LM13700 weirdness
in [Design]
|
Prev: SPSC2 Does anybody make a micro with 3 DACs
Next: Ocean County NJ Family-Law and Cyber-Law Lawyer Charles Novins Continues To Defend Against Net Abusers
From: Jim Thompson on 25 Apr 2010 20:15 On Sun, 25 Apr 2010 19:57:59 -0400, Bitrex <bitrex(a)de.lete.earthlink.net> wrote: >Joerg wrote: >> Bitrex wrote: >>> Phil Hobbs wrote: >>>> On 4/24/2010 6:25 PM, Bitrex wrote: >>>>> Phil Hobbs wrote: >>>>>> On 4/24/2010 2:23 AM, Bitrex wrote: >>>>>>> I'm using an LM13700 as a VCA in a 9V battery powered circuit, >>>>>>> using an >>>>>>> opamp buffered virtual ground. The circuit works fine when powered >>>>>>> off >>>>>>> the battery, but I wanted to test it using different supply voltages. >>>>>>> When I hooked up the supply to the 15 volt breaboard power supply, I >>>>>>> noticed that the LM13700 was oscillating at about 150 Hz when >>>>>>> there was >>>>>>> no input signal - the oscillation stops when signal is applied and >>>>>>> the >>>>>>> VCA turns on. I couldn't seem to get it to settle down with the usual >>>>>>> supply bypass capacitors, whatever values I tried. Maybe it's just a >>>>>>> consequence of sloppy breadboard layout? The other ICs in the circuit >>>>>>> (mostly TL084s) work OK regardless of supply choice. Suggestions >>>>>>> would >>>>>>> be appreciated! >>>>>> >>>>>> >>>>>> Motorboating? >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>>> >>>>> >>>>> Maybe, but I've tried decoupling the supply in various ways and it just >>>>> doesn't seem to be having any effect on the oscillation. I thought that >>>>> perhaps it might be due to the higher gain of the LM13700 at the higher >>>>> supply voltage, but today I tried disconnecting the signal going into >>>>> the control current input, which caused the frequency of the >>>>> oscillation >>>>> to go up to about 80 Khz. With the control current input disconnected, >>>>> the gain should theoretically be zero... >>>>> >>>> >>>> It's really unlikely to be a bypassing issue at 150 Hz. >>>> >>>> Motorboating is due to accumulated phase _lead_ at low frequency, >>>> rather than lag at high frequency. Only happens in AC-coupled >>>> stages, so if your amp is DC-coupled, that isn't it. >>> >>> Here's a link to the relevant portion of the schematic, though it >>> isn't completed yet: >>> http://i227.photobucket.com/albums/dd240/bitrex2007/DualVCA.jpg >>> >>> Signal comes in from the upper left and is divided down by the 100k >>> and 100 ohm resistors, and the control signal comes in from the bottom >>> right and is made into two signals 180 degrees out of phase for the >>> two VCAs. >>> Output will be at the upper right. Right now for testing I just have >>> one current source/VCA/output pair hooked up on the breadboard using >>> signal generators as inputs. The only AC coupling will be at the >>> input and output of the circuit. >>> >>> >>>> It may also be that your feedback loop becomes unstable when the very >>>> low operating current makes its gain bandwidth get really really low. >>>> Why not post a schematic? >>> >>> Since the LM13700 is being used as a VCA there's no feedback loop >>> around it, exactly; but I think I understand your point - if the gain >>> is very low the bandwidth will be wide and make the amp prone to >>> oscillation. There's bandwidth limiting in the input stage of the >>> actual circuit but for test purposes I was just driving it off a >>> signal generator. I'll try