference, please don't use JPG for schematics. It introduces artifacts which make editing a pain. PNG is better, preferably without anti-aliasing on the text. I like to stick to monochrome myself, but that's preference.Perhaps something like this?The gates will now be biased negative, with respect to the source.

author: iMo
author_rank: Super Contributor
timestamp: June 05, 2025, 02:47:11 pm
content: This may work with the 2N7002 as well (after minor changes in the 2N7002 biasing).https://w7zoi.net/hycas-pcb.html

author: MariuszD
author_rank: Regular Contributor
timestamp: June 07, 2025, 11:54:39 am
content: Quote from: AlexRG on June 03, 2025, 08:38:06 amI found your circuit interesting for analysis. But i don't understand how it is possible for AGC to work with such small voltage/current variation. My simulation shows that it doesn't work at all.

author: shabaz
author_rank: Frequent Contributor
timestamp: June 07, 2025, 12:07:50 pm
content: His other thread concerns transmission. It seems he's building a transceiver, and who knows for what purpose.Certainly not likely to be legal.

author: AlexRG
author_rank: Regular Contributor
timestamp: June 07, 2025, 12:35:35 pm
content: Quote from: MariuszD on June 07, 2025, 11:54:39 amLet's imagine that the maximum gain of the RF amplifier is 23. The amplitude of the reference voltage is 2.8 V. We increase the reference voltage to 2.85 V, the gain decreases from 23 to 16 times. Then we increase the reference voltage to 2.9 V, the gain decreases from 16 to 2 times.The receiver will have a reference voltage source, by adjusting which you need to set the maximum gain of the RF amplifier. For example, it will be 2.8 V. (In reality, this voltage may be different, depending on the type of 2N7002 transistors).When this voltage is 2.8 V, the RF amplifier gain is maximum. When a constant AGC voltage of 100 mV comes from the amplitude detector, the RF amplifier gain drops to 2 times. The higher the constant AGC voltage, the lower the gain.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 08, 2025, 08:47:35 am
content: Quote from: AlexRG on June 07, 2025, 12:35:35 pmPreviously, you wrote that a change of 2.65V-2.75V is sufficient, which is why I didn’t observe a significant change in gain in the simulation.Now it works.Quote from: shabaz on June 07, 2025, 12:07:50 pmThe fact that you don't know why he does it is not a crime.Not everyone from Russia is in engaged in war.Not guilty until proven guilty.

author: shabaz
author_rank: Frequent Contributor
timestamp: June 08, 2025, 09:21:21 am
content: I do not need to prove a thing. People can make their own decisions what to do. It's clear he did try to mislead:https://www.eevblog.com/forum/beginners/tl5001-as-a-audio-pwm/msg5940366/#msg5940366You are acting as his useful idiot, and that's on you.

author: Simon
author_rank: Unknown
timestamp: June 08, 2025, 03:19:45 pm
content: Quote from: shabaz on June 08, 2025, 09:21:21 amWhy don't you shut up? As you said, people can make up their own minds. Look at the bigger picture. If I am not mistaken what is being made is not the most advanced thing in the world, it's not like many others could not just do this without all this messing about, so what is your problem exactly?

author: shabaz
author_rank: Frequent Contributor
timestamp: June 08, 2025, 04:14:35 pm
content: Quote from: Simon on June 08, 2025, 03:19:45 pmYes, youaremistaken.If you had examined more closely before interjecting, you would have seen that the OP is not actually a beginner - he is more than aware about intermods, and his PCB layout shows awareness of how to construct for RF.You should have asked yourself, why then does he need help, if it's as simple as you appear to suggest?The answer is, he is specifically using parts never intended for RF purposes, even though jellybean RF-appropriate parts exist. That then makes it non-trivial because you won't find many online examples, ChatGPT will be less helpful, because there are not as many examples designed in this manner. If it's so easy, let's hear your expert opinions on the subject matter.Then, pile on the fact that AlexRG did mislead regarding frequency of operation, he is squarely targeting within commonly-used military spectrum (there is no indication he is licensed), his transmitter is clearly for a high-power since he needs to feed it from a likely high power PWM-driven buffer, and it doesn't take a genius to figure out that at a minimum this is likely to be illegal, if he has no license.With respect, you can kindly shut up first if you're not going to make the effort to understand what's going on.You could have thought to yourself, here's a person who clearly offers help, has a history of that, and indeed did help AlexRG on his other post, until he realized that the two concurrent threads had warning signs, by indeed looking at the bigger picture, and that surely he must have had a reason to speak up, whether you agree with it or not. I had thought better of you.

author: Simon
author_rank: Unknown
timestamp: June 08, 2025, 05:09:40 pm
content: we are getting numerous complaints about you from multiple people, if you don't want to help then don't. It is not apparent what the final intentions are so that is that. You are acting like this will win a war, I'm sure that if this is so crucial plenty of others can do it. This is you last warning!

author: shabaz
author_rank: Frequent Contributor
timestamp: June 08, 2025, 06:01:45 pm
content: Quote from: Simon on June 08, 2025, 05:09:40 pmMy responses were on-topic. I had to go into some detail simply because you'd incorrectly assumed that the questions were simplistic and that some bigger picture had been missed.I have not been contacted by any moderator to let me know about any complaint, fair or otherwise.If you or others have pent up frustration regarding complaints you have not told me about (and I'm sure I could complain about others), they can't have been all that significant that no-one made the effort to notify me.Your "final intentions" comment makes zero sense, I'm not hiding a thing, not even my name, and I'm not going anywhere, I'm here to stay.I don't use a fake name, throwaway e-mail or anything, and I help if I can, and stay out of all the nonsense that some people write. If you're going to choose to interject, how about actually supporting the people who make some effort to provide information, versus those who have demonstrably been misleading as mentioned above?