LM13700 weirdness
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From: Bitrex on 24 Apr 2010 22:34 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. > > Cheers > > Phil Hobbs > > >
From: Bitrex on 25 Apr 2010 01:37 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. I meant to say from output to input... :) I connected up a feedback loop of a parallel RC from output to input of the VCA and it has made things settle down. Originally it didn't have one - I hadn't seen one in the schematics of similar circuits I've seen but perhaps my breadboard layout makes in necessary.
From: Joerg on 25 Apr 2010 10:03 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? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Phil Hobbs on 25 Apr 2010 14:55 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? > Only if you want decent distortion performance...and then it becomes more like a current conveyor--current-in, current-out. I love LM13700s. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Bitrex on 25 Apr 2010 19:57
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. |