putting a low pass filter on the input of >>> the test circuit and see if that helps. >>> >> >> Besides feedback, aren't you supposed to tie the diode tap to the >> positive supply via a resistor? >> > >I haven't decided what to do with those, yet. The trade off for the >improvement in distortion is that those diodes are supposed to be run at >a relatively high current to get it...in this application (run from a >battery) I don't know if the improvement in distortion is worth the >extra couple of mA. Then again it's only about as much as an extra op >amp section to linearize each LM13700 and I'm already using 10 sections >in the design...eh. > > > > > Never mind that your loop gain is all over the place ?:-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Jamie on 26 Apr 2010 09:18 Bitrex wrote: > Phil Hobbs wrote: > >> On 4/24/2010 6:25 PM, Bitrex wrote: >> >>> Phil Hobbs wrote: >>> >>>> On 4/24/2010 2:23 AM, Bitrex wrote: >>>> >>>>> I'm using an LM13700 as a VCA in a 9V battery powered circuit, >>>>> using an >>>>> opamp buffered virtual ground. The circuit works fine when powered off >>>>> the battery, but I wanted to test it using different supply voltages. >>>>> When I hooked up the supply to the 15 volt breaboard power supply, I >>>>> noticed that the LM13700 was oscillating at about 150 Hz when there >>>>> was >>>>> no input signal - the oscillation stops when signal is applied and the >>>>> VCA turns on. I couldn't seem to get it to settle down with the usual >>>>> supply bypass capacitors, whatever values I tried. Maybe it's just a >>>>> consequence of sloppy breadboard layout? The other ICs in the circuit >>>>> (mostly TL084s) work OK regardless of supply choice. Suggestions would >>>>> be appreciated! >>>> >>>> >>>> >>>> Motorboating? >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> >>> Maybe, but I've tried decoupling the supply in various ways and it just >>> doesn't seem to be having any effect on the oscillation. I thought that >>> perhaps it might be due to the higher gain of the LM13700 at the higher >>> supply voltage, but today I tried disconnecting the signal going into >>> the control current input, which caused the frequency of the oscillation >>> to go up to about 80 Khz. With the control current input disconnected, >>> the gain should theoretically be zero... >>> >> >> It's really unlikely to be a bypassing issue at 150 Hz. >> >> Motorboating is due to accumulated phase _lead_ at low frequency, >> rather than lag at high frequency. Only happens in AC-coupled stages, >> so if your amp is DC-coupled, that isn't it. > > > Here's a link to the relevant portion of the schematic, though it isn't > completed yet: > http://i227.photobucket.com/albums/dd240/bitrex2007/DualVCA.jpg > > Signal comes in from the upper left and is divided down by the 100k and > 100 ohm resistors, and the control signal comes in from the bottom right > and is made into two signals 180 degrees out of phase for the two VCAs. > Output will be at the upper right. Right now for testing I just have > one current source/VCA/output pair hooked up on the breadboard using > signal generators as inputs. The only AC coupling will be at the input > and output of the circuit. > Maybe it has nothing to do with it how ever, due to the nature of that device, my self, I wouldn't tie (-) input together directly, I would use a R branch/T to join them.. The inputs could be causing each other trouble...
