Very Low Noise Amplifier Design for UltraSonics Band (30-40)KHz - Page 1
Forum section: electronics / rf-microwave

author: trentender
author_rank: Contributor
timestamp: October 04, 2023, 02:45:58 pm
content: Doing some rf receiver design across the band (30-40)KHz-(I know,not really rf, more audio and ultrasonic engineering) and need a very low noise pre-amplifier with high gain (20db+) but with Vcc in the range (2.5V to 3.2V) and maximum dc current of 0.30mA.Minimum signal level is -101dbm (50R) and need to get that up to >-80dbm (50R).Do not have much pcb area to work in so need to minimise size of components.Initial thoughts was to go via an op-amp solution but working at such low currents and low voltages I think the noise performance will be poor so maybe not a good choice.That leaves a discrete transistor solution.Also forgot to mention that this pre-amplifier also needs to be tuned since there is a very large unwanted signal (MSF) at 60KHz which I do not want to amplify. So looking to design a discrete transistor amplifier,possibly two stages,Vcc as low as 2.5V, maximum dc current of 0.30mA and a Noise Figure of probably no more than 3dB.Pin minimum about -101dbm (50R) and Pin maximum about -80dbm (50R) plus 60KHz MSF signal need to be filtered away as much as possible. So not asking for much really,anyone got any bright ideas!!!!!!

author: LM21
author_rank: Regular Contributor
timestamp: October 04, 2023, 03:11:19 pm
content: A low noise opamp?

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2023, 03:20:57 pm
content: My first guess, without doing the required calculations, is that in that frequency range with a tuned input circuit, I would use a good JFET input device.This would require that the tuned circuit step up the impedance to get a noise match with the FET input.A quick look in an old noise book shows a 2N4869 havingeNof about 3 nV/(rt Hz) andiNof about 5 fA/(rt Hz) at 30 kHz.Therefore, the optimum noise-match source impedance (for lowest noise figure) would be 600 k\$\Omega\$, which is a bit high.However, if your source impedance is lower than that, you can assume the voltage noise dominates.

author: RoV
author_rank: Regular Contributor
timestamp: October 04, 2023, 08:21:06 pm
content: Check "The Art Of Electronics" 3rd ed, tables 8.3a/b at pages 522-523, listing a choice of low-noise BJT and JFET/CMOS input opamps.BJT input types are generally less noisy at low input impedance, but a CMOS part may be a better compromise at very low voltage/current.For example, the CMOS TLV2221 works at 2.7-12 V with Iq=0.11 mA and has an en=19 nV/sqrt(Hz) @ 1 kHz. GBW is not high, 0.51 MHz, so you won't get a high gain from a single stage.The BJT LT1783 works at 2.5-18 V with Iq=0.23 mA and has an en=20 nV/sqrt(Hz) @1 kHz and an in=0.14 pA/sqrt(Hz), but with a higher GBW=1.3 MHz (still not very high).However, your requirements are not very clear to me. -101 dBm is quite a strong signal, not requiring an NF of 3 dB unless you need a very high SNR with the signal spread on a wide bandwidth. Besides, normally sources at low frequency are noisy by themselves, so they don't need a specially low noise amplifier.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2023, 08:33:22 pm
content: Note that a good discrete JFET is quite a bit quieter than those integrated op amps at 30 kHz.

author: szoftveres
author_rank: Regular Contributor
timestamp: October 04, 2023, 09:22:28 pm
content: Since 1) it's pretty low frequency and 2) a jfet has many megaohms impedance at audio frequencies, I wonder if using a step-up voltage transformer is an option.Matching 50 ohms source to the -pretty much- several pF capacitance and many megaohms of Z with an appropriate step-up transformer should increase the Vrms quite a bit at the gate of the jfet, at which point the signal level should be sufficient to overcome any noise figure.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2023, 09:36:26 pm
content: He needs to tune his amplifier, anyway, so the input resonant circuit can be used as a step-up impedance transformation.Note that the input capacitance of the JFET then becomes part of the resonant circuit.

author: magic
author_rank: Super Contributor
timestamp: October 04, 2023, 10:27:29 pm
content: Aren't BJT the right choice for low noise from low source impedance, and the right choice for low noise at low bias (OP wants 300μA for the whole circuit), and the right choice for doing anything at low voltage at all?BTW, my head hurts from all those decibels. Can anyone translate -100dBm from 50Ω into volts RMS, I'm lazyQuote from: trentender on October 04, 2023, 02:45:58 pm3dB referred to your -100dBm signal or 3dB above Johnson noise of 50Ω?The latter may be hard or impossible on your power budget, methinks.

author: szoftveres
author_rank: Regular Contributor
timestamp: October 05, 2023, 04:59:09 am
content: Quote from: TimFox on October 04, 2023, 09:36:26 pmI think they want to amplify 30-40kHz, that's Q=3; hopefully I'm missing something (e.g. they want to be able to narrow-band tune it across 30-40kHz).