# i have a problem to design wien-bridge oscillator

Discussion in 'The Projects Forum' started by flugman, May 21, 2008.

1. ### flugman Thread Starter New Member

May 21, 2008
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hi i have a big problem to design wien-bridge oscillator.

i've made it with ua741 and simulated it with pspice.

(attached some figures.)

i wanted to get oscillation freqeuncy in 100kHz and 10V amplitude but i could't have that.

so i guessed there is a simple problem that ua741 is not ideal.

however, i don't know why that circuit in not in oscillation frequency 100kHz.

this circuit oscillated in about 70kHz.

help me to compose it!!!

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2. ### Audioguru New Member

Dec 20, 2007
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The lousy old 741 opamp is 40 years old this year.
Its output drops above only 9kHz.
Its gain is 200,000 from DC to about 2Hz then drops at 6dB/octave until its gain at 1MHz is 1.

You need a newer high frequency opamp.

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3. ### flugman Thread Starter New Member

May 21, 2008
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i should use ua741 because the professor designated it

so i must use ua741 to design it.

do you know excess phase shift at 100kHz with ua741??

i just got a hint that excess phase shift would cause distortioness.

i heard there is about 0.1 rad excess phase shift at 10kHz.

if i get information about excess phase shift at 100kHz, i can solve

this problem.

4. ### Audioguru New Member

Dec 20, 2007
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The amplifier in the Wien Bridge oscillator needs a gain of slightly more than 3. The open loop gain of a 741 opamp at 100kHz is about 8 and its phase shift is 90 degrees so it has a pretty high phase shift of about 40 degrees (just a guess) when its gain is reduced to 3. Your frequency-determining resistors are already a heavy load on the opamp so decrease the value of the capacitors so that it oscillates at 100kHz.

At 100kHz its max output level is less than only 4V p-p with a horrible 10% distortion.

5. ### hgmjr Moderator

Jan 28, 2005
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If you decide to build such a circuit from actual components, you will need to consider some sort of amplitude stabilization method to maintain a consistent operation of of the oscillator over a wide range of environmental variations (i.e. temperature, supply voltage, component aging)

hgmjr