# Wien bridge oscillator help...

#### nlansley

Jan 9, 2023
8
Hi all, im wondering if someone could point me in the right direction.

i have been tasked with designing a wien bridge oscillator circuit that is required to have an output of 14.6kHz.

the circuit must also use a 741 op amp with a +/- 12v.

However after using the formula 1/(2*pi*CR) to calculate component values, when testing, the output frequency seems limited to around 11kHz?

is there anyway that would help me to achieve the desired output frequency as it seems that adjusting the RC circuit values doesnt have much of an effect?

#### crutschow

May 7, 2021
794
Post a schematic of your circuit.

Feb 19, 2021
677
For a 5V sine out you are right at the limits of the 741 slew rate :

Regards, Dana.

#### AnalogKid

Jun 10, 2015
2,843
For a 5V sine out you are right at the limits of the 741 slew rate :

ak

Feb 19, 2021
677

Regards, Dana.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,478
Why need a calculator?
The slew rate of a sine wave V = A × sin(2 × Pi × f × t) is max for t=0 (zero crossing) with SlewRate = A × 2 × Pi × f
Where:
V = voltage
A = amplitude
f = frequency
t = time

SlewRate = 5 V × 2 × Pi × 14600 Hz = 458672 V/s = 0.458672 V/µs (differences to post #3 probably due to rounding errors)

The slew rate of a 741 opamp is 0.5 V/µs (datasheet) so it should be barely able to handle a 5 V sine at 14.6 kHz but no higher amplitude.

Feb 19, 2021
677
Actually the slew rate spec in a 741 datasheet is a "typical" spec at room T and nominal
V. So no guarantees that part would not be slew limited at 14.6 Khz.

Might be prudent to use a 1 or 5 V / uS part (as its probably also typical spec).

Regards, Dana.

#### nlansley

Jan 9, 2023
8
cheers for the info,

the output that i am looking for can be less than the 5v though it just has to meet the criteria of a minimum amplitude of 0.5v pk-pk.

to change the amplitude of the output of this circuit does the supply to the opamp need to be limited?

#### nlansley

Jan 9, 2023
8
This is the design im using

#### Attachments

• Screenshot 2023-01-11 124329.png
21.9 KB · Views: 4

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,478
This is the design im using
Looks like the opamp is supplied qith Vcc (12 V?) on both the pos. and neg. supply terminal. This will not work. You need a pos. and a neg. supply voltage,

does the supply to the opamp need to be limited?
No.
The amplitude can be limited by the addition of diodes. See e.g. this discussion.

#### nlansley

Jan 9, 2023
8
Looks like the opamp is supplied qith Vcc (12 V?) on both the pos. and neg. supply terminal. This will not work. You need a pos. and a neg. supply voltage,
sorry for the confusion, top voltage is Vee (-12v). will limiting the output amplitude help achieve the frequency required due to it not being as close to the slew rate for this Amp?

Feb 19, 2021
677
Just do the slew rate calculation....

Regards, Dana.

#### AnalogKid

Jun 10, 2015
2,843
Why need a calculator?

For my library collection. I've seen many calculator sites come and go, so I like to have two or three links for each type.

ak

Moderator
Nov 17, 2011
13,478

#### nlansley

Jan 9, 2023
8
Thanks for the help!

I have finally got the circuit to output the correct frequency, this is with using Zeners to regulate the output amplitude but i also had to use a Potentiometer in series with my resistor in the RC networks to achieve the correct frequency.

the total resistance with the Pot and the fixed resistor is the same as one single resistor however the output frequency is very different.

what is the reason for this?

Feb 19, 2021
677
How much of a deviation in freq ? What freqs are you seeing ?

Post a pic of your prototype and schematic.....

Regards, Dana.

#### nlansley

Jan 9, 2023
8
with a fixed resistor we were getting around 13kHz but with a pot and fixed resistor we could get the required 14.6kHz. this testing was simulated on proteus

Feb 19, 2021
677
Seems like a large deviation. Cant rack that up to stray C at this freq. Which R

Obviously check the sim is using same R total in the applicable circuit branch.

What does the spice model for the pot look like ?

Regards, Dana.

Last edited:

#### nlansley

Jan 9, 2023
8
this is the revised circuit. the inclusion of RV1 and Rv2 in series with the resistors is what resulted in the frequency change even though in theory the total resistance in the filter networks is still 3.3k. however with just a standard 3.3k resistor in place, the desired frequency couldnt be attained.

#### Attachments

• mod.png
21.9 KB · Views: 4

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,478
RV1 and Rv2 in series
There's no RV1 or RV2 in your schematic.

the total resistance in the filter networks is still 3.3k.

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