Wien Bridge + Power Amplifier producing 26V AC with 400hz

Discussion in 'The Projects Forum' started by aboutsc, Jan 23, 2012.

Nov 8, 2010
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Hi all,

I am doing an aircraft project to produce a 400hz and 26V ac output signal. I have transformed the 230V ac/60hz into 13V ac.

I had tried to use the NI Multisim to simulate the circuit that i am going to build. However, I was not able to produce a nice sine wave with 400hz and 26Vpp. Instead, I kept on getting a straight line. May anyone tell me how to achieve?

The resistance and capacitance values that I used in the Wien Bridge Oscillator and Class A Amplifier are as following:

Wien Bridge:
Rf =2k ohms, Ri= 1k ohms giving Av=3 (1+Rf/Ri)
R= 39k ohms, C= 0.01μF
Vcc = ±13V

Class A:
Rb1=7.8kΩ, Rb2=1.5kΩ
Rc=4.7kΩ, Re=1kΩ
Vcc = ±13V

Hope you can help me with this! Thank you.

2. PaulEE Member

Dec 23, 2011
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Maybe I'm missing something, but wein bridge oscillators run on DC...

Nov 8, 2010
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Thanks for your reply. I do supply 13V DC for the Wien Bridge Oscillator. Do I make anything wrong?

4. PaulEE Member

Dec 23, 2011
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Can you post a screenshot of your schematic? It's a lot easier for us to troubleshoot if we have something to visualize the circuit with. Thanks

5. crutschow Expert

Mar 14, 2008
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3,359
Also post the simulation file, if you can.

6. Audioguru New Member

Dec 20, 2007
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896
The gain in a Wien Bridge oscillator must be a little more than 3 for it to start.
If your resistors are not absolutely perfect then they might not have a ratio of a little more than 3. Then an automatic-gain-control keeps the amplitude away from clipping.

Maybe your simulation program needs the circuit to be kicked to get it started because a real circuit starts by amplifying noise.

7. PaulEE Member

Dec 23, 2011
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Well said.

Nov 8, 2010
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Thank you for your reply. I have changed the resistor value like you have mentioned earlier. I got the sine wave and 17Vpp/400hz.

However, I still cannot amplify the input signal to 26V AC/400hz. Maybe I did something wrong with the value?

Please find the attached file for your reference. Hope you can hope! Thank you!!

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9. t06afre AAC Fanatic!

May 11, 2009
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not all of us do have the multisim software. Can you post a PDF or PNG file of your schematics.

Nov 8, 2010
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Thank you for the reminder. I have attached the pictures.

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11. Audioguru New Member

Dec 20, 2007
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Your 741 opamp does not have a negative supply voltage at its pin 4. I assume that it is -10V then the output is clipping badly at 17V p-p as shown on your 'scope.
If it has an added automatic gain control circuit then its output can be set to a level a little below clipping.

The transistor has a supply of only 10V so its output level is low. I can't see the value of its load resistor R10 that also reduces its output level.

Nov 8, 2010
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I have added in the -13V to op-amp's pin 4 and manage to get a stable frequency at 400hz.

I have discovered that the sine wave is chopped off. The Vpp value is 24V which is ±12V. When this signal 24V AC/400hz was input into the class amplifier, the signal is becoming half-wave rectify. I cannot understand this. It looks like becoming a DC signal?!

Under the oscilloscope channel A is the output signal from the Wien Bridge Oscillator and Channel D is the output signal from class A amplifier.

I think I am quite confused with the class A amplifier.

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13. Audioguru New Member

Dec 20, 2007
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Of course the transistor is turned off when its input signal goes negative. It doesn't have an input coupling capacitor.

Why is there a 1uF capacitor in series with 50 ohms between the collector and emitter of the transistor?

Why are you measuring the signal at the emitter of the transistor?

A Class-A amplifier usually has a load resistor that is double or higher than its collector resistor. It is capacitor-coupled to its collector and connected to ground.

I simulated your class-A transistor with an input coupling capacitor, proper bias resistors and a 10k load resistor. Its output clips when its input level is only about 1.7V peak and then its maximum output level is about 4.3V peak.

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14. PaulEE Member

Dec 23, 2011
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Audio,

What simulation software is that? LTSpice?

And, as for the oscillator amplitude, (for the original poster), is there any particular reason you're using a class-a transistor output? Perhaps you could achieve the amplitude you want with another opamp and then use a common-collector transistor stage to lower the output impedance?

Just a thought.

Paul

15. Audioguru New Member

Dec 20, 2007
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Yes, my simulation software is LTspice IV.

PaulEE likes this.

Nov 8, 2010
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There isn't any particular reason of using a Class a amplifier. But I am still stuck in getting the correct amplitude. My Vcc supply is too low for the sine wave to swing up to 26Vrms AC. I have tried Class AB as well. But both of them are not working properly. Maybe I use the wrong value for the resistors?

I wish to try other ways to achieve for the amplitude. I have no problem getting the 400Hz but the amplitude gives me a big headache.

So, Paul you are mentioning about of using another op-amp and then use a common-collector transistor stage to lower the output impedance. Can you tell me a bit more on how to do with these? I do not really understand these.

Anyway, Thank you people who have helped me!

17. Audioguru New Member

Dec 20, 2007
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Do you want a sine-wave of 26V p-p into a 50 ohms load? Then you need a class-AB power amplifier with an output power of 1.7W.

The class-AB power amplifier will use 1 transistor for voltage gain and use a complementary pair of emitter-followers for current gain. It will use a plus and minus 17V power supply.

Here is a simple amplifier that produces more than 26V p-p into 50 ohms when its input is 17V p-p. I used transistors that are too smnall so you should use medium power transistors like TIP31 and TIP32 for the outputs. I used a single 34V supply but you can use a plus and minus 17V supply.

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18. PaulEE Member

Dec 23, 2011
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If you want a 26Vrms (1.414 * 26v = peak), you must increase the power supply rail. Hooking up an output stage with a 13v power supply will never equal 26vrms or 1.414*26vpeak...

When you string op-amp stages together, the overall gain, from input of first op-amp stage to output of second op-amp stage, the gain is the product of the two. So, say your first stage has a gain of +10 and the second +20, your overall gain would be +200 v/v.

Now, suppose you put a 0.13Vrms signal into the first op-amp of the example above. If your op-amps have a +13v supply on each stage, the output signal will clip because the op-amp can't physically go to 26v like the 200*.13v equation says it should.

A common-emitter amplifier is a one-transistor amplifier with (usually) biasing resistors and an emitter/collector resistor. It has moderate input impedance and moderate output impedance. It has voltage and current (power) gain.

A common-collector amplifier is one-transistor amplifier with biasing resistors and an emitter resistor. It has moderate to high input impedance and generally low output impedance. It has current gain.

When you model an amplifier, you must look at both the output impedance of the source you're inputting, the input impedance of the amplifier, the output impedance of the amplifier, and the impedance of the load. The first pair of values form a voltage divider, as do the second pair.

The common-collector amplifier makes a moderate to high source impedance (an op-amp output pin, for example) look low from the load's point-of-view. This allows more current to be driven into the load...in your case, 50 ohms?

Is this making sense?

To get a signal out of an op-amp, or any other transistor amplifier, that is at or near 26Vrms, you need an equivalently large (actually slightly larger) supply voltage on your circuit.