All, I found this all thread with the goal to understand and to build a telecom from fundamental components. Thank you for contibuting this info here. After reading the above ideas by all and measurements by #12, I see some important aspects I don't understand and which would help this a more complete discussion:
- Given the speaker's resistance (4 Ohms, 8 Ohms etc), there should be no issues in amplifying the sound by a high-Zin opamp. But as someone mentioned, the speaker may not be as easy to model as a voltage source with Rout=8Ohms. What is the model of a speaker? From theory thevenin/norton equivalent transformations, we can consider a it as either voltage source or current source. But the Ro stays the same in both representations. Let's say Ro=8 Ohms
- #12's testing showed that indeed, the signal output begins to drop if speaker is loaded with >100Ohms, => Zo_speaker (output _impedance_) is somewhere on the order of 10s of Ohms.
- I see that google provides hits with a speaker-to-microphone converter schematics that all use a common-base amplifier stage. If we use a common-base stage, the transistor input impedance (looking into the emitter) =1/gm. If speaker is viewed as a current source, then the Zin_amp needs to be << 8 Ohms, => 1/gm << 8, => gm >> 0.125 S or Mhos. This is because current will flow to a lower impedance node and we want it to flow into the amplifier, not its own Ro. So that could work if transistor is biased with plenty of current for a large gm. Seems feasible, but it's close.
- But why is common-gate current-input amp any easier/better than connecting the same speaker to an opamp? In this case, the speaker is a voltage source with very low output impedance so any basic opamp would do. Opamp Zin >>> 8Ohms (usually 100MOhm or GOhms), so a good power transfer is guaranteed without any design effort.
- What is it about speaker's properties that makes it a better match with a transistor emitter node? Is it its inductiveness, which will make the signal ring/resonate when the speaker sees a capacitive component at the opamp's input? And maybe the low impedance of the common-base will dampen the unwanted ringing?
I would appreciate any feedback. Thank you!
Moderator edit: new thread created from this.
- Given the speaker's resistance (4 Ohms, 8 Ohms etc), there should be no issues in amplifying the sound by a high-Zin opamp. But as someone mentioned, the speaker may not be as easy to model as a voltage source with Rout=8Ohms. What is the model of a speaker? From theory thevenin/norton equivalent transformations, we can consider a it as either voltage source or current source. But the Ro stays the same in both representations. Let's say Ro=8 Ohms
- #12's testing showed that indeed, the signal output begins to drop if speaker is loaded with >100Ohms, => Zo_speaker (output _impedance_) is somewhere on the order of 10s of Ohms.
- I see that google provides hits with a speaker-to-microphone converter schematics that all use a common-base amplifier stage. If we use a common-base stage, the transistor input impedance (looking into the emitter) =1/gm. If speaker is viewed as a current source, then the Zin_amp needs to be << 8 Ohms, => 1/gm << 8, => gm >> 0.125 S or Mhos. This is because current will flow to a lower impedance node and we want it to flow into the amplifier, not its own Ro. So that could work if transistor is biased with plenty of current for a large gm. Seems feasible, but it's close.
- But why is common-gate current-input amp any easier/better than connecting the same speaker to an opamp? In this case, the speaker is a voltage source with very low output impedance so any basic opamp would do. Opamp Zin >>> 8Ohms (usually 100MOhm or GOhms), so a good power transfer is guaranteed without any design effort.
- What is it about speaker's properties that makes it a better match with a transistor emitter node? Is it its inductiveness, which will make the signal ring/resonate when the speaker sees a capacitive component at the opamp's input? And maybe the low impedance of the common-base will dampen the unwanted ringing?
I would appreciate any feedback. Thank you!
Moderator edit: new thread created from this.