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I cannot get the second BJT amplification of Experiment 11 of Charles Platt's Make: Electronics to work, and I'm all out of clues. This question is very similar, but not identical as my circuit works fine up to the first amplification. This question is also about experiment 11, but does not seem related.

The first 2N2222 is fed the output of the 2N6027, which discharges the 0.0047uF cap with a frequency in the kHz range; this amplifies this signal. If I hook up a speaker between the first 2N2222's emitter and ground, I hear a buzzing sound, just as the book says.

Now, if I attempt to amplify this buzzing with the second 2N2222 as shown in the circuit, the speaker just cackles a bit. Initially I thought I might have overloaded the speaker, but no matter how much I experiment with the various resistors, I don't here any buzzing.

Platt: experiment 11

I guess this is something that can easily be analyzed with an oscilloscope, but unfortunately I don't have access to one yet.

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  • \$\begingroup\$ Please draw schematics so one doesn't have to tilt one's head to understand them easily. High power voltage should be at top, low power voltages (ground in your case) at bottom, and logical flow of signals left to right. Surely this circuit wasn't laid out this way in the book you got it from, right? Also label the battery voltage! \$\endgroup\$ Jul 10, 2014 at 21:32
  • \$\begingroup\$ I found a very similar question I missed in my first search, it contained a full circuit diagram. I updated the question to use that circuit. \$\endgroup\$
    – Duoran
    Jul 10, 2014 at 22:54

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I think the main problem is that your circuit isn't DC-biased near the middle of its operating range. Your darlington drive requires two junction drops of DC on the input before anything happens. However, a small voltage change on the base of Q1 can cause a large voltage change accross the speaker. As a result, you can't really bias this circuit reliably without some feedback.

The simplest way to add bias is to put a voltage divider from the top of the the speaker to the base of Q1. Figure the base of Q1 needs to be around 1.3 V. You don't show what the power voltage is, so I'm going to do the example assuming 12 V. You want the junction between the speaker and R3 to be about half way from either limit, so around 6 V. To make 1.3 V from 6 V, the top resistor needs to be about 3.6x the bottom one. 36 kΩ for the top and 10 kΩ for the bottom should get you somewhere into a workable range. Once you get things basically working, you can tweek the divider to get 1/2 the supply voltage on the top of the speaker.

To prevent whatever is happening on the other end of R1 from interfering with the DC bias of this section, put a DC blocking cap in series with it. Let's say the lowest frequency you care about is 20 Hz, so 1 µF will do nicely.

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