operational-amplifier
This is a schematic for the ZX sound module:
My understanding is that the first part is a high-pass filter followed by a non-inverting amplifier.
But isn't the second op-amp a comparator? It seems to me that it would output a HIGH if V- < 0.01*Vcc, or LOW otherwise. So, I am just wondering how it outputs a "0-5V varying output" as described in the note above the schematic.
There is a reason non-inverting 1st order high-pass filters are sometimes used and this is to preserve the dc level in the circuit. It's horses for courses - some designs will want to preserve the DC level, some won't care and some won't want to. It's not a black and white sort of thing.
When it comes to a 2nd order high-pass filter the non-inverting configuration (Sallen Key type) will usually be preferred and this doesn't preserve the dc level: -
Again horses for courses.
Figure 1. The various filter componenents.
Figure 2. An active high-pass filter. Source: Electronics Tutorials.
The RC networks (2-3) and (4-5) form active band-pass filters.
One friend suggested that a differentiator forms a band-pass filter and an integrator + differentiator combination cancels each other out, leaving you with a linear amplifier with a band-pass filter.
The integrator and differentiator would only cancel out if they had the same cut-off frequencies. They don't so you get a band-pass filter.
If that's that case, why would we want that?
The datasheet explains on page 3: The magnitude of the Doppler Shift is proportional to reflection of transmitted energy and is in the range of microvolts (µV). A high gain low frequency amplifier is usually connected to the IF terminal in order to amplify the Doppler shift to a processable level (see Annex 1). Frequency of Doppler shift is proportional to velocity of motion. Typical human walking generates Doppler shift below 100 Hz.
Best Answer
The 1st stage not only amplifies the mic input but rectifies it - you get half wave rectified audio superimposed on the 0V rail. There are no diodes for the rectification - the 1st stage relies on the op-amp being biased at 0V and it's output cannot go below 0V therefore the output is 0V for no signal and as signal rises, the output develops positive pulses equivalent to the half cycles of the input signal.
The 2nd stage is a comparator (like you assumed) with a threshold voltage set at about 72mV (1k and 68k on a 5V rail) BUT I think they have the op-amp drawn the wrong way round so I'm going to look a little further into this one.
Aha: -
Pin 6 and Pin 5 are incorrectly shown in their diagram. The above picture is the proper pin-out and the guys making the module have slipped-up in their diagram showing pin 6 as +Vin!!