In my previous blog post on designing light communication receiver circuit using pulse frequency modulation (PFM) technique, I said that because of noise in the output signal I had to use unity gain Sallen-key 2nd order filter. And after the filtering, the message signal was weak and so I had to some kind of amplifier circuit. Here I will be writing about the BJT amplifier circuit I designed and added to the light communication receiver circuit.
The complete receiver circuit with the amplifier circuit is shown below.
This is a Pulse Frequency Modulation (PFM) Audio Receiver. It is designed to take high-frequency light pulses from a photodiode and convert them back into a clean, audible signal for a speaker.
I think this is a sophisticated receiver design because it uses a "hybrid" approach: Op-Amps for precision filtering (the Sallen-Key filter) and Discrete BJTs for high-gain pre-amplification.
1. The Input Stage (Light-to-Voltage)
D2 (Photodiode) & R7: This is the "eye" of the circuit. Light pulses hit the photodiode, creating tiny bursts of current. R7 converts that current into a voltage.
C5 & R8: These act as a high-pass filter to block DC sunlight/ambient light, letting only the fast PFM pulses pass through.
2. The PFM Demodulator (U3:A & U3:B)
U3:A (Transimpedance/Buffer): This stage stabilizes the weak signal. With R9 at 500MΩ, you have massive sensitivity to even the tiniest flickers of light.
U3:B (Sallen-Key Low Pass Filter): This is the "brain." Its job is to remove the $25\text{kHz}$ carrier pulses (the "noise") and leave behind only the smooth audio sine wave. It turns the "digital-like" pulses back into "analog" sound.
These are built using single TL072 operational amplifier.
3. The BJT Pre-Amplifier (Q1)
Why it's here: The signal coming out of the filter is very clean but very weak (only a few millivolts).
Operation: This is a Voltage Divider Bias amplifier.
R14 ($6.8\text{k}\Omega$): This is the load. Making it larger (up from 4.5k) gives the extra voltage swing needed.
C13 ($100\mu\text{F}$): This is the Bypass Capacitor. It "shorts" the emitter to ground for audio, boosting the gain to over $100\times$.
C12 ($1\text{nF}$): This is the Miller Capacitor which is used to kill any leftover high-frequency "hiss" to keep the audio signal smooth.
4. The Final Power Stage (U4 LM386)
RV4 (Volume Pot): This allows you to control how much of the boosted signal reaches the power amp.
U4 (LM386): Unlike the other stages, this is a Current Amplifier. It takes the high-voltage signal and provides the "muscle" (amps) needed to physically move the speaker cone (LS1).
C9 ($220\mu\text{F}$): This blocks DC voltage from the speaker so only the AC sound wave reaches it, preventing the speaker from burning out.
Here I wanted to talk about the BJT amplifier design. I tried to use op-amp based amplifier but somehow it did not produce the high voltage gain I needed. I have to try it again; it should work but I opted to go with discrete bipolar transistor-based amplifier design.
Below is figure showing only the BJT amplifier.
The BJT is built around the BJT transistor 2N3904. It is biased using voltage divider biasing technique and I used the BJT amplifier design calculator to calculate the resistor and capacitors values. The resistors R1,R2, RC and RE are biasing resistor to bias the transistor in its active region while the capacitor C1, C2 are coupling capacitor and C3 is the emitter bypass capacitor.
The following shows the signal waveform of the pulsed PFM signal(yellow), then the signal at the input of the BJT amplifier(blue), the signal at the output of the BJT amplifier(pink) and the output of the LM386 audio amplifier(green).
The gain of the BJT amplifier was around 124. Here I used single tone 1khz sine wave for testing and building the circuit.
