[Moved] Increasing frequency in a Wien's Bridge

[kodiak]

Greetings, guys! I'm trying to increase the frequency of a Wien's Bridge up to at least 600kHz. As R1=R2 and C1=C2, the frequency should be given by \[\frac{1}{2ΠRC}\], but this relation does not fit the simulated frequency when it goes higher than 14k~15kHz. I'm aware that the Wien's Bridge was not meant to work with frequencies this high, but I just can't understand why it stops working. The following circuit should oscilate in 1.4MHz, but in the simulation it seems to oscilate in 440kHz.

I would think that this would be related to the Opamp I'm using, but I have already made a TL072 oscilate in MHz with a different oscilation circuit. So, would you have any idea of what's going on with the circuit that does not allow it to oscilate faster? What could I do?
Thank you.

 

[KlausST]

Hi,

Yes, i thi k it is the Opamp.
Go through it's datasheet and find charts giving open loop gain and - more important - phase shift against frequency.

For the oscillation you need a phase shift of 180 degree. With low frequencies the opamp has about zero phase shift, so you don't need to take this into account. But with higher frequencies there is a significant phase shift.

Klaus

 

[LvW]

The oscillation condition requires a loop gain with a phase shift of ZERO deg.
For the WIEN type oscillator this is achieved with a bandpass (zero phase) and a non-inverting opamp (also zero phase). Hence, there is no 180deg phase shift at all.
For R1=R2 and C1=C2 you need an opamp gain slightly largerb than "3". Hence, the used opamp must be able to provide this gain at the desired frequency.
But there is another parameter that - in many cases - limits the use for higher frequencies: SLEW RATE.
Therefore, select an opamp with very high slew rates (large signal bandwidth) - for example, a Current-Feedback Amplifier (CFA).

More than that, if the used capacitors are in the pF range, you must be aware that parasitic capacitances play a certain role.

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A rough calculation gives a minimum slew rate (for 5 Volts output amplitude) of app. SR=20V/µs.

 

[Audioguru]

Most opamps cannot drive the very low value 150 ohm resistors in your circuit since their minimum allowed load is 2000 ohms. Some opamps can drive a load of 600 ohms. But if you increase the value of the resistors then you must decrease the value of the capacitors down to almost only stray capacitance.