Oh, Ron H, I forgot to say that I have distortion at the output of TL072 when I increase the modulating amplitude or reduce the resistor between pin 2 and 3, so can you direct me or show me a circuit (probably based on transistors) to incrase the amplitude ofthe output of the modulator before TL072 and then reduce the volume of modulating signal or increase the Resistor? Is it possible at all?

Thanks

You need to understand that there will be (approximately) zero AC (signal) voltage at the outputs of the modulator, since they are connected to the summing nodes (virtual AC grounds) of the op amps. If you don't understand why this is the case, perhaps one of the moderators can point you to a tutorial on op amps. If you want more output from the op amps, increase the resistors Rf (12k in my schematic). Higher supply voltage will help, but you will still have to increase Rf.
The schematic and waveform files were the full width of my screen when I posted them. This forum always seems to shrink pictures, and I have no idea how to make them larger. When I post on the Electro-Tech forum, they come out full size. Apparently, shrinking them saves server space, but it also makes them unreadable in some cases. Can one of the moderators or administrators comment on this?

 

If you don't understand why this is the case, perhaps one of the moderators can point you to a tutorial on op amps.

Happy to oblige.
http://www.allaboutcircuits.com/vol_3/chpt_8/index.html

.

 

Here is an attempt to make the amplifier portion readable. The modulator circuit is unchanged from the original.

 

Here is an attempt to make the amplifier portion readable. The modulator circuit is unchanged from the original.

Thanks Sir,

What is your idwa about this?
It has a higher output amplitude.

 

Thanks Sir,

What is your idwa about this?
It has a higher output amplitude.

That general concept will work if you add 3k in series between point 'a' and the 100nF cap which is shown connected to 'a'. Without the resistor, you will be driving the second op amp into nonlinear behavior. However, your feedback resistors are much too large. I told you that 12k is optimum for the input signal levels I gave you. If you don't have enough output, you need to raise the input signal levels. Low signal amplitudes can't be compensated for by making the feedback resistors large, because the TL072 gain-bandwidth product is too low. With Rf=390k and Rs=3k (the resistor from 1496 pin 6), your loop gain is around 130. The GBW product of TL072 is 3MHz, so your bandwidth will be about 3Mhz/130=23kHz. This won't work well with your 40kHz carrier frequency.
Why are you tackling something so complicated? You are obviously in way over your head here. You should take baby steps before attempting to run a marathon.

 

That general concept will work if you add 3k in series between point 'a' and the 100nF cap which is shown connected to 'a'. Without the resistor, you will be driving the second op amp into nonlinear behavior. However, your feedback resistors are much too large. I told you that 12k is optimum for the input signal levels I gave you. If you don't have enough output, you need to raise the input signal levels. Low signal amplitudes can't be compensated for by making the feedback resistors large, because the TL072 gain-bandwidth product is too low. With Rf=390k and Rs=3k (the resistor from 1496 pin 6), your loop gain is around 130. The GBW product of TL072 is 3MHz, so your bandwidth will be about 3Mhz/130=23kHz. This won't work well with your 40kHz carrier frequency.
Why are you tackling something so complicated? You are obviously in way over your head here. You should take baby steps before attempting to run a marathon.

Yes, you are right, I am a baby in electronics, but unfortunately I have to to make this circuit at the moment.

Well, the Rf is 100k and I forgot to reflect this in my pic.

I noticed that my modulator chip has a problem, and then I replaced it with a new one, so your original circuit is working ok (with Rf=100k) I need a resonable gain so I prefer to use 100k instead of 12k.

besides,I am using 2 330k resistors for modulator chip(pins 1 and 4), that's why I am thinking that I would need less input amplitude for modulating signal.
by the why I changed the Re of the modulator to 470 ohm.

What are you meaning of: 'The GBW product of TL072 is 3MHz, so your bandwidth will be about 3Mhz/130=23kHz. This won't work well with your 40kHz carrier frequency.'
what is the relation of band with with carrier freq here!?

Thanks

P.s. Are you thinking if I find the Lm8272 then I will have a higher output at my transducer? If so I'll try to find it.

 

Yes, you are right, I am a baby in electronics, but unfortunately I have to to make this circuit at the moment.

Well, the Rf is 100k and I forgot to reflect this in my pic.

I noticed that my modulator chip has a problem, and then I replaced it with a new one, so your original circuit is working ok (with Rf=100k) I need a resonable gain so I prefer to use 100k instead of 12k.

What is the amplitude of your 40kHz carrier input?

besides,I am using 2 330k resistors for modulator chip(pins 1 and 4), that's why I am thinking that I would need less input amplitude for modulating signal.

That's way too high! The typical bias current on pins 1 and 4 is 12uA, 30uA max. 12uA times 330k is about 4V. The voltage divider that biases these resistors (1k, 820, 1.3k) provides a nominal bias voltage of 3.85V. This means you are getting almost no current through your modulator, and this is why you need such high value feedback resistors on your op amps. The pin 1,4 resistors should be no larger than 1k.

by the why I changed the Re of the modulator to 470 ohm.

What are you meaning of: 'The GBW product of TL072 is 3MHz, so your bandwidth will be about 3Mhz/130=23kHz. This won't work well with your 40kHz carrier frequency.'
what is the relation of band with with carrier freq here!?

All op amps have a gain-bandwidth product (GBW). The product of gain and bandwidth is approximately constant for any given op amp. This means that if you set the gain higher, the bandwidth will drop proportionally. The TL072 has a GBW of 3MHz. The gain of your output stage is 100k/3k=33.3, where 3k is the resistor from pin 6 to +12V. This means that your bandwidth is about 90kHz, which is OK. You should need to reduce Rf and Re once you reduce the pin 1,4 resistor values.

Thanks

P.s. Are you thinking if I find the Lm8272 then I will have a higher output at my transducer? If so I'll try to find it.

Probably not.

 

What is the amplitude of your 40kHz carrier input?

That's way too high! The typical bias current on pins 1 and 4 is 12uA, 30uA max. 12uA times 330k is about 4V. The voltage divider that biases these resistors (1k, 820, 1.3k) provides a nominal bias voltage of 3.85V. This means you are getting almost no current through your modulator, and this is why you need such high value feedback resistors on your op amps. The pin 1,4 resistors should be no larger than 1k.
All op amps have a gain-bandwidth product (GBW). The product of gain and bandwidth is approximately constant for any given op amp. This means that if you set the gain higher, the bandwidth will drop proportionally. The TL072 has a GBW of 3MHz. The gain of your output stage is 100k/3k=33.3, where 3k is the resistor from pin 6 to +12V. This means that your bandwidth is about 90kHz, which is OK. You should need to reduce Rf and Re once you reduce the pin 1,4 resistor values.

Probably not.

Sorry Ron H,
There is a mistype, my mean is 330Ohm not 330K for pins 1 and 4.

well, the carrier is a constant square wave(nearly 50% to 52% duty cycle) at 300mVp-p

Thank for describing of GBW, but why you think 23K of bandwidth is not ok while I need 2 to 3KHz of bandwidth?

 

Sorry Ron H,
There is a mistype, my mean is 330Ohm not 330K for pins 1 and 4.

well, the carrier is a constant square wave(nearly 50% to 52% duty cycle) at 300mVp-p

Thank for describing of GBW, but why you think 23K of bandwidth is not ok while I need 2 to 3KHz of bandwidth?

You need 2 to 3kHz of bandwidth, but it is centered at the carrier frequency, which is 40kHz. The op amp has to pass this frequency (actually, to the upper sideband).
330 ohms may be loading your source. Where does your modulating signal come from? Where does your carrier come from?