I came across a AM demodulation circuit that uses an emitter follower circuit before the envelope detector. I wanted to check whether using emitter follower would improve the demodulation circuit. I know that in electronics circuit, the input impedance should always be greater than the output impedance, by a factor of 10, to avoid loading effect. If this is not the case, the signal magnitude will be reduced in going through the circuit. A solution the loading effect is to use emitter follower circuit.
When we take the output from the emitter as output of the stage then the circuit is called emitter follower circuit. The advantage of using it is that it draws less power from the source because its input impedance is high while its output impedance is low. Emitter follower provides current gain but no voltage gain, but overall will provide power gain.
Below is the circuit diagram of the AM demodulator circuit designed with emitter follower circuit.
Note that I have modified this circuit from the AM demodulation circuit that I implemented with emitter follower.
The received AM signal(\(V_{am}\) with carrier frequency of 100KHz(in this example) enters the emitter follower circuit made up of transistor 2N3904 Q1, biasing resistors (R4,R5,R6,R3). The capacitor C2 and C3 are the input and output coupling capacitor. Note that lower value of C3 will decrease the amplitude of the output signal from the emitter follower. So the RF signal that is AM signal enters the emitter follower out of the output coupling capacitor C3 into the envelope detector circuit which is made up of diode R1, D1(1N4148), C and RL. The resistors R1, RL along with capacitor C determines the timing of charging and discharging which is explained in details in the guide designing an Envelope Detector Circuit for AM Demodulation. The value of this resistors and capacitor can be quickly calculated with the Online Envelope Detector Calculator. The output of the envelope detector circuit then enters the 2nd order Low Pass filter made up of the resistors and capacitors R2,C1 and R8,C4 with cutoff frequency of around 22KHz(I think the AM message bandwidth is 3KHz and so the cutoff frequency should be changed here). You can use the online LPF filter calculator to calculate the values of the resistors and capacitors. There is a resistor R7 which I added to pull up the dc voltage before the diode in order to bias the diode properly. Without it I found out that the output from the diode and after the low pass filters, the magnitude of the rectified signal and the filtered output are low.
The following shows the signal waveform of the transmitted message signal(\(V_{tx}\)), the AM signal transmitted/received(\(V_{am}\)), the diode rectified signal(\(V_{d}\)) and the received and recovered message signal(\(V_{rx}\)). Comparing the signals waveforms transmitted message signal(\(V_{tx}\)) and the received and recovered message signal(\(V_{rx}\)), we can see that proper demodulation of AM signal is achieved.
The following video demonstrates how this AM diode demodulator together with emitter follower works.
