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I designed a single stage transistor amplifier as shown in this schematic:

enter image description here

I constructed this on a breadboard and applied the DC bias voltage to check the biasing conditions. Prior to this, I made the following assumptions: 1. The collector current ( also almost equal to emitter current) is 2mA. 2. The small signal gain of the transistor is 100. 3. The collector to emitter saturation voltage is 0.2V.

Now with these assumptions, I calculated the resistor values Rc and Re. As per my analysis, I had predicted that the voltage drop across Rc, Rc and Vce would be 3V. But in practice, when I measured the DC voltages, I found the voltage across Rc to be 4.1V, that across emitter was 4.2 volts and the voltage Vce was 0.7 volts.

This meant that the transistor was being driven into saturation and when I connected to a speaker, the output was attenuated rather than amplification. Where did I go wrong in my analysis?

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  • \$\begingroup\$ I think your biasing voltage is too high. Try replacing R2 with a 20K pot. then test with the pot in a few diff. positions to see if you can get a desirable respose. \$\endgroup\$ – Robherc KV5ROB Jan 30 '16 at 6:48
  • \$\begingroup\$ I tried this now. The result is the same. Vce remains at 0.7V \$\endgroup\$ – Sâu Jan 30 '16 at 6:53
  • \$\begingroup\$ With R2 turned down to 0, Vce should have jumped to almost 9V \$\endgroup\$ – Robherc KV5ROB Jan 30 '16 at 6:57
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    \$\begingroup\$ He's saying that with R2 turned down to zero the base voltage should drop to zero, and the transistor will turn off. The emitter voltage should then drop to zero and the collector voltage go to 9 V. Report back on Vb, Ve and Vc with R2 turned down to zero. It sounds as though R2 isn't connected properly. Add photo of breadboard to original post. \$\endgroup\$ – Transistor Jan 30 '16 at 8:10
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    \$\begingroup\$ Double-check that you have connected the transistor correctly. It's easy to get it wrong and Vce=0.7V sounds suspiciously like Vbe or Vbc for a forward biassed diode. \$\endgroup\$ – Brian Drummond Jan 30 '16 at 12:51
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Reduce Re so it is much less than Rc then reduce R2 so it is much less than R1 .Now you will be able to get a reasonable Vce so your amplifier will work .If you want to drive a speaker then put a complementry emitter follower between the output and the speaker.

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  • \$\begingroup\$ ....much less than Rc? I would not suggest to reduce Re because for a good DC stability - as a rule of thumb - the DC drop across Re should be app. 20...30 % of Vcc. (Now, it is 33%). It is sufficient to increase R1 or reduce R2 accordingly (by app. 10%). \$\endgroup\$ – LvW Jan 30 '16 at 11:26

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