From: Jamie on 26 Apr 2010 09:20 Joerg wrote: > Bitrex wrote: > >> Phil Hobbs wrote: >> >>> On 4/24/2010 6:25 PM, Bitrex wrote: >>> >>>> Phil Hobbs wrote: >>>> >>>>> On 4/24/2010 2:23 AM, Bitrex wrote: >>>>> >>>>>> I'm using an LM13700 as a VCA in a 9V battery powered circuit, >>>>>> using an >>>>>> opamp buffered virtual ground. The circuit works fine when powered >>>>>> off >>>>>> the battery, but I wanted to test it using different supply voltages. >>>>>> When I hooked up the supply to the 15 volt breaboard power supply, I >>>>>> noticed that the LM13700 was oscillating at about 150 Hz when >>>>>> there was >>>>>> no input signal - the oscillation stops when signal is applied and >>>>>> the >>>>>> VCA turns on. I couldn't seem to get it to settle down with the usual >>>>>> supply bypass capacitors, whatever values I tried. Maybe it's just a >>>>>> consequence of sloppy breadboard layout? The other ICs in the circuit >>>>>> (mostly TL084s) work OK regardless of supply choice. Suggestions >>>>>> would >>>>>> be appreciated! >>>>> >>>>> >>>>> >>>>> Motorboating? >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>>> >>>> >>>> Maybe, but I've tried decoupling the supply in various ways and it just >>>> doesn't seem to be having any effect on the oscillation. I thought that >>>> perhaps it might be due to the higher gain of the LM13700 at the higher >>>> supply voltage, but today I tried disconnecting the signal going into >>>> the control current input, which caused the frequency of the >>>> oscillation >>>> to go up to about 80 Khz. With the control current input disconnected, >>>> the gain should theoretically be zero... >>>> >>> >>> It's really unlikely to be a bypassing issue at 150 Hz. >>> >>> Motorboating is due to accumulated phase _lead_ at low frequency, >>> rather than lag at high frequency. Only happens in AC-coupled >>> stages, so if your amp is DC-coupled, that isn't it. >> >> >> Here's a link to the relevant portion of the schematic, though it >> isn't completed yet: >> http://i227.photobucket.com/albums/dd240/bitrex2007/DualVCA.jpg >> >> Signal comes in from the upper left and is divided down by the 100k >> and 100 ohm resistors, and the control signal comes in from the bottom >> right and is made into two signals 180 degrees out of phase for the >> two VCAs. >> Output will be at the upper right. Right now for testing I just have >> one current source/VCA/output pair hooked up on the breadboard using >> signal generators as inputs. The only AC coupling will be at the >> input and output of the circuit. >> >> >>> It may also be that your feedback loop becomes unstable when the very >>> low operating current makes its gain bandwidth get really really low. >>> Why not post a schematic? >> >> >> Since the LM13700 is being used as a VCA there's no feedback loop >> around it, exactly; but I think I understand your point - if the gain >> is very low the bandwidth will be wide and make the amp prone to >> oscillation. There's bandwidth limiting in the input stage of the >> actual circuit but for test purposes I was just driving it off a >> signal generator. I'll try putting a low pass filter on the input of >> the test circuit and see if that helps. >> > > Besides feedback, aren't you supposed to tie the diode tap to the > positive supply via a resistor? > Why do that?, if you did, you won't be able to detect the local broadcast station! Gotta love those detectors.
From: Bitrex on 26 Apr 2010 06:07 Jim Thompson wrote: > On Sun, 25 Apr 2010 19:57:59 -0400, Bitrex > <bitrex(a)de.lete.earthlink.net> wrote: > >> Joerg wrote: >>> Bitrex wrote: >>>> Phil Hobbs wrote: >>>>> On 4/24/2010 6:25 PM, Bitrex wrote: >>>>>> Phil Hobbs wrote: >>>>>>> On 4/24/2010 2:23 AM, Bitrex wrote: >>>>>>>> I'm using an LM13700 as a VCA in a 9V battery powered circuit, >>>>>>>> using an >>>>>>>> opamp buffered virtual ground. The circuit works fine when powered >>>>>>>> off >>>>>>>> the battery, but I wanted to test it using different supply voltages. >>>>>>>> When I hooked up the supply to the 15 volt breaboard power supply, I >>>>>>>> noticed that the LM13700 was