On 7/5/2015 3:18 PM, piglet wrote:> On 05/07/2015 19:59, bitrex wrote: >> >> The LM13700 is a very useful chip and works at low supply voltages as >> well. It's unfortunate they don't make better OTAs that would be >> suitable for much higher frequency applications. > > You might find the LM13700 especially useful in your Theremin project > for the volume control circuitry. That VCA stage can be a problem area > in Theremin design. I find the old RCA design which modulates tube > filament power to vary the volume delightfully zany! > > piglet > >Which design was that? I can't imagine modulating filament power at low audio frequencies could be very great for it...O_o
Opamp frequency mixer
Started by ●July 4, 2015
Reply by ●July 5, 20152015-07-05
Reply by ●July 5, 20152015-07-05
>Which design was that? I can't imagine modulating filament power at low >audio frequencies could be very great for it...O_oNo kidding. Running a tube in the cathode-emission-limited condition is for a good time, not a long time. (Rectifiers are an exception.) Cheers Phil Hobbs
Reply by ●July 5, 20152015-07-05
On Sat, 04 Jul 2015 15:36:16 -0700, the renowned John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:> > >I'm just finishing this one > >http://www.amazon.com/Much-ADO-about-Almost-Nothing/dp/0615139957/ref=sr_1_1?ie=UTF8&qid=1436049038&sr=8-1&keywords=much+ado+almost+nothingThis is his too.. I've not seen it. http://www.amazon.com/Circumstantial-Evidence-John-Penter/dp/B002JSC26G -- Best regards, Spehro Pefhany Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8 Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
Reply by ●July 5, 20152015-07-05
On Sunday, July 5, 2015 at 11:59:27 AM UTC-7, bitrex wrote:> There's a very simple circuit for a four-quadrant multiplier in the > LM13700 application notes. I'm not sure if it will work at a few > hundred kHz, however.Two-quadrant multiplication with an OTA is more than enough, and it'll work fine at megahertz frequencies, because you don't care about power gain. Feed your low-level signal into the diff inputs, and modulate the bias current with the LO.> The LM13700 is a very useful chip and works at low supply voltages as > well. It's unfortunate they don't make better OTAs that would be > suitable for much higher frequency applications.They do: TI's OPA861, for example. They just have such opaque datasheets and descriptions, that it's hard to recognize the face under the greasepaint and red nose. <http://www.ti.com/lit/ds/sbos338g/sbos338g.pdf>
Reply by ●July 5, 20152015-07-05
On 7/5/2015 6:26 PM, whit3rd wrote:> On Sunday, July 5, 2015 at 11:59:27 AM UTC-7, bitrex wrote: > >> There's a very simple circuit for a four-quadrant multiplier in the >> LM13700 application notes. I'm not sure if it will work at a few >> hundred kHz, however. > > Two-quadrant multiplication with an OTA is more than enough, and it'll work > fine at megahertz frequencies, because you don't care about > power gain. Feed your low-level signal into the diff inputs, > and modulate the bias current with the LO.OTAs are just an input diff pair and four current mirrors: one to turn the bias current input (I_ABC in the LM13700 datasheet) into the tail current for the diff pair, and three more to take the diff pair's collector currents and mirror them from the supply rails. (There's a hard-to-avoid asymmetry, because the diff pair collectors always sink current, so the + current source only needs one mirror, whereas the - current source needs two in cascade.)> >> The LM13700 is a very useful chip and works at low supply voltages as >> well. It's unfortunate they don't make better OTAs that would be >> suitable for much higher frequency applications.I still have a couple of dozen VTC VA713 and VA2713s from the early '90s. They're a lot like the LM13700 except a hundred or so times faster. Long gone, of course. :(> > They do: TI's OPA861, for example. They just have such opaque > datasheets and descriptions, that it's hard to recognize the > face under the greasepaint and red nose. > > <http://www.ti.com/lit/ds/sbos338g/sbos338g.pdf>The OPA861 isn't an OTA, despite the name--it's intended as an improved BJT. It lacks differential inputs, for a start. TI rented the name for a completely different device, drat them. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●July 5, 20152015-07-05
