LM13700 weirdness
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From: Bitrex on 24 Apr 2010 02:23 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!
From: Phil Hobbs on 24 Apr 2010 17:10 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 -- 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 24 Apr 2010 18:25 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...
From: Phil Hobbs on 24 Apr 2010 19:52 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... > Motorboating is due to accumulated phase _lead_ at low frequency, rather than lag at high frequency. Only happens in AC-coupled stages. 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? 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: Phil Hobbs on 24 Apr 2010 19:53
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. 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? 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 |