Getting the best noise figure with a generic transistor - Page 1
Forum section: electronics / rf-microwave

author: profdc9
author_rank: Frequent Contributor
timestamp: November 30, 2021, 05:01:20 pm
content: I would like to use a generic widely available transistor such as 2N3904 or 2N2222 in the lower HF (3 MHz to 14 MHz) as a low-noise preamplifier after a double balanced ring mixer in an HF radio. I know this is not an ideal situation, but given this limitation, what would be the best approach? For example, 2N3904 has a noise figure of about 5 dB around 100 Hz. To achieve something close to this, would I use a common-emitter amplifier with a small amount of emitter degeneration? The source impedance is 51 ohms with a 2.2 uF capacitor to ground as a low-pass filter (which is 723 ohms at 100 Hz).

author: TimFox
author_rank: Super Contributor
timestamp: November 30, 2021, 06:00:46 pm
content: I don't have good datasheets handy for these BJTs, but the noise figure quoted at 100 Hz depends on the source impedance and the collector current.In general, the optimal source impedance for a BJT is lower than that for a FET, where the optimal source impedance is that which gives the lowest noise figure.Good datasheets (hard to find now) gave contours of noise figure as a function of collector current on one axis and source impedance on the other axis, with different graphs at several discrete frequencies.

author: UR5FFR
author_rank: Contributor
timestamp: November 30, 2021, 06:57:37 pm
content: Hi,At low band, dynamic range is much more important than noise. Also postmixer amplifier need to have good isolation between input and output. Circuit solutions may be different depending on the class of your receiver.Very low noise have Norton amplifier (common base with transformer feedback). But it have poor isolation.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 30, 2021, 08:50:24 pm
content: search "rohm 2n3904.html"Most data sheets are abbreviated too much but this one is complete, including a set of noise characteristics.I am also presently working on a demodulator for If of HF radio for ft8 modeThis one uses MC1496 Gilbert cell in full diff. mode into a diff amp using AD8055 followed by Sallen Key LPF using another AD8055,giving audio about 0dBu into the sound card.

author: David Hess
author_rank: Super Contributor
timestamp: November 30, 2021, 11:56:19 pm
content: I have only done this at lower frequencies but the ratio of the input voltage noise density and input current noise density defines the optimum source impedance for lowest noise, and adjusting the collector current alters this ratio so it can be adjusted for lowest noise in a given design. The input current noise depends on the base current so transistors with higher hfe are ultimately lower noise, and they are sometimes advertised that way.Ignore low frequency noise specifications because they include flicker noise which does not apply at higher frequencies.So you can calculate the input noise voltage from the collector current, and you can calculate the input current noise from the base current, which is the collector current divided by the hfe. And from those, you can calculate the optimum source impedance.There is also the base spreading resistance which is a significant source of noise at low impedance, but this would need to be measured in most cases; few transistors specify it.

author: profdc9
author_rank: Frequent Contributor
timestamp: December 01, 2021, 02:18:44 am
content: I was reading this document that seems to explain a bit:http://www.janascard.cz/PDF/Design%20of%20ultra%20low%20noise%20amplifiers.pdfLook at eqn (3)Ic = (kT/q) Sqrt/rsThis would suggest that the relevant parameter is hFe. It would seem that the equivalent base spreading resistance isn't accounted for, and this is why transistors like the 2N3904 don't make particularly good LNAs? So for example if rs=51 ohms, and rbb=500 ohms, the optimal collector current is dominated by the base resistance, and for example hFe=200 would mean thatIc = (0.026 V) Sqrt / (500 ohms) = 735 uA.I was looking at the 2N4401 for a common transistor that has some specifications for noise figure and optimal source impedance:https://www.onsemi.com/pdf/datasheet/2n4401-d.pdfBased on the data sheet, for 51 ohms it would seem that the ideal collector current would be on the order of a few mA. Is there anything know about how 2N4401 compares to other generic transistors (e.g. 2N3904, 2N2222)?

author: David Hess
author_rank: Super Contributor
timestamp: December 01, 2021, 12:36:48 pm
content: Quote from: profdc9 on December 01, 2021, 02:18:44 amThat agrees with what I said.QuoteThe base spreading resistance limits low noise performance at low impedances, and common transistors have a pretty high base spreading resistance. This also explains why "common" "low noise" transistors like the KSA992/KSC1845 and 2N5087/2N5089 have such high hfe, and at lower currents, but they pay for this with lower Ft and lower maximum voltage.Integrated transistors like the LM394 and MAT-01 have much lower base spreading resistance because of how they are constructed. Ring emitter transistors may have much lower base spreading resistance, and I know from reports that "super e-line" type transistors from Zetex also have low base spreading resistance, just a couple ohms. Others (Motorola/On and Fairchild) now make transistors like these; they are distinguished by high hfe at high current and low saturation voltages.QuoteThey are all about the same because their construction is all about the same, but of course the 2N4401 and 2N2222 are larger than the 2N3904 so have a lower base spreading resistance. The BC327/BC337 is another set sometimes used for lower noise because they are larger.I suspect the more modern interdigitated structure of the 2N4401 would yield lower noise than the 2N2222, but have not verified it.The old Fairchild and National discrete semiconductor databooks have information on the processes used for various transistor families which might be helpful.

author: RoV
author_rank: Regular Contributor
timestamp: December 01, 2021, 10:17:36 pm
content: Quote from: profdc9 on November 30, 2021, 05:01:20 pmPlease clarify the transistor operating frequency, since it is after the mixer. Is it audio, in a direct conversion receiver? Or some intermediate frequency?

author: G0HZU
author_rank: Super Contributor
timestamp: December 01, 2021, 11:27:27 pm
content: Yes, I wondered the same. Is it audio or RF? I've used the 2N3904 for both but usually for RF stuff. I have a nice RF spectrum analyser here that can measure noise figure quite accurately up to about 3GHz. I have another microwave analyser that is also quite good at baseband measurements for gain and noise figure. I can easily test some simple AF or RF amplifier circuits for you using a 2N3904 or the other devices mentioned in the thread.For small signal stuff at RF the 2N3904 is generally the best of the jellybean parts. It is made with a different process and you can tell by looking at the shape of the die that it should perform better at RF than the other jellybean parts.

author: mawyatt
author_rank: Super Contributor
timestamp: December 01, 2021, 11:40:15 pm
content: Quote from: profdc9 on November 30, 2021, 05:01:20 pmAt 3-14MHZ you should be able to build this mixer in 8 phase mode with standard logic (clocks) and discrete NMOS sets (switches) with outstanding results.Best,Best,

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 02, 2021, 12:13:01 am
content: Quote from: G0HZU on December 01, 2021, 11:27:27 pmOK,I would like comments on noise and bandwidth for my 2n3904 circuit, but I don't want to divert profdc9's thread by posting circuits unless agreed.I have a small board "Fet_Bi Hi_Z to 50 Ohm buffer 200kHz~20MHz " using a 2N4416 source follower / 2N3904 emitter follower that I have made a few of for hobby, and about to update the artwork for full surface mount ( it is presently only capacitors surface mount). It has low parts count and works down to 3.2V battery.I have done qucs analysis and test measurements but I can't test noise figure here.