Why Not Use a Diode to Block DC

I'm Bob newhamel today I'm going to answer a question from a viewer who goes by the name of mons and his question is on the video about small signal amplifiers and the question is this is probably a stupid question but why do we use a capacitor instead of a diode to prevent the DC from going from the circuit to the microphone and that is not a stupid question it's actually a fairly common question which is why I'm going to make a video to answer it so let's take a look at what he's talking about I'm not going to show the whole circuit I'm going to just show the part in question if you want to see the whole video you can see it linked below but we have a small signal amplifier there's the input to the transistor we have some bias resistors here to get the transistor to the voltage that we need on the base of the transistor and out here we have a microphone going to ground that goes to ground too and what we don't want is this DC voltage going through the microphone we don't want any current going through the microphone we just want the current from the microphone at least the signal from so what do we do we put a capacitor here and so that blocks the direct current from going through the microphone but the alternating current because the current is constantly changing when we have a constant because the voltage is constantly changing when we have a constantly changing voltage we get a constant current through the capacitor watch my video on how capacitors work to know more about that but in a nutshell whenever the voltage changes across a capacitor we get a short period of time when current flows through it as things equalize and then it stops flowing but with the microphone if we have a constantly changing voltage here because we're speaking into it or whatever we're going to get a constantly changing voltage across the capacitor so we get current going this way and then that way back and forth through the capacitor so even though the direct current cannot make it through the capacitor the signal from the microphone goes right through essentially since this voltage goes up and down it causes this voltage to go up so even though we have these bias resistors holding this at a particular voltage the voltage coming from the microphone and then coming right through the capacitor