For learning purposes I mounted basic audio amplifier on breadboard, schematic below:
Basically it works, meaning voltage has been slighly icreased as well as current and as a result I hear the sound in small 8 Ohm, 0.5W speaker. Nevertheless I have a few questions. Thanks in advance for detailed answers:
1.Looking at the schamatic we see point B (red dot). If I remove the C2 capacitor I have no output signal. My explanation to this is, without C2 audio signal is "shoted" to ground immediately before it reaches Q1 base. With C2 however, its right plate is constantly charged and discharged and therefore the signal has the chance to reach Q1 base. Is that good explanation?
2.Even though I use samll values resistor across the circuit I have small voltage and current amplification. Especially I would expect high current gain but it's not so. It's enough to put ~100 Ohm on the emitter terminal and I don't hear anything form the speaker. Below scope capture shows voltage levels having the gain of max 3. It did not help if I powered the circuit from 9V battery instead of 3. (changing base bias of cource). So why I was able to achieve only tiny gain?
- I am a bit suprised that it works at all, since audio input signal has positve as well as negative components. While positive components are ok, negative swings should stop transistor operation. My explanation is that the negative swings on left capacitor plate rises right capaciotr plate to positive value with magnitute two times bigger and therefore traniststor is ON. Does it make sense?
Blue signal – Audio_Out
Yellow signal – Audio_In
Best Answer
C2 is your input coupling capacitor meaning it allows your AC signal to pass thru and not any DC current. If you connect your source directly to the rest of the circuit (direct coupling) without C2, you will effectively be shorting the base to ground thru your source and the transistor will no longer be in forward-active mode, which is why you get no signal.
This circuit doesn't stand a chance of driving an 8 ohm speaker. You are going to see extreme loading effects. For starters, output impedance is too high and it can't provide enough current to the load. You will need a power amp to drive a speaker. A push-pull stage might work if you design it properly.
The purpose of your resistive divider at the base is to bias the base above ground at a certain DC voltage. Coupling thru C2 allows you to superimpose your AC signal on top of that DC bias voltage. So you have a little room to swing negative. Might make it more clear if you look at the AC equivalent circuit.