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I am working on a simple FM radio receiver. Built the circuit and it works very well. Learned few new things, and got some questions, especially the output stage. The Op-amp 2822 is a dual op-amp, but the output is only mono. One of the op-amp output is connected to one side of the speaker and other output to other side. The speaker doesn't connect to the ground in this setup, Very strange. there is Also a 10u electrolytic capacitor connected to the inverting inputs of these 2 op-amp. will a normal 104 ceramic cap works? and also what are the purpose of C11, C12, C14 and R6?

Another thing is CF2 and CF1; Are they ceramic resonators? but one has 3 leads while other only 2. and do they have polarity?

last very interesting thing is from the input stage. There are 2 resistors connected to the collector of the 9018, and there is a biased resistor 150k connected in between. what is the benefit of this setup? (instead of using one collector resistor and connect the biased resistor to the collector directly. )

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  • \$\begingroup\$ I'm giving a +1 for a well presented and researched question. \$\endgroup\$
    – ChrisR
    Commented Jan 27, 2016 at 13:17

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there is Also a 10u electrolytic capacitor connected to the inverting inputs of these 2 op-amp. will a normal 104 ceramic cap works?

C16 couples the inverting inputs of the audio amplifiers together for AC signals. Yes you can use a ceramic cap also as long as it is also 10 uF.

and also what are the purpose of C11, C12, C14 and R6?

All these Caps have one pin to ground, the other to the signal node so they for filtering high frequency noise out of the signal.

R6 has probably to do with the volume adjust. Not how R6 is 10 k but the potmeter is 50 k. Maybe the potmeter is not a logarithmic one like you would normally use for audio. Then a cheap trick to fake a more logarithmic behavior is to load the potmeter with a relatively low resistance, R6 in this case.

Another thing is CF2 and CF1; Are they ceramic resonators?

CF1 (3 pins) is (probably) a SAW (Surface Acoustic Wave) filter. It behaves lika a bandpass filter but with much narrower bandwidth than you could easily get from an LC filter. Note how the bottom pin of CF2 is actually connected to the positive supply rail, the other pins are input and output.

CF2 (2 pins) can be either a crystal or a ceramic resonator. Since it has a 330 ohms resistor in series I conclude it's a resonator. Crystals are more expensive and placing a resistor in series with a crystal deteriorates it's properties. I think here this resonator is used as a filter for the detector output, see the CD2003 datasheet.

"There are 2 resistors connected to the collector of the 9018, and there >is a biased resistor 150k connected in between. what is the benefit of >this setup?"

It's a CE stage (Common Emitter), part of the output signal is fed through that 150k resistor back to the input. This provides (local) feedback. It reduces the gain of that stage but improves linearity and bandwidth. Exactly the same way as when using feedback with an opamp.

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  • \$\begingroup\$ Thank you. After more digging, Could CF1 be a Crystal ceramic filters or maybe SAW filter is the same thing? (can't find any parts from ebay using search word SAW filter) Never heard of these names either. Google doesn't show any detail information as well. According to the circuit they are connected to Fm Out and Fm In, so it much be some kind of filter. I also found a wiki entry: en.wikipedia.org/wiki/Crystal_filter, could CF1 be it? "A crystal filter is very often found in the intermediate frequency (IF) stages of high-quality radio receivers." \$\endgroup\$
    – user83582
    Commented Jan 27, 2016 at 11:50
  • \$\begingroup\$ That Crystal filter consists of many resonators to make a filter. This SAW filter is already a filter by itself in one component. Note how at CF1 (Cermic Filter) it says: L10.7A, this refers to the IF frequency of 10.7 MHz which is used in many FM radios. SAW filters can be expensive, also we have full on-chip solutions these days so you won't see many SAW filters in modern products. 20 years ago though, it was quite common to use a SAW filter as IF channel filter. \$\endgroup\$
    – Bimpelrekkie
    Commented Jan 27, 2016 at 12:21
  • \$\begingroup\$ CF1 is a ceramic filter rather than a SAW device. CF2 may be a phase shifter required for the quadrature detector (FM demodulator) - a kind of ceramic filter but optimised for linear phase across the IF passband. \$\endgroup\$
    – user16324
    Commented Jan 27, 2016 at 13:10
  • \$\begingroup\$ @BrianDrummond According to muRata, see: murata.com/~/media/webrenewal/support/library/catalog/products/… page 3, a crystal filter is a piezoelectrical device. I think it operates in the same way as a SAW filter. In my opinion: SAW filter = ceramic filter. Same thing different names. But if I'm wrong please explain the difference as I could not find any. \$\endgroup\$
    – Bimpelrekkie
    Commented Jan 27, 2016 at 13:23
  • \$\begingroup\$ There are similarities but piezoelectric effects are usually bulk rather than surface properties and I think that's the difference. \$\endgroup\$
    – user16324
    Commented Jan 27, 2016 at 13:27
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The opamp outputs are connected as a 'bridge'.

The caps cause a 180 degree phase change in the inputs, and the outputs are therefore 180 degrees out of phase as well.

You can therefore get double the voltage swing on the loudspeaker for the same battery voltage.

P = v^2/R

Double the voltage, 4 times the power into the same load! (provided you can manage the current)

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    \$\begingroup\$ Correct and there's also another advantage to a bridge, you don't need a large output capacitor for the speaker. This can save space and/or cost. \$\endgroup\$
    – Bimpelrekkie
    Commented Jan 27, 2016 at 12:26
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    \$\begingroup\$ Note the TDA2822 is NOT strictly an opamp but at audio power amp, datasheet here : st.com/web/en/resource/technical/document/datasheet/… \$\endgroup\$
    – user16324
    Commented Jan 27, 2016 at 13:06

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