Pls correct me what is wrong with this FM transmitter

[audioguru]

Hi David the Bear,
R2 should be 47k, not 4k7.
R1 should be 200 ohms not 220n.

I don't knpow what U1B and the pile of parts connected to it are for. They bias pin 2 of U1A at +2.5V which is not necessary since U1A is simply a follower for the DC voltage at its input pin 3. C12 causes its high frequency AC gain to be its maximum (about 32 times at 53kHz) which you probably do not want.
Instead, the input pin 3 of U1A needs to be biased at +2.5V.

The modulation input of the FM oscillator is only maybe 50mV, not 3.8V so the opamp does not need any gain.

I don't know if U1A will oscillate when it tries to drive the 470pF capacitor C5 as its load.

 

[davidbear]

Hi Audioguru,

Clearly, I was wrong, I did not learn anything about your circuit (I am clearly not attacking it from the correct prospective or perhaps I have insufficient background, maybe you will continue to help?). Let me try to address some of the things that you bring up. I was under the impression that R2 and C5 were forming a low pass filter, thus I lowered the value of R2 so that 57 kHz could pass. I was also under the impression that R1 was being used to limit the emitter current.

I had been under the impression in your circuit that Q1 was set up as a common emitter with a gain of about 20. I also thought that the electret mic put out about 0.2Vpp. Thus, I figured that my op amp need to provide the same. As far as all of the cabbage around U1B/U1A is concerned, I was trying to take from p.6 of Texas Instruments Single Supply Op-Amp Application note SLOA058 https://www.electro-tech-online.com/custompdfs/2010/06/SingleSupply.pdf. The output of U1B becomes the virtual ground for U1A. The 1nF capacitors are trying to prevent RF from feeding back into the circuit.

It would appear that you are correct about driving the 470 pF load. The microchip datasheet from a related op amp (https://www.electro-tech-online.com/custompdfs/2010/06/22189a.pdf) suggests a ~800-1000 ohm resistor be added in series with the capacitor.

 

[audioguru]

I was under the impression that R2 and C5 were forming a low pass filter, thus I lowered the value of R2 so that 57 kHz could pass.

No.
R2 in your latest schematic is the base bias resistor for the oscillator transistor that must be 47k for the transistor to work. I said that the collector resistor of the preamp transistor in parallel with the base bias resistor of the oscillator transistor at AC frequencies made a 17kHz lowpass filter. I said that the extremely low output impedance of an opamp (that has plenty of gain at 53kHz but yours doesn't) would make a very high cutoff frequency.

I was also under the impression that R1 was being used to limit the emitter current.

No.
R1 allows a point where positive feedback can be applied to the emitter of the oscillator transistor. Without R1 then the emitter has an impedance that is much too low to feed the positive feedback.

I had been under the impression in your circuit that Q1 was set up as a common emitter with a gain of about 20.

No.
Q1 has a gain of 20 only at low audio frequencies. C4 increases the gain to about 5 times at 15kHz for pre-emphasis.

I also thought that the electret mic put out about 0.2Vpp.

No.
An electret mic has an output of about 10mV (0.01V) into a 10k load when you talk at a normal conversation voice level about 10cm from the mic.

Thus, I figured that my op amp need to provide the same. As far as all of the cabbage around U1B/U1A is concerned, I was trying to take from p.6 of Texas Instruments Single Supply Op-Amp Application note SLOA058 https://www.electro-tech-online.com/custompdfs/2010/06/SingleSupply-1.pdf. The output of U1B becomes the virtual ground for U1A.

No.
EDIT: U1A doesn't need a virtual ground opamp circuit. Just 3 resistors plus a filter capacitor will work instead.
Texas Instruments are very WRONG. The (+) input of the opamp must have a bias voltage, not the (-) input.

The 1nF capacitors are trying to prevent RF from feeding back into the circuit.

No.
C12 causes the gain to keep increasing at high frequencies because it shorts the (-) input of the opamp to ground.

It would appear that you are correct about driving the 470 pF load. The microchip datasheet from a related op amp (https://www.electro-tech-online.com/custompdfs/2010/06/22189a-1.pdf) suggests a ~800-1000 ohm resistor be added in series with the capacitor.

Then the circuit will not have much gain at 53kHz.
You should use an opamp that works well at high audio frequencies and has a low output impedance like an MC34071. It doesn't oscillate when its output drives a capacitor as high as 10nF. It also works fine with a supply as low as 5V.

 

[davidbear]

Thanks again. I think that I made most of the changes that you suggested, including changing the op-amp. I hope this simple design will do the trick. I was comparing different op amps at digikey, and I was unable to find a search parameter that would tell me if the op amp handled capacitive loads. Is there a method other than pouring over tens of datasheets to get that info? In addition to the MC34071, I found the TS922 and LM7321 op-amps, which are rail-to-rail op-amps, would these work? Not that I would change away from the MC34071 in this project, it would seem to be perfect, cheap and effective.

I am back to trying to figure out R1, why should it be 200 ohms now instead of the 220 ohms in your original mod4? I am further trying to figure out how to go from base voltage to frequency modulation. I recognize that the CE capacitance changes with the base voltage as the audio signal enters Q2, but how much does it change? Where would I find that information? The 2n3904 datasheet talks about capacitance as a function of reverse bias (is this reverse bias?), but we are taking about tenths of picoFarads, is that enough? What is the equation that turns that capacitance into frequency in this circuit?

Audioguru, thanks again for your patience. Should you ever need to know something about biochemistry please don't hesitate to ask.

 

[audioguru]

You had R1 as 220n instead of 220 ohms. When I told you I made a typo error and typed 200 ohms by mistake.

The collector-base junction of a transistor is reverse-biased and the datasheet for the 2N3904 shows how much (actually how little) its capacitance is changed by voltage.