oscillating at about 150 Hz when >>>>>>>> there was >>>>>>>> no input signal - the oscillation stops when signal is applied and >>>>>>>> the >>>>>>>> VCA turns on. I couldn't seem to get it to settle down with the usual >>>>>>>> supply bypass capacitors, whatever values I tried. Maybe it's just a >>>>>>>> consequence of sloppy breadboard layout? The other ICs in the circuit >>>>>>>> (mostly TL084s) work OK regardless of supply choice. Suggestions >>>>>>>> would >>>>>>>> be appreciated! >>>>>>> >>>>>>> Motorboating? >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>>> >>>>>> Maybe, but I've tried decoupling the supply in various ways and it just >>>>>> doesn't seem to be having any effect on the oscillation. I thought that >>>>>> perhaps it might be due to the higher gain of the LM13700 at the higher >>>>>> supply voltage, but today I tried disconnecting the signal going into >>>>>> the control current input, which caused the frequency of the >>>>>> oscillation >>>>>> to go up to about 80 Khz. With the control current input disconnected, >>>>>> the gain should theoretically be zero... >>>>>> >>>>> It's really unlikely to be a bypassing issue at 150 Hz. >>>>> >>>>> Motorboating is due to accumulated phase _lead_ at low frequency, >>>>> rather than lag at high frequency. Only happens in AC-coupled >>>>> stages, so if your amp is DC-coupled, that isn't it. >>>> Here's a link to the relevant portion of the schematic, though it >>>> isn't completed yet: >>>> http://i227.photobucket.com/albums/dd240/bitrex2007/DualVCA.jpg >>>> >>>> Signal comes in from the upper left and is divided down by the 100k >>>> and 100 ohm resistors, and the control signal comes in from the bottom >>>> right and is made into two signals 180 degrees out of phase for the >>>> two VCAs. >>>> Output will be at the upper right. Right now for testing I just have >>>> one current source/VCA/output pair hooked up on the breadboard using >>>> signal generators as inputs. The only AC coupling will be at the >>>> input and output of the circuit. >>>> >>>> >>>>> It may also be that your feedback loop becomes unstable when the very >>>>> low operating current makes its gain bandwidth get really really low. >>>>> Why not post a schematic? >>>> Since the LM13700 is being used as a VCA there's no feedback loop >>>> around it, exactly; but I think I understand your point - if the gain >>>> is very low the bandwidth will be wide and make the amp prone to >>>> oscillation. There's bandwidth limiting in the input stage of the >>>> actual circuit but for test purposes I was just driving it off a >>>> signal generator. I'll try putting a low pass filter on the input of >>>> the test circuit and see if that helps. >>>> >>> Besides feedback, aren't you supposed to tie the diode tap to the >>> positive supply via a resistor? >>> >> I haven't decided what to do with those, yet. The trade off for the >> improvement in distortion is that those diodes are supposed to be run at >> a relatively high current to get it...in this application (run from a >> battery) I don't know if the improvement in distortion is worth the >> extra couple of mA. Then again it's only about as much as an extra op >> amp section to linearize each LM13700 and I'm already using 10 sections >> in the design...eh. >> >> >> >> >> > > Never mind that your loop gain is all over the place ?:-) > > ...Jim Thompson Even with the smiley, such a comment from an experienced designer makes me wonder if it's just a joke about VCAs being inherently distorting or if I've done something blindingly wrong! :(
From: Jim Thompson on 26 Apr 2010 09:58
