LM13700 weirdness

Motorboating?

Cheers

Phil Hobbs

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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

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

Here's a link to the relevant portion of the schematic, though it isn't completed yet:

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.

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.

Besides feedback, aren't you supposed to tie the diode tap to the positive supply via a resistor?

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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

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

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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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! :(

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...

Why do that?, if you did, you won't be able to detect the local broadcast station! Gotta love those detectors.

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

Thanks! I will definitely do that. I'm curious as to the origin of these loop gain variations - in an LM13700 the gain is controlled by sinking a current into a base current compensated current mirror, the other half of which is connected to the input differential pair to control the gain. It would _seem_ that there's little room for things to go wrong...

The gain varies ?:-) ...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

voltages.

usual

circuit

just

that

higher

Well, right, I guess I thought that you were implying that it varies _unpredictably_ at low currents. I think I see how that could happen, in a perfect world the transconductance would just be I_0/2*VT, where I_0 is the reference current...but of course even the Wilson current mirror in the IC is less than perfect at low currents.