On 7/5/2015 12:39 PM, Clifford Heath wrote:> On 06/07/15 02:22, Clifford Heath wrote: >> On 05/07/15 07:06, Jim Thompson wrote: >>> On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com> >>> wrote: >>>> On 2015-07-04 1:30 PM, bitrex wrote: >>>>> Can anyone suggest an opamp mixer topology for the low 100s of kHz? >>>> If it absolutely has to be semiconductors I'd use the MC1496. >>> I'd go with the MC1494... if you can get it. It has the level >>> shifting built-in... quite adequate for audio, and immensely eases the >>> pain of doing your own level-shifting. >> >> If you can't get MC1494 but can arrange one bipolar drive signal, you >> can make the rest of a Gilbert cell very easily using the matched >> transistors in a LM3046, which are readily available, cheap and >> sufficiently retro. You even get a spare transistor you could use for >> the phase splitter. > > Forgot to mention, the Ft of the LM3046 is circa 350MHz, so no problem > operating at HF. And there's always the rather faster and more expensive > HFA3101 if you need to operate up to 1GHz :). But the LM3046 is one of > my jelly-beans for discrete stuff up to 50MHz. It can go higher, but > it's less trouble to design using faster transistors. >If you feel like showing me an example of the kind of circuit you have in mind, that would be great... ;-)
Reply by ●July 5, 20152015-07-05
On 7/5/2015 12:39 PM, Clifford Heath wrote:> On 06/07/15 02:22, Clifford Heath wrote: >> On 05/07/15 07:06, Jim Thompson wrote: >>> On Sat, 04 Jul 2015 13:53:15 -0700, Joerg <news@analogconsultants.com> >>> wrote: >>>> On 2015-07-04 1:30 PM, bitrex wrote: >>>>> Can anyone suggest an opamp mixer topology for the low 100s of kHz? >>>> If it absolutely has to be semiconductors I'd use the MC1496. >>> I'd go with the MC1494... if you can get it. It has the level >>> shifting built-in... quite adequate for audio, and immensely eases the >>> pain of doing your own level-shifting. >> >> If you can't get MC1494 but can arrange one bipolar drive signal, you >> can make the rest of a Gilbert cell very easily using the matched >> transistors in a LM3046, which are readily available, cheap and >> sufficiently retro. You even get a spare transistor you could use for >> the phase splitter. > > Forgot to mention, the Ft of the LM3046 is circa 350MHz, so no problem > operating at HF. And there's always the rather faster and more expensive > HFA3101 if you need to operate up to 1GHz :). But the LM3046 is one of > my jelly-beans for discrete stuff up to 50MHz. It can go higher, but > it's less trouble to design using faster transistors. >Unfortunately, however, it seems like the MC1494, 1496, LM3046, and so on are all non-stock just about everywhere.
Reply by ●July 6, 20152015-07-06
Digikey shows 6000-odd of the MC1496 in stock. http://www.digikey.com/product-detail/en/MC1496DR2G/MC1496DR2GOSCT-ND/1139685?WT.z_cid=ref_octopart_dkc_buynow&site=us Cheers Phil Hobbs
Reply by ●July 6, 20152015-07-06
On 05/07/2015 22:58, Phil Hobbs wrote:>> Which design was that? I can't imagine modulating filament power at low >> audio frequencies could be very great for it...O_o > > No kidding. Running a tube in the cathode-emission-limited condition is for a good time, not a long time. (Rectifiers are an exception.) > > Cheers > > Phil Hobbs >No cathode. Directly heated, the thermal TC of a cathode would respond far too slowly to volume hand movement. piglet
Reply by ●July 6, 20152015-07-06
On 05/07/2015 22:45, bitrex wrote:> > Which design was that? I can't imagine modulating filament power at low > audio frequencies could be very great for it...O_oThe filament runs at the volume oscillator frequency (ca 400khz in the original 1929 RCA) via tuned circuit like a slope detector so that varying the volume oscillator frequency by hand movement varies filament emission and varies current drive to one of the AF stages. Its 90 year old VCA. Zany. Schematics and descriptions here: http://www.pavekmuseum.org/theremin/theop.html http://www.theremin.us/RCA/rca_theremin.html piglet