On Mon, 26 Apr 2010 06:07:04 -0400, Bitrex <bitrex(a)de.lete.earthlink.net> wrote: >Jim Thompson wrote: >> On Sun, 25 Apr 2010 19:57:59 -0400, Bitrex >> <bitrex(a)de.lete.earthlink.net> wrote: >> >>> Joerg wrote: >>>> Bitrex wrote: >>>>> Phil Hobbs wrote: >>>>>> On 4/24/2010 6:25 PM, Bitrex wrote: >>>>>>> Phil Hobbs wrote: >>>>>>>> On 4/24/2010 2:23 AM, Bitrex wrote: >>>>>>>>> I'm using an LM13700 as a VCA in a 9V battery powered circuit, >>>>>>>>> using an >>>>>>>>> opamp buffered virtual ground. The circuit works fine when powered >>>>>>>>> off >>>>>>>>> the battery, but I wanted to test it using different supply voltages. >>>>>>>>> When I hooked up the supply to the 15 volt breaboard power supply, I >>>>>>>>> noticed that the LM13700 was oscillating at about 150 Hz when >>>>>>>>> there was >>>>>>>>> no input signal - the oscillation stops when signal is applied and >>>>>>>>> the >>>>>>>>> VCA turns on. I couldn't seem to get it to settle down with the usual >>>>>>>>> supply bypass capacitors, whatever values I tried. Maybe it's just a >>>>>>>>> consequence of sloppy breadboard layout? The other ICs in the circuit >>>>>>>>> (mostly TL084s) work OK regardless of supply choice. Suggestions >>>>>>>>> would >>>>>>>>> be appreciated! >>>>>>>> >>>>>>>> Motorboating? >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>>> >>>>>>> Maybe, but I've tried decoupling the supply in various ways and it just >>>>>>> doesn't seem to be having any effect on the oscillation. I thought that >>>>>>> perhaps it might be due to the higher gain of the LM13700 at the higher >>>>>>> supply voltage, but today I tried disconnecting the signal going into >>>>>>> the control current input, which caused the frequency of the >>>>>>> oscillation >>>>>>> to go up to about 80 Khz. With the control current input disconnected, >>>>>>> the gain should theoretically be zero... >>>>>>> >>>>>> It's really unlikely to be a bypassing issue at 150 Hz. >>>>>> >>>>>> Motorboating is due to accumulated phase _lead_ at low frequency, >>>>>> rather than lag at high frequency. Only happens in AC-coupled >>>>>> stages, so if your amp is DC-coupled, that isn't it. >>>>> Here's a link to the relevant portion of the schematic, though it >>>>> isn't completed yet: >>>>> http://i227.photobucket.com/albums/dd240/bitrex2007/DualVCA.jpg >>>>> >>>>> Signal comes in from the upper left and is divided down by the 100k >>>>> and 100 ohm resistors, and the control signal comes in from the bottom >>>>> right and is made into two signals 180 degrees out of phase for the >>>>> two VCAs. >>>>> Output will be at the upper right. Right now for testing I just have >>>>> one current source/VCA/output pair hooked up on the breadboard using >>>>> signal generators as inputs. The only AC coupling will be at the >>>>> input and output of the circuit. >>>>> >>>>> >>>>>> It may also be that your feedback loop becomes unstable when the very >>>>>> low operating current makes its gain bandwidth get really really low. >>>>>> Why not post a schematic? >>>>> Since the LM13700 is being used as a VCA there's no feedback loop >>>>> around it, exactly; but I think I understand your point - if the gain >>>>> is very low the bandwidth will be wide and make the amp prone to >>>>> oscillation. There's bandwidth limiting in the input stage of the >>>>> actual circuit but for test purposes I was just driving it off a >>>>> signal generator. I'll try putting a low pass filter on the input of >>>>> the test circuit and see if that helps. >>>>> >>>> Besides feedback, aren't you supposed to tie the diode tap to the >>>> positive supply via a resistor? >>>> >>> I haven't decided what to do with those, yet. The trade off for the >>> improvement in distortion is that those diodes are supposed to be run at >>> a relatively high current to get it...in this application (run from a >>> battery) I don't know if the improvement in distortion is worth the >>> extra couple of mA. Then again it's only about as much as an extra op >>> amp section to linearize each LM13700 and I'm already using 10 sections >>> in the design...eh. >>> >>> >>> >>> >>> >> >> Never mind that your loop gain is all over the place ?:-) >> >> ...Jim Thompson > >Even with the smiley, such a comment from an experienced designer makes >me wonder if it's just a joke about VCAs being inherently distorting or >if I've done something blindingly wrong! :( I don't know that VCA's are "inherently distorting", but the loop gain variation can be wild, particularly at very low currents. Download the LoopGain checker from my website and simulate. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy |