LM675: OP AMP oscillates briefly then oscillation die out and outputs DC component only

Ismail Hegazi

Part Number: LM675
Other Parts Discussed in Thread: OPA548

Hi,

I have a non-inverting op amp circuit shown in the image below:

The circuit works very well when connected to a high R load. However, when connected to a lower R load, the output oscillates briefly and then outputs a 24 DC signal.

The rails on the OP amp are +48V and 0V

Input signal is a 700 Hz, 20 VPP sinusoid (Connected to pin Vdrive), and a 10 V DC offset.

The video attached shows the response. In the video I have the load connected to a 20 ohm resistor.

Any ideas why this is happening?

Thanks.

over 1 year ago

0 Ismail Hegazi over 1 year ago

Prodigy 70 points

I should add more details. My driving circuit is a simple oscillator passing through a buffer:

I am noticing as well when the load is connected, the oscillator output dies out as well. I wonder if the issue is with the oscillator as opposed to the non-inverting op amp...

0 Raymond Zhang1 over 1 year ago in reply to Ismail Hegazi

TI__Guru 55471 points

Hi Ismail,

The LM675 is a decompensated amplifier, and it is required to have noise gains ≥ 10V/V, as prescribed in the STABILITY section of the datasheet. Unfortunately, there is no pspice simulation model for the IC.

For LM675, you would need something similar to the OPA548 in order to be stable, where noise gain is >= 10V/V. Since I do not have LM675 PSpice model, I use OPA548 for the simulation instead, which OPA548 is able to drive the 20Ω load. So to make it stable, you would need to increase the noise gains in LM675 above 10V/V.

OPA548 vs LM675 02022023.TSC

Here is the previous link that it may help to explain some of the issues.

https://e2e.ti.com/support/amplifiers-group/amplifiers-internal/f/amplifiers-internal-forum/186539/lm675-spice-model-or-equivalent?tisearch=e2e-sitesearch&keymatch=lm675%252525252520simulation#

If you have additional questions, please let us know.

Best,

Raymond

0 kai klaas69 over 1 year ago in reply to Ismail Hegazi

Guru 140200 points

Hi Ismail,

I'm not sure what you are doing. But usually a Wien bridge oscillator needs an amplitude stabilization (AGC automatic gain control) like shown in these threads:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1020764/tlv2471-3mhz-wien-bridge-sine-wave-oscillator

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/894414/tlv2471-wrong-ground-connection

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/822864/tina-spice-opa454-wein-bridge-oscillator

By the way, has this thread to do with these threads?

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1129077/opa549-why-doesn-t-this-circuit-oscillate-operate-as-intended/4188840?tisearch=e2e-sitesearch&keymatch=wien#4188840

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1169602/lm675-distortion-noise-only-when-load-is-connected/4400712?tisearch=e2e-sitesearch&keymatch=wien#4400712

Kai