digital-logicfetlogic-levelpower electronics
I have been using the BSS138 level converter circuit for a while now and today it struck me I get how the voltage is stepped up by the FET or the BSS138 when you are applying say 3.3 V on the LV side.
What happens in the FET to reverse that and make it bidirectional?
Let's start with the electret microphone basics.
The actual microphone is formed by a (very very) thin sheet of mylar film that is charged. This is connected to the metal body of the electret capsule but insulated from the pick-up plate. The mylar film and the pick-up plate form a capacitor. The charge on the mylar film produces a voltage difference between it and the pick-up plate. (V = Q/C)
As the mylar film is moved by the sound waves it changes the distance between it and the pick-up plate and changes the capacitance value as the distance between the 'plates' increases and decreases.
This is effectively what your electret microphones consist of.
Most electrets contain a FET which converts the very high impedance of the 'capacitor' voltage into a more useful form. It consists of a FET with the gate connected to the pick-up plate, the drain connection as a '+ pin' and the source connected to the ground (metal body).
You could add a FET but given that electrets are so inexpensive, by the time and money you would spend adding a FET to your electrets it would be a lot easier and cheaper and more reliable just to buy a new electret microphone with the FET built into it.
TL;DR Your frequency is too high for that method of level shifting.
Old thread but on the off chance that someone stumbles across this like I did while looking for answers here's what is likely wrong:
I built my own level shifter based on the sparkfun level shifter which you are using. Like you I am attempting to shift 3.3V up to 5V at a frequency of 1MHz. I used a slightly different transistor/MOSFET and mine was just outputting 0V when on.
Changing my resistor values down to 100 ohms improved the output (I got spikes that went to about 3.3V so still not what was desired).
I then tried reducing the frequency to 1 KHz and the level shifter worked perfectly.
Basically I think the pwm turns the gate on (output goes low) and when it turns off again it takes awhile (too long at high freq) for the output to become high again. The resistor is just too slow. Smaller resistor values help but at 1 MHz I've found it's not quite good enough.
Alternate options to consider include:
Best Answer
When the HV side goes low, the body diode becomes forward biased, pulling down the source to a lower voltage than the gate.
Provided the gate is pulled up to a sufficiently high voltage for that new source voltage to bias the channel into conduction, the gate pullup takes over and fully enhances the channel.