High Input Impedance Single Supply Amp ?
in [Design]
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From: Jan Panteltje on 13 Jun 2010 15:07 On a sunny day (Sun, 13 Jun 2010 11:03:49 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <756a16d2rt3bm8u7johsh5sviag4dpb7ei(a)4ax.com>: >At 10 pF each, and a 300 ohm source, rolling at 20 KHz, you could use >2650 BF862s in parallel, for a voltage noise density of 16 pv/rthz. >Audiophools would line up for that. You would need a good, low-noise, >40 amp power supply. Some thoughts, not saying that any of this matters, but anyways: First frequency characteristic, a resistive terminated 300 ohm inductor (say 'generator') behaves different from a open generator. The loading of the membrane: If you load the thing with any impedance, then you sort of put the brakes on the movement of the membrane. How did the manufacturer specify the load when measuring the frequency characteristic. Dynamic range, as Phil pointed out it is microvolts to volts, when mike close the the food opening. A Simple JFET follower is not very linear over that range. Differential input, maybe you want to drive the mike into some diff amplifier. All that taken into account I think the OPs idea of a TIL071 ?? or whatever JFET opamp is not a bad idea, but have not done the math. It would certainly score better in linearity than that example circuit diagram somebody showed here driving that mike into a BJT emitter (but good impedance matching). As for the minimum noise level, do not get carried away, these mikes pick up all sort of hum from anything near, plus the hum and noise some run of the mill microphone cables pick up. And also keep in mind the sort of usage, sort of performance the mike is used for, so I think super low noise amps with thousands of fets as you suggest is not realistic for this purpose. But indeed maybe it would sell, maybe 300 nuvistors in parallel... Then finally there is the parametric amplifier, and related to that, I used one of these long time ago directly in the collector of a simple one transistor 100 MHz oscillator, enough signal to FM modulate it via Vce changes to hear a watch ticking in the same room via the radio in the other :-) So.... What is the SNR of FM broadcast radio? do not remember, but I remember that experiment:-) +9V tuning | trimmer mike 100 pF | ----------- | | |/ )_____| == === )--------------------------------------------- antenna |10p | ) 7 turns air space 8 mm diameter ---100k ---- | c ---- b e Some NPN | | /// Something like this it was IIRC :-)
From: Dave Platt on 13 Jun 2010 15:25 In article <756a16d2rt3bm8u7johsh5sviag4dpb7ei(a)4ax.com>, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >The best uncooled microwave amps are probably phemts, with noise >figures around 0.3 dB. The matching networks are essentially step-up >transformers, giving free, almost noiseless voltage gain before the >fet gate. Is this the common-base "Norton Amplifier" configuration? or something using a similar trick? >The mmics I've tested tend to have input impedances in the 30-40 ohm >range; I've only found one that actually hits 50 ohms. I wonder if >that's the optimum nf point, or whether there's some other reason to >make them that low. Was that a measurement at DC, or at the operating frequency? If the former, the designers might be assuming that there will be some parasitic inductance in the input lead... a bit of shunt capacitance added right at the end of the input trace could then result in a 50-ohm resistive input Z and thus a good match for the trace? -- Dave Platt <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: John Larkin on 13 Jun 2010 15:58 On Sun, 13 Jun 2010 12:25:21 -0700, dplatt(a)radagast.org (Dave Platt) wrote: >In article <756a16d2rt3bm8u7johsh5sviag4dpb7ei(a)4ax.com>, >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>The best uncooled microwave amps are probably phemts, with noise >>figures around 0.3 dB. The matching networks are essentially step-up >>transformers, giving free, almost noiseless voltage gain before the >>fet gate. > >Is this the common-base "Norton Amplifier" configuration? or >something using a similar trick? I think people usually ground the source and drive the gate through a matching network. CEL/NEC make nice parts, and I think they have example circuits on their data sheets. > >>The mmics I've tested tend to have input impedances in the 30-40 ohm >>range; I've only found one that actually hits 50 ohms. I wonder if >>that's the optimum nf point, or whether there's some other reason to >>make them that low. > >Was that a measurement at DC, or at the operating frequency? Wideband, measured with low-level TDR. The ERA-type mmics make nice wideband pulse amplifiers. The Zin is typically 30-40 ohms, pretty much resistive, so added reactance won't help. The SGA3586 is the only mmic I've found that hits 50 real ohms; you can tune device current to nail it exactly. I have a graph somewhere... I used them as the front ends for a 2-axis delay-line imaging microchannel plate detector (say that fast 5 times) for the ill-fated atom-probe project. John
From: George Herold on 13 Jun 2010 19:59 On Jun 13, 11:25 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 13 Jun 2010 07:30:13 -0700 (PDT), «Leo» <leo2...(a)gmail.com> > wrote: > > > > > > >On 12 jun, 14:09, «Leo» <leo2...(a)gmail.com> wrote: > >> I need to make a high input impedance amplifier for a mic preamp with > >> a single supply. I was thinking of using a TL071 (non-inverting) opamp > >> with high resistance bias resistors (to bias the opamp to half the > >> supply voltage) so the input impedance results is aprox. 1MegOhm. But > >> I don't know if such a scheme would work. I figured since the input > >> bias currents in the JFET input opamps are very low (max 200pA @ 25ºC, > >> 7nA full range) I could bias it with two 2.2MegOhm resistors. Also I > >> think adding high value resistors increases noise at the input...and > >> the overall gain of the circuit would be kind of large (~1000, or > >> perhaps larger), so it would give me a large noise at the output. The > >> input from the mic is in the micro-volt range. > > >> I don`t know if a discrete bipolar solution would be better...or any > >> other clever circuit configuratios for that matter... > > >In reply to all. > > >I know there are mic preamps everywhere on the net. I just wanted to > >build my own. > > >I have a mic that is a SM58 look alike, so the specs could be much > >worse than those of the SM58, and I don't have the mean to measure the > >specs. > > >My ultimate question would be, what input impedance should I aim at > >for my preamp ? > > High. You don't want to push much opamp current noise into the 300 > ohms of the mike, so lots of transistors and "low noise" opamps are > out. Consider a BF862 jfet maybe, or even a few in parallel, if you > want extreme quiet. > > There are some jfet opamps in the 4 nv/rthz noise range, but a BF862 > is 0.8. You could usually get by running one at Idss, especially if > you'd be willing to trim a resistor. A pseudo-inductor drain load > might work too. > > A thread on dynamic microphone noise, in the context of thermodynamics > and conservation of energy, would be interesting. Or maybe it's been > done in another group. You might like "The Physics of Vibration" vol. I by A. B. Pippard. George H. > > John- Hide quoted text - > > - Show quoted text -
From: Phil Allison on 13 Jun 2010 20:31
"Paul Keinanen Da Smug Pedant from Hell" >>>My ultimate question would be, what input impedance should I aim at >>>for my preamp ? > > According to the Shure SM58 specs, the rated output impedance is 150 > ohms, actual 300 ohms (whatever that means). ** Clearly this wanking fool has never even seen an SM58. > > For best noise performance, power matching should be used i.e. a load > resistance of 150-300 ohms. ** Absolute, 100% BULLSHIT !!!!!!!!!!!!!!! > In RF design, power matching is the accepted practice for lowest noise > figures ** Absolutely does NOT apply to the case of a mic pre-amp. ..... Phil |