Hi all

Im trying to create a wien bridge oscillator with a frequency of 14.6kHz. i have to use a 741 op amp with a supply of +/-12v.

using the formula to derive the values for RC when simulated the frequency is only about 13kHz.

what is the reason for this and how do i correct this to obtain the correct frequency?

i have attached an image for the simulation circuit that i am testing

 

Use a better op-amp, a 741 is probably adding some phase shift of its own at that frequency.

 

My calculator predicts 14.468 KHz. R1 should be twice R4 at 100K and what is that Bat1 for?

 

I have made R1 slightly more than R4 just to maintain oscillation, when I do it directly as double it decays. And I’ve used the battery to introduce noise to start the signal for the circuit, however when simulated 13kHz is achieved

 

The most critical parameter for 14kHz is the slew rate of the 741 device (as small as 0.5V/µs).
You must use another opamp with a slewr rate of at least some volts/µs.
I suppose that the signal you have observed is somewhat distorted in triangular shape, correct?

 

hi nath,
Please post the LTSpice asc file so that we can compare the simulation results.
E

 

I have made R1 slightly more than R4 just to maintain oscillation, when I do it directly as double it decays. And I’ve used the battery to introduce noise to start the signal for the circuit, however when simulated 13kHz is achieved

I observed similar conditions when actually breadboardlng the circuit and the frequency was almost exactly 13Khz. using a 741.
I suppose the values for R2 and R3 could be reduced 12% and see if that makes any difference.

 

The antique 741 opamp design is 54 years old and it should be buried.

 

You are not far off, resistors are typically quite close tolerance but capacitors can be +/- 20% of nominal value! How about just changing resistor values to get the frequecy you want?

 

hi, im hoping some one can shed some light on this for me.

i have been tasked with creating a wien bridge oscillator circuit to create a 14.6kHz output. i have designed my circuit using 1/2piCR and got the values for my components. yet when i run the simulation in LTspice my output frequency is almost 10%out at 13.2kHz. does anyone know why this is and how to rectifiy?

i have attached my LTspice files and a screenshot of my circuit

 

You are not far off, resistors are typically quite close tolerance but capacitors can be +/- 20% of nominal value! How about just changing resistor values to get the frequecy you want?

i wanted to keep these at design value if possible

 

hi nath,
The FFT shows 13.89kHz.
E

 

hi nath,
Have you observed that the form factor, of 1, for a Sine wave is not correct for your plotted Sine wave.?
E

 

hi nath,
Have you observed that the form factor, of 1, for a Sine wave is not correct for your plotted Sine wave.?
E

Hi, sorry what do you mean by this?

 

The antique 741 opamp design is 54 years old and it should be buried.

Then professors will have to rewrite their 50-year-old notes.

 

Why do you have two 1n4148 diodes in parallel ?

Les.

 

Why do you have two 1n4148 diodes in parallel ?

Les.

This is for amplitude control

 

I would have expected them to be in reverse in opposite polarity to each other for amplitude control.

Les.

 

i wanted to keep these at design value if possible

I understand, but in the real world component values, especially capacitors, have wide tolerances - you could spend time trying to get all the capacitors right but it's quicker and easier to change resistor values. You can fine tune resistor values relatively easily by selecting a resistor a little larger than required and putting resistors in parallel with it which have two orders of magnitude higher value. In the days when people wanted accurate restance decade boxes this is how it was done inexpensively. I'd also add, an engineer should always look for the simplest solution to a problem!

 

hi nath,
Rerun this with the 741 and the AD8030
Note the frequency shift in your favour.
The 741 is unsuitable for this application.
E