Re: Op amp oscillator operating point

On Jul 24, 12:13 pm, Stephen-I-am <step...@xxxxxxxxxxxxxx> wrote:
I simulated this op amp oscillator with LTSPICE but was a bit
surprised that the output was a square wave rather than a sinusoid. It
is a dual supply op amp with 3 RC networks between the output and the
inverting terminal.

To make it I simulated the RC network cascade and saw where the
transfer function phase crossed 180 degrees. I was expecting
oscillation to happen there at 3.9 kHz, but it actually occurred at
3.04 kHz. I had assumed that the op amp doesn't add phase, but maybe I
should look at the phase transfer function in the circuit with the
caps removed.

Perhaps I need to shoot for a higher frequency if I want a sinusoid?

R1 Vout 3 10k
C1 3 0 10n
R2 2 1 10k
C2 1 0 10n
R3 3 2 10k
C3 2 0 10n
XU1 0 1 VCC VEE Vout LT1001
V1 VCC 0 2.5
V2 0 VEE 2.5
.tran 2m startup
.options method=trap
.lib LTC.lib
.backanno
.end

For oscillators, you need to force the time step on the .tran card.
Use all 4 parameters.

The very fact that the op amp is of finite bandwidth means it will add
phase lag.
.

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