High Input Impedance Single Supply Amp ?
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From: «Leo» on 12 Jun 2010 13:09 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...
From: John Larkin on 12 Jun 2010 13:27 On Sat, 12 Jun 2010 10:09:33 -0700 (PDT), �Leo� <leo2100(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 @ 25�C, >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... What sort of microphone is it? What's its impedance? Is it DC continuous? Do you have access to both ends? John
From: «Leo» on 12 Jun 2010 14:14 On 12 jun, 14:27, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sat, 12 Jun 2010 10:09:33 -0700 (PDT), «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... > > What sort of microphone is it? What's its impedance? Is it DC > continuous? Do you have access to both ends? > > John I don't have the specs, but it is a SM58 kind of mic. Cheaper, with worse specs I imagine. Here's the SM58 http://www.shure.com/americas/products/microphones/sm/sm58-vocal-microphone
From: Fred on 12 Jun 2010 14:32 On Jun 12, 1:14 pm, «Leo» <leo2...(a)gmail.com> wrote: > On 12 jun, 14:27, John Larkin > > > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > On Sat, 12 Jun 2010 10:09:33 -0700 (PDT), «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... > > > What sort of microphone is it? What's its impedance? Is it DC > > continuous? Do you have access to both ends? > > > John > > I don't have the specs, but it is a SM58 kind of mic. Cheaper, with > worse specs I imagine. Here's the SM58http://www.shure.com/americas/products/microphones/sm/sm58-vocal-micr... http://sound.westhost.com/project66.htm http://www.jensen-transformers.com/apps_sc.html http://orca.st.usm.edu/~jmneal/preamp1.htm http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm
From: John Larkin on 12 Jun 2010 14:42 On Sat, 12 Jun 2010 11:14:59 -0700 (PDT), �Leo� <leo2100(a)gmail.com> wrote: >On 12 jun, 14:27, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sat, 12 Jun 2010 10:09:33 -0700 (PDT), �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 @ 25�C, >> >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... >> >> What sort of microphone is it? What's its impedance? Is it DC >> continuous? Do you have access to both ends? >> >> John > >I don't have the specs, but it is a SM58 kind of mic. Cheaper, with >worse specs I imagine. Here's the SM58 >http://www.shure.com/americas/products/microphones/sm/sm58-vocal-microphone TL071 is noisy, 18 nv/rthz. You can get down to numbers like 4 with better hi-z opamps, 0.8 with a discrete jfet like BF862. There are tons of mic preamp circuits around already. It's been done to death. John
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