Welcome back to electronics.

This is Dr. Robinson.

In this lesson, we are going to look at a diode envelope detector circuit.

In your previous lesson, you were introduced to voltage regulators and

our objectives for today's lesson are to introduce amplitude modulation and

demodulation and introduce an envelope detector circuit.

Amplitude Modulation is the modification of the amplitude of a waveform

by variation of a second waveform.

Now these wave forms can be any sort of wave form, a triangle wave,

a square wave, but let me show you how we would form an amplitude modulated

signal from two cosine waves.

Here are the equation of a wayform that I'm referring to as the carrier signal.

You can see that it is a cosine function that varies between plus 1 and

minus 1 at a frequency of omega c.

We multiply that by an amplitude A so

the overall carrier signal varies between plus A and minus A.

Now here's a second cosine function that I'm referring to as the message signal.

Again, it has an amplitude, this time equal to k and

it's varying at a frequency of omega m, the message frequency.

Now if we combine these two signals in this way,

we form an AM modulated signal where the AM modulated

signal s(t) is equal to 1+m of t, all times c of t.

Now in a cosine function like this,

the quantity here in this position is the amplitude.

And in this case, it's a constant a, it doesn't vary with time.

But in the AM signal, we can consider this entire quantity here