Electronic – Addition of HF Capacitor to Constant Current Source

Some comments: I constructed initially with only one transistor (BC107) and tested by giving audio signal as input. The output from the 8 ohm speaker was pretty much distorted, which I think was due to impedance mismatch. This is NOT caused by impedance mismatch. An 8 ohm speaker is a very low impedance, one tiny BC107 will have BIG trouble to drive it. Not at low volumes but at even a moderate volume, a BC107 wil not cut it. What happens when the BC107 reaches its limits ? I starts to clip meaning it cannot provide more signal to the speaker. This clipping you will hear as distortion.

Concerning impedance mismatch: You do not need impedance matching to drive a speaker. 99.99% of audio amplifiers have a very low output impedance. When loaded with an 8 ohm load that means there is actually a large impedance mismatch ! But that is OK since what we want is to put a voltage across the speaker. You usually only need impedance matching for high-frequency signals like more than 10 MHz or so OR for transporting audio signals over long a distance (phonelines).

Now you have oscillations and I think that is because you increased the gain of your amplifier (since you added an extra stage). I hope you agree that the base of Q1 is the input of your amplifier. Note how the voltage at this input is just a divided down version of the supply voltage. If the supply voltage varies, the voltage at the input will also vary !

Now if there is a signal going to the speaker, the second stage draws current. I guess you're feeding this amplifier with a battery so the supply voltage will drop a little when there is current being drawn. This small drop in supply voltage will also appear at the input and it will be amplified and fed to the speaker. Then the speaker will draw some current. See the there is a loop here ? For an oscillator you need a loop. You build one.

I suggest to decrease the gain, you can do that not by changing the value of Cb but by placing a resistor in series with Cb. A value of 180 ohms is a good starting point.

What is wrong with this circuit?

Typically a circuit like this has a gain that is roughly numerically the collector impedance divided by emitter impedance. So if you had 2k2 in the collector and 220 ohms in the emitter you'd get a voltage gain of ten.

This type of circuit is usually pushed to have much higher gains by bypassing the emitter resistor with a capacitor. This makes the emitter impedance very low at audio frequencies but also significantly increases distortion.

Then if you really want things to go wrong you make collector impedance something really low by hanging a 4 ohm speaker on it. Collector impedance (AC) falls from 2k2 to about 4 ohms and you have virtually no gain and lots of distortion.

Reducing the value of emitter bypass capacitor reduces distortion but also reduces the gain. How do I find the correct value for getting good performance at audio frequencies?

Use a resistor in series with C1 to control the AC gain.

Using this single stage I am able to get very less audio output. Its almost inaudible. If I use a second stage, I get highly distorted output. However it is audible to certain extent. What should I do to reduce the distortion without affecting the gain?

See my observations above - don't hang a speaker on the collector - use an emitter follower or better still get an audio amplifier IC like the LM386.

Can you please provide the design steps involved in choosing the capacitor values?

I think this has been done.