I've been playing around with designs for IR voice communicators, I thought it would be fun and informative to start with something basic, like the AM transmitter/receiver pair given by Forrest Mims, which I've attached. I built both circuits, using the same components listed (in particular, the 741), even though better options now exist. As I've built and tested the circuits, I have found myself stumped by three questions about the receiver circuit that I'm very much hoping you can help me with.
The first is a feature of the design. The 741 can only swing within a couple volts of the negative rail, so by tying the non-inverting input to ground, the output will be stuck at about (say) 2V unless the inverting input swings *below* ground. And this seems to be what happens when I measure it. Why not bias the inverting input to half the supply to allow symmetric swing? Am I missing something here?
My second question is about the role of the capacitor C1 at the inverting input. Here's how I'm seeing it, but please tell me if and how I go wrong. When phototransmitter Q1 is out of conduction, C1 charges exponentially toward 9V through R1. If Q1 begins to conduct, the collector voltage drops suddenly, as the current flows through R1. This happens faster than C1 can discharge, so the voltage at the 741's inverting input drops by the same amount. (This voltage rises as C1 discharges.) If the drop is sufficient, then the inverting input actually drops below ground allowing the 741's output to rise. And this change is transmitted through the potentiometer R3 into the 386's input. Is this the correct way of seeing it? If not, why not?
My third question deals with the LM386. In the version I built, the input to 386 look good on the scope but the output does not vary. Perhaps I fried the chip, but what may be the problem is that I don't exactly understand the relationship between the 386's output and its input. I've gone over the data sheet, and I've bypassed the power supply pins properly (I'm pretty sure). I understand that the 386 output is self-centering at half the supply voltage, but how does the output voltage vary with the input when the inverting input is grounded? For instance, does the average voltage map to Vs/2 with deviations from that average amplified? If not, what determines the output's deviation from Vs/2? If so, over what time scale is that average taken? And I know that the 386 does not act like a standard op-amp, but then what is the difference between the inverting and noninverting input?
I'd appreciate any help you can offer on these questions. Thanks so much.
The first is a feature of the design. The 741 can only swing within a couple volts of the negative rail, so by tying the non-inverting input to ground, the output will be stuck at about (say) 2V unless the inverting input swings *below* ground. And this seems to be what happens when I measure it. Why not bias the inverting input to half the supply to allow symmetric swing? Am I missing something here?
My second question is about the role of the capacitor C1 at the inverting input. Here's how I'm seeing it, but please tell me if and how I go wrong. When phototransmitter Q1 is out of conduction, C1 charges exponentially toward 9V through R1. If Q1 begins to conduct, the collector voltage drops suddenly, as the current flows through R1. This happens faster than C1 can discharge, so the voltage at the 741's inverting input drops by the same amount. (This voltage rises as C1 discharges.) If the drop is sufficient, then the inverting input actually drops below ground allowing the 741's output to rise. And this change is transmitted through the potentiometer R3 into the 386's input. Is this the correct way of seeing it? If not, why not?
My third question deals with the LM386. In the version I built, the input to 386 look good on the scope but the output does not vary. Perhaps I fried the chip, but what may be the problem is that I don't exactly understand the relationship between the 386's output and its input. I've gone over the data sheet, and I've bypassed the power supply pins properly (I'm pretty sure). I understand that the 386 output is self-centering at half the supply voltage, but how does the output voltage vary with the input when the inverting input is grounded? For instance, does the average voltage map to Vs/2 with deviations from that average amplified? If not, what determines the output's deviation from Vs/2? If so, over what time scale is that average taken? And I know that the 386 does not act like a standard op-amp, but then what is the difference between the inverting and noninverting input?
I'd appreciate any help you can offer on these questions. Thanks so much.
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